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Configuring Source-Route Bridging

Configuring Source-Route Bridging

This chapter describes source-route bridging (SRB) configuration tasks. For a discussion of remote source-route bridging (RSRB) configuration tasks, refer to the "Configuring Remote Source-Route Bridging" chapter in this publication.

For a complete description of the commands mentioned in this chapter, refer to the "Source-Route Bridging Commands" chapter in the Bridging and IBM Networking Command Reference.

SRB Configuration Task List

Perform the tasks in the following sections to configure SRB:

See the end of this chapter for "SRB Configuration Examples."

The Cisco IOS software issues a warning if a duplicate bridge definition exists in a router. You must remove an old bridge definition before adding a new bridge definition to a router configuration.

Configure Source-Route Bridging

Our implementation of source-route bridging enables you to connect two or more Token Ring networks using either Token Ring or Fiber Distributed Data Interface (FDDI) media.

As designed by IBM and the IEEE 802.5 committee, when a router is configured as a source-route bridge, bridged traffic does not pass across non-Token Ring media, and only those protocols that are not being routed are source-route bridged. For example, if IPX routing is enabled on a router that is configured for source-route bridging, IPX datagrams will not be source-route bridged. However, datagrams for other nonrouted protocols will be source-route bridged. Our implementation of source-route bridging extends this definition.

A dual-port bridge is the simplest possible source-route bridging configuration. When configured as a dual-port bridge, the access server or router serves to connect two Token Ring LANs. One LAN is connected through one port (Token Ring interface), and the other LAN is connected through the other port (also a Token Ring interface). Figure 44 shows a dual-port bridge.


Figure 44: Dual-Port Bridge


A dual-port bridge is a limitation imposed by IBM Token Ring chips; they can only process two ring numbers. If you have a router with two or more Token Ring interfaces, you can work around the two-ring number limitation. You can configure your router as multiple dual-port bridges or as a multiport bridge using a virtual ring.

You can define several separate dual-port bridges in the same router. However, the routers on the LANs cannot have any-to-any connectivity; that is, they cannot connect to every other router on the bridged LANs. Only the routers connected to the dual-port bridge can communicate with one another. Figure 45 shows two separate dual-port bridges (T0-T2 and T1-T3) configured on the same router.


Figure 45: Multiple Dual-Port Bridges


A better solution for overcoming the two-ring number limitation of IBM Token Ring chips is to configure a multiport bridge using a virtual ring. A virtual ring on a multiport bridge allows the router to interconnect three or more LANs with any-to-any connectivity; that is, connectivity between any of the routers on each of the three LANs is allowed. A virtual ring creates a logical Token Ring internal to the Cisco IOS software, which causes all the Token Rings connected to the router to be treated as if they are all on the same Token Ring. The virtual ring is called a ring group. Figure 46 shows a multiport bridge using a virtual ring.


Figure 46: Multiport Bridge Using a Virtual Ring


To take advantage of this virtual ring feature, each Token Ring interface on the router must be configured to belong to the same ring group. For information about configuring a multiport bridge using a virtual ring, see the "Configure a Multiport Bridge Using a Virtual Ring" section later in this chapter.

Our implementation of SRB expands the basic functionality to allow autonomous switching of SRB network traffic for FDDI interfaces (see Figure 47), adding counters to SRB accounting statistics, and implementing process-level switching of SRB over FDDI. This functionality provides a significant increase in performance for Token Rings interconnected across an FDDI backbone.


Note Autonomous FDDI SRB is supported on Cisco 7000 series routers where autonomous switching is possible.

Figure 47: Autonomous FDDI SRB


You can configure the Cisco IOS software for source-route bridging by performing the tasks in one of the first three sections and optionally, the tasks in the last section:

Configure a Dual-Port Bridge

A router equipped with Token Ring cards is by default a Token Ring host, and SRB is disabled by default. To configure a dual-port bridge that connects two Token Rings, you must enable source-route bridging on each of the Token Ring interfaces that connect to the two Token Rings. To enable source-route bridging, perform the following task in interface configuration mode for each of the Token Ring interfaces:
Task Command

Enable local source-routebridging on a Token Ring interface.

source-bridge local-ring bridge-number target-ring

For multiple dual-port source-route bridges, repeat this task for each Token Ring interface that is part of a dual-port bridge. If you want your network to use only SRB, you can connect as many routers via Token Rings as you need. Remember, source-route bridging requires you to bridge only Token Ring media.


Note Ring numbers need to be unique across interfaces and networks, so that when you enable source-route bridging over an interface, the local and target rings are defined. Each node on the network will know if it is the target of explorer packets sent on the network.

Configure a Multiport Bridge Using a Virtual Ring

To configure a source-route bridge to have more than two network interfaces, you must perform the following tasks in the specified order:

Once you have completed these tasks, the router acts as a multiport bridge not as a dual-port bridge.


Note Ring numbers need to be unique across interfaces and networks.

Define a Ring Group in SRB Context

Because all IBM Token Ring chips can only process two ring numbers, we have implemented the concept of a ring group or virtual ring. A ring group is a collection of Token Ring interfaces in one or more routers that share the same ring number. This ring number is used just like a physical ring number, showing up in any route descriptors contained in packets being bridged. Within the context of a multiport bridge that uses SRB rather than RSRB, the ring group resides in the same router. See the "Configuring Remote Source-Route Bridging" chapter to compare ring groups in the SRB and RSRB context.

A ring group must be assigned a ring number that is unique throughout the network. It is possible to assign different Token Ring interfaces on the same router to different ring groups, if, for example, you plan to administer them as interfaces in separate domains.

To define or remove a ring group, perform one of the following tasks in global configuration mode:
Task Command

Define a ring group.

source-bridge ring-group ring-group [virtual-mac-address]

Remove a ring group.

no source-bridge ring-group ring-group [virtual-mac-address]

Enable SRB and Assign a Ring Group to an Interface

After you have defined a ring group, you must assign that ring group to those interfaces you plan to include in that ring group. An interface can only be assigned to one ring group. To enable any-to-any connectivity among the end stations connected through this multiport bridge, you must assign the same target ring number to all Token Ring interfaces on the router.

To enable SRB and assign a ring group to an interface, perform the following task in interface configuration mode:
Task Command

Enable source-route bridging and assign a ring group to a Token Ring interface.

source-bridge local-ring bridge-number target-ring

Configure FDDI SRB

To configure autonomous FDDI SRB, perform the following tasks, beginning in global configuration mode:
Task Command

Configure an FDDI interface.

interface fddi slot/port 1

Enable source-route bridging.

source-bridge local-ring bridge-number target-ring

Enable autonomous switching.

source-bridge route-cache cbus

1This command is documented in the "Interface Commands" chapter of the Configuration Fundamentals Command Reference.


Note The multiring command and the LAN Network Manager are not supported on FDDI.

Configure Fast-Switching SRB over FDDI

Fast-Switching SRB/FDDI enhances performance. For example, if you want to use access-lists, fast-switching SRB/FDDI provides fast performance and access-list filters capability.

To configure fast-switching SRB/FDDI perform the following tasks, beginning in global configuration mode:
Task Command

Configure an FDDI interface.

interface fddi slot/port1

Enable source-route bridging.

source-bridge local-ring bridge-number target-ring

Enable source-bridge spanning.

source-bridge spanning

Enable fast-switching.

source-bridge route-cache

Enable the collection and use of RIF information.

multiring protocol-keyword

1This command is documented in the "Interface Commands" chapter of the Configuration Fundamentals Command Reference.

Enable the Forwarding and Blocking of Spanning-Tree Explorers

When trying to determine the location of remote destinations on a source-route bridge, the source device will need to send explorer packets. Explorer packets are used to collect RIF information. The source device can send spanning-tree explorers or all-routes explorers. Note that some older IBM devices generate only all-routes explorer packets, but many newer IBM devices are capable of generating spanning-tree explorer packets.

A spanning-tree explorer packet is an explorer packet that is sent to a defined group of nodes that comprise a statically configured spanning tree in the network. In contrast, an all-routes explorer packet is an explorer packet that is sent to every node in the network on every path.

Forwarding all-routes explorer packets is the default. However, in complicated source-route bridging topologies, using this default can generate an exponentially large number of explorers that are traversing the network. The number of explorer packets becomes quite large because duplicate explorer packets are sent across the network to every node on every path. Eventually each explorer packet will reach the destination device. The destination device will respond to each of these explorer packets. It is from these responses that the source device will collect the RIF and determine which route it will use to communicate with the destination device. Usually, the route contained in the first returned response will be used.

The number of explorer packets traversing the network can be reduced by sending spanning-tree explorer packets. Spanning-tree explorer packets are sent to specific nodes; that is, to only the nodes on the spanning tree, not to all nodes in the network. You must manually configure the spanning-tree topology over which the spanning-tree explorers are sent. You do this by configuring which interfaces on the routers will forward spanning-tree explorers and which interfaces will block them.

To enable forwarding of spanning-tree explorers on an outgoing interface, perform the following task in interface configuration mode:
Task Command

Enable the forwarding of spanning-tree explorer packets on an interface.

source-bridge spanning


Note While enabling the forwarding of spanning-tree explorer packets is not an absolute requirement, it is strongly recommended in complex topologies. Configuring an interface to block or forward spanning-tree explorers has no effect on how that interface handles all-routes explorer packets. All-routes explorers can always traverse the network.

To block forwarding of spanning tree explorers on an outgoing interface, perform the following task in interface configuration mode:
Task Command

Block spanning-tree explorer packets on an interface.

no source-bridge spanning

Enable the Automatic Spanning-Tree Function

The automatic spanning-tree function supports automatic resolution of spanning trees in SRB networks, which provides a single path for spanning explorer frames to traverse from a given node in the network to another. Spanning explorer frames have a single-route broadcast indicator set in the routing information field. Port identifiers consist of ring numbers and bridge numbers associated with the ports. The spanning-tree algorithm for SRB does not support Topology Change Notification BDPU.


Note Although the automatic spanning-tree function can be configured with SR/TLB, the SRB domain and transparent bridging domain have separate spanning trees. Each Token Ring interface can belong to only one spanning tree. Only one bridge group can run the automatic spanning-tree function at a time.

To create a bridge group that runs an automatic spanning-tree function compatible with the IBM SRB spanning-tree implementation, perform the following task in global configuration mode:
Task Command

Create a bridge group that runs the automatic spanning-tree function.

bridge bridge-group protocol ibm

To enable the automatic spanning-tree function for a specified group of bridged interfaces, perform the following task in interface configuration mode:
Task Command

Enable the automatic spanning-tree function on a group of bridged interfaces.

source-bridge spanning bridge-group

To assign a path cost for a specified interface, perform the following task in interface configuration mode:
Task Command

Assign a path cost for a specified group of bridged interfaces.

source-bridge spanning bridge-group path-cost path-cost


Note Ports running IEEE and IBM protocols form a spanning tree together on the LAN, but they do not mix in the router itself. Make sure the configurations are correct and that each LAN runs only one protocol.

See the end of this chapter for an example of source-route bridging with the automatic spanning-tree function enabled.

Limit the Maximum SRB Hops

You can minimize explorer storms if you limit the maximum number of source-route bridge hops. For example, if the largest number of hops in the best route between two end stations is six, it might be appropriate to limit the maximum source-route bridging hops to six to eliminate unnecessary traffic. This setting affects spanning-tree explorers and all-routes explorers sent from source devices.

To limit the number of SRB hops, perform one of the following tasks in interface configuration mode:
Task Command

Control the forwarding or blocking of all-routes explorer frames received on this interface.

source-bridge max-hops count

Control the forwarding or blocking of spanning-tree explorer frames received on this interface.

source-bridge max-in-hops count

Control the forwarding or blocking of spanning-tree explorer frames sent from this interface.

source-bridge max-out-hops count

Configure Bridging of Routed Protocols

Source-route bridges use MAC information, specifically the information contained in the routing information field (RIF), to bridge packets. A RIF contains a series of ring and bridge numbers that represent the possible paths the source node might use to send packets to the destination. Each ring number in the RIF represents a single Token Ring in the source-route bridged network and is designated by a unique 12-bit ring number. Each bridge number represents a bridge that is between two Token Rings in the SRB network and is designated by a unique 4-bit bridge number. The information in a RIF is derived from explorer packets traversing the source-route bridged network. Without the RIF information, a packet could not be bridged across a source-route bridged network.

Unlike source-route bridges, Level 3 routers use protocol-specific information (for example, Novell IPX or XNS headers) rather than MAC information to route datagrams. As a result, the Cisco IOS software default for routed protocols is to not collect RIF information and to not be able to bridge routed protocols. However, if you want the software to bridge routed protocols across a source-route bridged network, the software must be able to collect and use RIF information to bridge packets across a source-route bridged network. You can configure the software to append RIF information to routed protocols so that routed protocols can be bridged. Figure 48 shows a network topology in which you would want to use this feature.


Figure 48: Topology for Bridging Routed Protocols across a Source-Route Bridged Network


To configure the Cisco IOS software to bridge routed protocols, you must perform the task in the first section, and optionally, one or both of the tasks in the other sections as follows:

Enable Use of the RIF

You can configure the Cisco IOS software so that it will append RIF information to the routed protocols. This allows routed protocols to be bridged across a source-route bridged network. The routed protocols that you can bridge are as follows:

Enable use of the RIF only on Token Ring interfaces on the router.

To configure the Cisco IOS software to append RIF information, perform the following task in interface configuration mode:
Task Command

Enable collection and use of RIF information.

multiring {protocol-keyword [all-routes | spanning] | all | other}

For an example of how to configure the software to bridge routed protocols, see the "SRB and Routing Certain Protocols Example" section later in this chapter.

Configure a Static RIF Entry

If a Token Ring host does not support the use of IEEE 802.2 TEST or XID datagrams as explorer packets, you might need to add static information to the RIF cache of the router.

To configure a static RIF entry, perform the following task in global configuration mode:
Task Command

Enter static source-route information into the RIF cache.

rif mac-address rif-string {interface-name | ring-group ring}

Configure the RIF Timeout Interval

RIF information that can be used to bridge routed protocols is maintained in a cache whose entries are aged.


Note The rif validate enable commands have no effect on remote entries learned over RSRB.

To configure the number of minutes an inactive RIF entry is kept in the cache, perform the following tasks in global configuration mode:
Task Command

Specify the number of minutes an inactive RIF entry is kept.

rif timeout minutes

Enable RIF validation for entries learned on an interface (Token Ring or FDDI).

rif validate-enable

Enable RIF validation on an SRB that is malfunctioning.

rif validate-enable-age

Enable synchronization of the RIF cache with the protocol route cache.

rif validate-enable-route-cache

Configure Translation between SRB and Transparent Bridging Environments

Source-route translational bridging (SR/TLB) is a Cisco IOS software feature that allows you to combine SRB and transparent bridging networks without the need to convert all of your existing source-route bridges to source-route transparent (SRT) nodes. As such, it provides a cost-effective connectivity path between Ethernets and Token Rings, for example.

When a router is configured for SR/TLB, the router operates in fast-switching mode by default, causing packets to be processed in the interrupt handler when the packets first arrive, rather than queuing them for scheduled processing. The no source-bridge transparent fastswitch command is provided to disable fast-switched SR/TLB, causing the router to handle packets by process switching. For more information on disabling fast-switched SR/TLB, refer to the "Disable Fast-Switched SR/TLB" section in this chapter.


Note When you are translationally bridging, you will have to route routed protocols and translationally bridge all others, such as LAT.

Overview of SR/TLB

You can bridge packets between an SRB domain and a transparent bridging domain. Using this feature, a software "bridge" is created between a specified virtual ring group and a transparent bridge group. To the source-route station, this bridge looks like a standard source-route bridge. There is a ring number and a bridge number associated with a ring that actually represents the entire transparent bridging domain. To the transparent bridging station, the bridge represents just another port in the bridge group.

When bridging from the SRB (typically, Token Ring) domain to the transparent bridging (typically, Ethernet) domain, the source-route fields of the frames are removed. The RIFs are cached for use by subsequent return traffic.

When bridging from the transparent bridging domain to the SRB domain, the router checks the packet to see if it has a multicast or broadcast destination or a unicast (single host) destination. If it is multicast, the packet is sent as a spanning-tree explorer. If it is a unicast destination, the router looks up the path to the destination in the RIF cache. If a path is found, it will be used; otherwise, the router will send the packet as a spanning-tree explorer.

An example of a simple SR/TLB topology is shown in Figure 49.


Figure 49: Example of a Simple SR/TLB Topology



Note The Spanning-Tree Protocol messages used to prevent loops in the transparent bridging domain are not passed between the SRB domain and the transparent bridging domain. Therefore, you must not set up multiple paths between the SRB and transparent bridging domains.

The following notes and caveats apply to all uses of SR/TLB:

Caution Bridging between dissimilar media presents several problems that can prevent communication from occurring. These problems include bit order translation (or usage of MAC addresses as data), maximum transmission unit (MTU) differences, frame status differences, and multicast address usage. Some or all of these problems might be present in a multimedia bridged LAN and prevent communication from taking place. Because of differences in the way end nodes implement Token Ring, these problems are most prevalent when bridging between Token Rings and Ethernets or between Token Ring and FDDI LANs.

We currently know that problems occur with the following protocols when bridged between Token Ring and other media: Novell IPX, DECnet Phase IV, AppleTalk, VINES, XNS, and IP. Further, problems can occur with the Novell IPX and XNS protocols when bridged between FDDI and other media. We recommend that these protocols be routed whenever possible.

To enable SR/TLB, you must perform the task in the following section:

In addition, you can also perform the tasks in the following sections:

Enable Bridging between Transparent Bridging and SRB

Before enabling bridging, you must have completely configured your router using multiport SRB and transparent bridging. Once you have done this, establish bridging between transparent bridging and source-route bridging by performing the following task in global configuration mode:
Task Command

Enable bridging between transparent bridging and SRB.

source-bridge transparent ring-group pseudo-ring bridge-number tb-group [oui]

Disable Fast-Switched SR/TLB

To disable fast-switched SR/TLB and cause the router to handle packets by process switching, perform the following task in global configuration mode:
Task Command

Disable fast-switched SR/TLB.

no source-bridge transparent ring-group fastswitch

Enable Translation Compatibility with IBM 8209 Bridges

To transfer data between IBM 8209 Ethernet/Token Ring bridges and routers running the SR/TLB software (to create a Token Ring backbone to connect Ethernets), perform the following task on each Token Ring interface in interface configuration mode:
Task Command

Move data between IBM 8209 Ethernet/Token Ring bridges and routers running translational bridging software.

ethernet-transit-oui [90-compatible | standard | cisco]

Enable Token Ring LLC2-to-Ethernet Conversion

The Cisco IOS software supports the following types of Token Ring to Ethernet frame conversions:

For most non-IBM hosts, Token Ring LLC2 frames can be translated in a straightforward manner into Ethernet 802.3 LLC2 frames. This is the default conversion in the Cisco IOS software.

However, many Ethernet-attached IBM devices use nonstandard encapsulation of LLC2 on Ethernet. Such IBM devices, including PS/2s running OS/2 Extended Edition and RT-PCs, do not place their LLC2 data inside an 802.3 format frame, but rather place it into an Ethernet Type 2 frame whose type is specified as 0x80d5. This nonstandard format is called 0x80d5, named after the type of frame. This format is also sometimes called RT-PC Ethernet format because these frames were first widely seen on the RT-PC. Hosts using this nonstandard 0x80d5 format cannot read the standard Token Ring LLC2 to Ethernet 802.2 LLC frames.

To enable Token Ring LLC2 to Ethernet LLC2 conversion, you can perform one or both of the following tasks:

Enable 0x80d5 Processing

You can change the Cisco IOS software's default translation behavior of translating Token Ring LLC to Ethernet 802.3 LLC to translate Token Ring LLC2 frames into Ethernet 0x80d5 format frames. To enable this nonstandard conversion, perform the following task in global configuration mode:
Task Command

Change the Ethernet/Token Ring translation behavior to translate Token Ring LLC2 frames into Ethernet 0x80d5 format frames.

source-bridge enable-80d5

Enable Standard Token Ring LLC2-to-Ethernet LLC2 Conversion

After you change the translation behavior to perform Token Ring LLC2 frames into Ethernet 0x80d5 format frames, some of the non-IBM hosts in your network topology might use the standard Token Ring conversion of Token Ring LLC2 to 802.3 LLC2 frames. If this is the case, you can change the translation method of those hosts to use the standard translation method on a per-DSAP basis. The translation method for all the IBM hosts would still remain as Token Ring LLC2 to Ethernet 0x80d5 translation.

To define non-IBM hosts in your network topology to use the standard translation method while the IBM hosts use the nonstandard method, perform the following task in global configuration mode:
Task Command

Allow some other devices to use normal LLC2/IEEE 802.3 translation on a per-DSAP basis.

source-bridge sap-80d5 dsap

Configure NetBIOS Support

NetBIOS is a nonroutable protocol that was originally designed to transmit messages between stations, typically IBM PCs, on a Token Ring network. NetBIOS allows messages to be exchanged between the stations using a name rather than a station address. Each station knows its name and is responsible for knowing the names of other stations on the network.


Note In addition to this type of NetBIOS, which runs over LLC2, we have implemented another type of NetBIOS that runs over IPX. For information on the IPX type of NetBIOS, refer to the chapter "Configuring Novell IPX" in the Network Protocols Configuration Guide, Part 2.

NetBIOS name caching allows the Cisco IOS software to maintain a cache of NetBIOS names, which avoids the high overhead of transmitting many of the broadcasts used between client and server NetBIOS PCs (IBM PCs or PS/2s) in an SRB environment.

When NetBIOS name caching is enabled, the software performs the following actions:

The software will time out the entries in the NetBIOS name cache after a specific interval of their initial storage. The timeout value is a user-configurable value. You can configure the timeout value for a particular Token Ring if the NetBIOS name cache is enabled on the interface connecting to that Token Ring. In addition, you can configure static name cache entries that never time out for frequently accessed servers whose locations or paths typically do not change. Static RIF entries are also specified for such hosts.

Generally, NetBIOS name caching is most useful when a large amount of NetBIOS broadcast traffic creates bottlenecks on WAN media connecting distant locations, and the WAN media is overwhelmed with this traffic. However, when two high-speed LAN segments are directly interconnected, the packet savings of NetBIOS name caching is probably not worth the processor overhead associated with it.


Note NetBIOS name caching is not recommended to be turned on in backbone routers, particularly if you have it enabled in all the routers connected to the backbone. NetBIOS caching should be distributed among multiple routers. NetBIOS name caching can be used only between Cisco routers that are running Software Release 9.1 or later.

To enable NetBIOS name caching, you must perform the tasks in the following sections:

In addition, you can configure NetBIOS name caching as described in the following sections:

Enable the Proxy Explorers Feature on the Appropriate Interface

In order to enable NetBIOS name caching on an interface, the proxy explorers feature must first be enabled on that interface. This feature must either be enabled for response to all explorer packets or for response to NetBIOS packets only.

To determine whether the proxy explorers feature has been enabled, perform the following task in EXEC mode:
Task Command

Determine whether or not the proxy explorers feature has been enabled

show startup-config1

1This command is documented in the "System Image, Microcode Image, and Configuration File Load Commands" chapter of the Configuration Fundamentals Command Reference.

To determine whether proxy explorers has been configured for response to all explorer packets, look in the configuration file for the source-bridge proxy-explorer entry for the appropriate interface. For example, if the appropriate interface is Token Ring 0, look for an entry similar to the following:

interface tokenring 0
source-bridge proxy-explorer

If that entry does not exist, look for the source-bridge proxy-netbios-only entry for the appropriate interface.

If neither entry exists, proxy explorers has not yet been enabled for the appropriate interface. To enable proxy explorers for response to all explorer packets, refer to the section "Configure Proxy Explorers" later in this chapter.

Otherwise, enable proxy explorers only for the NetBIOS name caching function by performing the following task in global configuration mode:
Task Command

Enable use of proxy explorers only for the NetBIOS name caching function and not for their general local response to explorers.

source-bridge proxy-netbios-only

Specify Timeout and Enable NetBIOS Name Caching

After you have ensured that the proxy explorers feature has been enabled for the appropriate interface, you can specify a cache timeout and enable NetBIOS name caching. To do this, perform the following tasks:
Task Command

Specify the timeout for entries in the NetBIOS name cache.

netbios name-cache timeout minutes

Enable NetBIOS name caching for the appropriate interfaces.

netbios enable-name-cache

Configure the NetBIOS Cache Name Length

To specify how many characters of the NetBIOS type name that the name cache will validate, perform the following global configuration task:
Task Command

Specify the number of characters of the NetBIOS type name to cache.

netbios name-cache name-len length

Enable NetBIOS Proxying

The Cisco IOS software can act as a proxy and send NetBIOS datagram type frames. To enable this capability, perform the following global configuration task:
Task Command

Enable NetBIOS proxying.

netbios name-cache proxy-datagram seconds

To define the validation time when the software is acting as a proxy for NetBIOS NAME_QUERY command or for explorer frames, perform the following global configuration task:
Task Command

Define validation time.

rif validate-age seconds

Create Static Entries in the NetBIOS Name Cache

If the router communicates with one or more NetBIOS stations on a regular basis, adding static entries to the NetBIOS name cache for these stations can reduce network traffic and overhead. You can define a static NetBIOS name cache entry that associates the server with the NetBIOS name and the MAC address. If the router acts as a NetBIOS server, you can specify that the static NetBIOS name cache is available locally through a particular interface. If a remote router acts as the NetBIOS server, you can specify that the NetBIOS name cache is available remotely. To do this, perform one of the following tasks in global configuration mode:
Task Command

Define a static NetBIOS name cache entry and specify that it is available locally through a particular interface.

netbios name-cache mac-address netbios-name
interface-name

Define a static NetBIOS name cache entry and specify that it is available remotely.

netbios name-cache mac-address netbios-name ring-group group-number

If you have defined a NetBIOS name cache entry, you must also define a RIF entry. For an example of how to configure a static NetBIOS entry, see the "NetBIOS Support with a Static NetBIOS Cache Entry Example" section later in this chapter.

Specify Dead-Time Intervals for NetBIOS Packets

When NetBIOS name caching is enabled and default parameters are set on the router (as well as the NetBIOS name server and the NetBIOS name client), approximately 20 broadcast packets per logon are kept on the local ring where they are generated. The broadcast packets are of the type ADD_NAME_QUERY, ADD_GROUP_NAME, and STATUS_QUERY.

The Cisco IOS software also converts pairs of FIND_NAME and NAME_RECOGNIZED packets received from explorers, which traverse all rings, to specific route frames that are sent only between the two machines that need to see these packets.

You can specify a query-timeout, or "dead-time" interval to prevent repeat or duplicate broadcast of these type of packets for the duration of the interval.

To specify dead time intervals, perform one or both of the following tasks in global configuration mode:
Task Command

Specify a dead time interval during which the Cisco IOS software drops any broadcast (NetBIOS ADD_NAME_QUERY, ADD_GROUP_NAME, or STATUS_QUERY) frames if they are duplicate frames sent by the same host.

netbios name-cache query-timeout seconds

Specify a dead time interval during which the software drops FIND_NAME and NAME_RECOGNIZED frames if they are duplicate frames sent by the same host.

netbios name-cache recognized-timeout seconds

Configure LAN Network Manager (LNM) Support

LAN Network Manager (LNM), formerly called LAN Manager, is an IBM product for managing a collection of source-route bridges. Using either a proprietary protocol or the Simple Network Management Protocol (SNMP), LNM allows you to monitor the entire collection of Token Rings that comprise your source-route bridged network. You can use LNM to manage the configuration of source-route bridges, monitor Token Ring errors, and gather information from Token Ring parameter servers.


Note LNM is supported on the 4/16-Mb Token Ring cards that can be configured for either 4- or 16-Mb transmission speeds. LNM support is not provided on CSC-R16M cards with SBEMON 2.0.

LNM is not limited to managing locally attached Token Ring networks; it also can manage any other Token Rings in your source-route bridged network that are connected through non-Token Ring media. To accomplish this task, LNM works in conjunction with the IBM Bridge Program. The IBM Bridge Program gathers data about the local Token Ring network and relays it back to LNM. In this manner, the bridge program becomes a proxy for information about its local Token Ring. Without this ability, you would require direct access to a device on every Token Ring in the network. This process would make managing an SRB environment awkward and cumbersome.

Figure 50 shows some Token Rings attached through a cloud and one LNM linking to a source-route bridge on each local ring.


Figure 50: LNM Linking to a Source-Route Bridge on Each Local Ring


If LNM requires information about a station somewhere on a Token Ring, it uses a proprietary IBM protocol to query to one of the source-route bridges connected to that ring. If the bridge can provide the requested information, it simply responds directly to LNM. If the bridge does not have the necessary information, it queries the station using a protocol published in the IEEE 802.5 specification. In either case, the bridge uses the proprietary protocol to send a valid response back to LNM, using the proprietary protocol.

As an analogy, consider a language translator who sits between a French-speaking diplomat and a German-speaking diplomat. If the French diplomat asks the translator a question in French for the German diplomat and the translator knows the answer, he or she simply responds without translating the original question into German. If the French diplomat asks a question the translator does not know how to answer, the translator must first translate the question to German, wait for the German diplomat to answer, and then translate the answer back to French.

Similarly, if LNM queries a source-route bridge in the proprietary protocol and the bridge knows the answer, it responds directly using the same protocol. If the bridge does not know the answer, it must first translate the question to the IEEE 802.5 protocol, query the station on the ring, and then translate the response back to the proprietary protocol to send to LNM.

Figure 51 illustrates requests from the LNM originating in an IBM proprietary protocol and then translated into IEEE 802.5 MAC-level frames.


Figure 51: LAN Network Manager Monitoring and Translating


Notice that the proprietary protocol LNM uses to communicate with the source-route bridge is an LLC2 connection. Although its protocol cannot be routed, LNM can monitor or manage anything within the SRB network.

How a Router Works with LNM

As of Software Release 9.0, Cisco routers using 4/16-Mbps Token Ring interfaces configured for SRB support the proprietary protocol that LNM uses. These routers provide all functions the IBM Bridge Program currently provides. Thus LNM can communicate with a router as if it were an IBM source-route bridge, such as the IBM 8209, and can manage or monitor any Token Ring connected to the router.

Through IBM Bridge support, LNM provides three basic services for the SRB network:

IBM Bridge support for LNM also allows asynchronous notification of some events that can occur on a Token Ring. Examples of these events include notification of a new station joining the Token Ring or of the ring entering failure mode, known as beaconing. Support is also provided for LNM to change the operating parameters in the bridge. For a complete description of LNM, refer to the IBM product manual supplied with the LNM program.

LNM support in our source-route bridges is a powerful tool for managing SRB networks. Through the ability to communicate with LNM and to provide the functionality of the IBM Bridge Program, our device appears as part of the IBM network. You therefore gain from the interconnectivity of our products without having to learn a new management product or interface.

When SRB is enabled on the router, configuring the Cisco IOS software to perform the functions of an IBM Bridge for communication with LNM occurs automatically. Therefore, if SRB has been enabled, you do not need to perform any tasks to enable LNM support. However, the LNM software residing on a management station on a Token Ring on the network should be configured to properly communicate with the router.

There are several options for modifying LNM parameters in the Cisco IOS software, but none are required for basic functionality. For example, because users can now modify the operation of the Cisco IOS software through SNMP as well as through LNM, there is an option to exclude a user from modifying the Cisco IOS software configuration through LNM. You also can specify which of the three LNM services (CRS, REM, RPS) the source-route bridge will perform.

To configure LNM support, perform the tasks in the following sections:

Configure LNM Software on the Management Stations to Communicate with the Router

Because configuring an LNM station is a fairly simple task and is well covered in the LNM documentation, it is not covered in depth here. However, it is important to mention that you must enter the MAC addresses of the interfaces comprising the ports of the bridges as adapter addresses. When you configure the router as a multiport bridge, configuring an LNM station is complicated by the virtual ring that is involved. The basic problem extends from the fact that LNM is designed to only understand the concept of a two-port bridge, and the router with a virtual ring is a multiport bridge. The solution is to configure a virtual ring into the LNM Manager station as a series of dual-port bridges.

Disable LNM Functionality

Under some circumstances, you can disable all LNM server functions on the router without having to determine whether to disable a specific server, such as the ring parameter server or the ring error monitor on a given interface.

To disable LNM functionality, perform the following task in global configuration mode:
Task Command

Disable LNM functionality.

lnm disabled

The command can be used to terminate all LNM server input and reporting links. In normal circumstances, this command should not be necessary because it is a superset of the functions normally performed on individual interfaces by the no lnm rem and no lnm rps commands.

Disable Automatic Report Path Trace Function

Under some circumstances, such as when new hardware has been introduced into the network and is causing problems, the automatic report path trace function can be disabled. The new hardware may be setting bit-fields B1 or B2 (or both) of the routing control field in the routing information field embedded in a source-route bridged frame. This condition may cause the network to be flooded by report path trace frames if the condition is persistent. The lnm pathtrace-disabled command, along with its options, allows you to alleviate network congestion that may be occurring by disabling all or part of the automatic report path trace function within LNM.

To disable the automatic report path trace function, perform the following task in global configuration mode:
Task Command

Disable LNM automatic report path trace function.

lnm pathtrace-disabled [all | origin]

Prevent LNM Stations from Modifying Cisco IOS Software Parameters

Because there is now more than one way to remotely change parameters in a router (either using SNMP or the proprietary IBM protocol), some method is needed to prevent such changes from detrimentally interacting with each other.You can prevent any LNM station from modifying parameters in the Cisco IOS software. It does not affect the ability of LNM to monitor events, only to change parameters on the router.

To prevent the modification of Cisco IOS software parameters by an LNM station, perform the following task in global configuration mode:
Task Command

Prevent LNM stations from modifying LNM parameters in the Cisco IOS software.

lnm snmp-only

Enable Other LRMs to Change Router Parameters

LNM has a concept of reporting links and reporting link numbers. A reporting link is simply a connection (or potential connection) between a LAN Reporting Manager (LRM) and a bridge. A reporting link number is a unique number used to identify a reporting link. An IBM bridge allows four simultaneous reporting links numbered 0 through 3. Only the LRM attached on the lowest-numbered connection is allowed to change LNM parameters in the router, and then only when that connection number falls below a certain configurable number. In the default configuration, the LRM connected through link 0 is the only LRM that can change LNM parameters.

To enable other LRMs to change router parameters, perform the following task in interface configuration mode:
Task Command

Enable a LRM other than that connected through link 0 to change router parameters.

lnm alternate number

Apply a Password to an LNM Reporting Link

Each reporting link has its own password that is used not only to prevent unauthorized access from an LRM to a bridge but to control access to the different reporting links. This is important because it is possible to change parameters through some reporting links.

To apply a password to an LNM reporting link, perform the following task in interface configuration mode:
Task Command

Apply a password to an LNM reporting link.

lnm password number string

Enable LNM Servers

As in an IBM bridge, the router provides several functions that gather information from a local Token Ring. All of these functions are enabled by default, but also can be disabled. The LNM servers are explained in the section "How a Router Works with LNM" earlier in this chapter.

To enable LNM servers, perform one or more of the following tasks in interface configuration mode:
Task Command

Enable the LNM Configuration Report Server (CRS).

lnm crs

Enable the LNM Ring Error Monitor (REM).

lnm rem

Enable the LNM Ring Parameter Server (RPS).

lnm rps

Change Reporting Thresholds

The Cisco IOS software sends a message to all attached LNMs whenever it begins to drop frames. The threshold at which this report is generated is based on a percentage of frames dropped compared with those forwarded. This threshold is configurable, and defaults to a value of 0.10 percent. You can configure the threshold by entering a single number, expressing the percentage loss rate in hundredths of a percent. The valid range is 0 to 9999.

To change reporting thresholds, perform the following task in interface configuration mode:
Task Command

Change the threshold at which the Cisco  IOS software reports the frames-lost percentage to LNM.

lnm loss-threshold number

Change an LNM Reporting Interval

All stations on a Token Ring notify the Ring Error Monitor (REM) when they detect errors on the ring. In order to prevent excessive messages, error reports are not sent immediately, but are accumulated for a short interval and then reported. A station learns the duration of this interval from a router (configured as a source-route bridge) when it first enters the ring. This value is expressed in tens of milliseconds between error messages. The default is 200, or 2 seconds. The valid range is 0 to 65535.

To change an LNM reporting interval, perform the following task in interface configuration mode:
Task Command

Set the time interval during which stations report ring errors to the Ring Error Monitor (REM).

lnm softerr milliseconds

Enable the RPS Express Buffer Function

The RPS express buffer function allows the router to set the express buffer bit to ensure priority service for frames required for ring station initiation. When this function is enabled, the router sets the express buffer bit in its initialize ring station response. This allows Token Ring devices to insert into the ring during bursty conditions.

To enable LNM to use the RPS express buffer function, perform the following task in interface configuration mode:
Task Command

Enable the RPS express buffer function.

lnm express-buffer

Monitor LNM Operation

Once LNM support is enabled, you can monitor LNM operation. To observe the configuration of the LNM bridge and its operating parameters, perform the following tasks in the EXEC mode:
Task Command

Display all configured bridges and their global parameters.

show lnm bridge

Display the logical configuration of all bridges configured in the router.

show lnm config

Display LNM information for an interface or all interfaces of the router.

show lnm interface [type number]

Display LNM information about a Token Ring or all Token Rings on the network.

show lnm ring [ring-number]

Display LNM information about a station or all stations on the network.

show lnm station [address]

Secure the SRB Network

This section describes how to configure three features that are used primarily to provide network security: NetBIOS access filters, administrative filters, and access expressions that can be combined with administrative filters. In addition, these features can be used to increase network performance because they reduce the number of packets that traverse the backbone network.

Configure NetBIOS Access Filters

NetBIOS packets can be filtered when transmitted across a Token Ring bridge. Two types of filters can be configured: one for source and destination station names and one for arbitrary byte patterns in the packet itself.

As you configure NetBIOS access filters, keep the following issues in mind:

To minimize any performance degradation, NetBIOS access filters do not examine all packets. Rather, they examine certain packets that are used to establish and maintain NetBIOS client/server connections, thereby effectively stopping new access and load across the router. However, applying a new access filter does not terminate existing sessions immediately. All new sessions will be filtered, but existing sessions could continue for some time.

For more information on access lists, refer to the "Configuring Traffic Filters" chapter of the Security Configuration Guide.

There are two ways you can configure NetBIOS access filters:

Configure NetBIOS Access Filters Using Station Names

To configure access filters using station names, you must do the following:

Step 1 Assign the station access list name.

Step 2 Specify the direction of the message to be filtered on the interface.

The NetBIOS station access list contains the station name to match, along with a permit or deny condition. You must assign the name of the access list to a station or set of stations on the network.

To assign a station access list name, perform the following task in global configuration mode:
Task Command

Assign the name of an access list to a station or set of stations on the network.

netbios access-list host name {permit | deny} pattern

When filtering by station name, you can choose to filter either incoming or outgoing messages on the interface. To specify the direction, perform one of the following tasks in interface configuration mode:
Task Command

Define an access list filter for incoming messages.

netbios input-access-filter host name

Define an access list filter for outgoing messages.

netbios output-access-filter host name

Configure NetBIOS Access Filters Using a Byte Offset

To configure access filters you must do the following:

Step 1 Assign a byte offset access list name.

Step 2 Specify the direction of the message to be filtered on the interface.

Keep the following notes in mind while configuring access filters using a byte offset:

The NetBIOS byte offset access list contains a series of offsets and hexadecimal patterns with which to match byte offsets in NetBIOS packets. To assign a byte offset access list name, perform the following task in global configuration mode:
Task Command

Define the byte offsets and patterns within NetBIOS messages to match with access list parameters.

netbios access-list bytes name {permit | deny} offset pattern


Note Using NetBIOS Byte Offset access filters disables the autonomous or fast switching of source-route bridging frames.

When filtering by byte offset, you can filter either incoming or outgoing messages on the interface. To specify the direction, perform one of the following tasks in interface configuration mode:
Task Command

Specify a byte-based access filter on incoming messages.

netbios input-access-filter bytes name

Specify a byte-based access filter on outgoing messages.

netbios output-access-filter bytes name

Configure Administrative Filters for Token Ring Traffic

Source-route bridges normally filter frames according to the routing information contained in the frame. That is, a bridge will not forward a frame back to its originating network segment or any other network segment that the frame has already traversed. This section describes how to configure another type of filter---the administrative filter.

Administrative filters can filter frames based on the following methods:

Whereas filtering by Token Ring address or vendor code causes no significant performance penalty, filtering by protocol type significantly affects performance. A list of SNAP (Ethernet) type codes is provided in the "Ethernet Type Codes" appendix in the Bridging and IBM Networking Command Reference.

Filter Frames by Protocol Type

You can configure administrative filters by protocol type by specifying protocol type codes in an access list. You then apply that access list to either IEEE 802.2 encapsulated packets or to SNAP-encapsulated packets on the appropriate interface.

The order in which you specify these elements affects the order in which the access conditions are checked. Each condition is tested in succession. A matching condition is then used to execute a permit or deny decision. If no conditions match, a deny decision is reached.


Note  If a single condition is to be denied, there must be an access-list command that permits everything as well, or all access is denied.

To filter frames by protocol type, perform the following task in global configuration mode:
Task Command

Create an access list for filtering frames by protocol type.

access-list access-list-number {permit | deny} {type-code wild-mask | address mask}

You can filter IEEE 802-encapsulated packets on either input or output. The access list you specify is the one you created that includes the protocol type codes.

To enable filtering on input or output, perform one of the following tasks in interface configuration mode:
Task Command

Enable filtering of IEEE 802-encapsulated packets on input by type code.

source-bridge input-lsap-list access-list-number

Enable filtering of IEEE 802-encapsulated packets on output by type code.

source-bridge output-lsap-list access-list-number

You can filter SNAP-encapsulated packets on either input or output. The access list you specify is the one you created that includes the protocol type codes.

To enable filtering on input or output, perform one of the following tasks in interface configuration mode:
Task Command

Filter SNAP-encapsulated packets on input by type code.

source-bridge input-type-list access-list-number

Filter SNAP-encapsulated frames on output by type code.

source-bridge output-type-list access-list-number

Filter Frames by Vendor Code

To configure administrative filters by vendor code or address, define access lists that look for Token Ring addresses or for particular vendor codes for administrative filtering. To do so, perform the following task in global configuration mode:
Task Command

Configure vendor code access lists.

access-list access-list-number {permit | deny} address mask

Filter Source Addresses

To configure filtering on IEEE 802 source addresses, assign an access list to a particular input interface for filtering the Token Ring or IEEE 802 source addresses. To do so, perform the following task in interface configuration mode:
Task Command

Enable filtering on IEEE 802 source addresses.

source-bridge input-address-list access-list-number

Filter Destination Addresses

To configure filtering on IEEE 802 destination addresses, assign an access list to a particular output interface. To do so, perform the following task in interface configuration mode:
Task Command

Enable filtering on IEEE 802 destination addresses.

source-bridge output-address-list access-list-number

Configure Access Expressions that Combine Administrative Filters

You can use access expressions to combine access filters to establish complex conditions under which bridged frames can enter or leave an interface. Using access expressions, you can achieve levels of control on the forwarding of frames that otherwise would be impossible when using only simple access filters. Access expressions are constructed from individual access lists that define administrative filters for the following fields in packets:


Note For any given interface, an access expression cannot be used if an access list has been defined for a given direction. For example, if an input access list is defined for MAC addresses on an interface, no access expression can be specified for the input side of that interface.

In Figure 52, two routers each connect a Token Ring to an FDDI backbone. On both Token Rings, SNA and NetBIOS bridging support is required. On Token Ring A, NetBIOS clients must communicate with any NetBIOS server off Token Ring B or any other, unpictured router. However, the two 3174 cluster controllers off Token Ring A must only communicate with the one FEP off of Token Ring B, located at MAC address 0110.2222.3333.

Without access expressions, this scenario cannot be achieved. A filter on Router A that restricted access to only the FEP would also restrict access of the NetBIOS clients to the FEP. What is needed is an access expression that would state "If it is a NetBIOS frame, pass through, but if it is an SNA frame, only allow access to address 0110.2222.3333."


Figure 52: Access Expression Example



Note Using access-expressions that combine access filters disables the autonomous or fast switching of source-route bridging frames.

Configure Access Expressions

To configure an access expression perform the following tasks:

When designing an access expression, you must create some phrase that indicates, in its entirety, all the frames that will pass the access expression. This access expression is designed to apply on frames coming from the Token Ring interface on Router A in Figure 52:

In Boolean form, this phrase can be written as follows:

The preceding statement requires three access lists to be configured:

The following configuration allows for all these conditions:

! Access list 201 passes NetBIOS frames (command or response)
access-list 201 permit 0xF0F0 0x0001
!
access-list 202 permit 0x0404 0x0001 ! Permits SNA frames (command or response)
access-list 202 permit 0x0004 0x0001 ! Permits SNA Explorers with NULL DSAP
!
! Access list 701 will permit the FEP MAC address
! of 0110.2222.3333
access-list 701 permit 0110.2222.3333

The 0x0001 mask allows command and response frames to pass equally.

Apply the access expression to the appropriate interface by performing the following task in interface configuration mode:
Task Command

Define a per-interface access expression.

access-expression {in | out} expression

Optimize Access Expressions

It is possible combine access expressions. Suppose you wanted to transmit SNA traffic through to a single address, but allow other traffic through the router without restriction. The phrase could be written as follows:

"Allow access if the frame is not an SNA frame, or if it is going to host 0110.2222.3333."

More tersely this would be:

"Not SNA or destined to 0110.2222.3333."

The access lists defined in the previous section create the following configuration:

interface tokenring 0
access-expression in ~lsap(202) | dmac(701)
!
access-list 202 permit 0x0404 0x0001 ! Permits SNA frames (command or response)
access-list 202 permit 0x0004 0x0001 ! Permits SNA Explorers with NULL DSAP
!
! Access list 701 will permit the FEP MAC address
! of 0110.2222.3333
access-list 701 permit 0110.2222.3333

This is a better and simpler access list than the one originally introduced and will probably result in better run-time execution as a result. Therefore, it is best to simplify your access expressions as much as possible before configuring them into the Cisco IOS software.


Note An "access-expression" type filter cannot exist with a "source-bridge" type filter on the same interface. The two types of filters are mutually exclusive.

Alter Access Lists Used in Access Expressions

Because access expressions are composed of access lists, special care must be taken when deleting and adding access lists that are referenced in these access expressions.

If an access list that is referenced in an access expression is deleted, the access expression merely ignores the deleted access list. However, if you want to redefine an access list, you can create a new access list with the appropriate definition and use the same name as the old access list. The newly defined access list replaces the old one of the same name.

For example, if you want to redefine the NetBIOS access list named MIS that was used in the preceding example, you would enter the following sequence of configuration commands:

!  Replace the NetBIOS access list
interface tokenring 0
access-expression in (smac(701) & netbios-host(accept))
no netbios access-list host accept permit CISCO*

Tune the SRB Network

The following sections describe how to configure features that enhance network performance by reducing the number of packets that traverse the backbone network:


Note In some situations, you might discover that default settings for LLC2 configurations are not acceptable. In such a case, you can configure LLC2 for optimal use. The chapter "Configuring LLC2 and SDLC Parameters" in this publication describes how you can use them to optimize your network performance.

Enable or Disable the Source-Route Fast-Switching Cache

Rather than processing packets at the process level, the fast-switching feature enables the Cisco IOS software to process packets at the interrupt level. Each packet is transferred from the input interface to the output interface without copying the entire packet to main system memory. Fast switching allows for faster implementations of local SRB between 4/16-Mb Token Ring cards in the same router, or between two routers using the 4/16-Mb Token Ring cards and direct encapsulation.

By default, fast-switching software is enabled when SRB is enabled. To enable or disable source-route fast-switching, perform one of the following tasks in interface configuration mode:
Task Command

Enable fast-switching.

source-bridge route-cache

Disable fast-switching.

no source-bridge route-cache


Note Using either NetBIOS Byte Offset access filters or access expressions that combine access filters disables the fast switching of source-route bridging frames.

Enable or Disable the Source-Route Autonomous-Switching Cache

Autonomous switching is a feature that enables the Cisco IOS software to transmit packets from the input ciscoBus card to the output ciscoBus card without any involvement on the part of the router processor.

Autonomous switching is available for local SRB between ciscoBus Token Ring (CTR) cards in the same router. Autonomous switching provides higher switching rates than does fast switching between 4/16-Mb Token Ring cards. Autonomous switching works for both two-port bridges and multiport bridges that use ciscoBus Token Ring cards.

In a virtual ring that includes both ciscoBus Token Ring and 4/16-Mb Token Ring interfaces, frames that flow from one CTR interface to another are autonomously switched, and the remainder of the frames are fast switched. The switching that occurs on the CTR interface takes advantage of the high-speed ciscoBus controller processor.

To enable or disable source-route autonomous switching, perform one of the following tasks in interface configuration mode:
Task Command

Enable autonomous switching.

source-bridge route-cache cbus

Disable autonomous switching.

no source-bridge route-cache cbus


Note Using either NetBIOS Byte Offset access filters or access-expressions that combine access filters disables the autonomous switching of SRB frames.

Enable or Disable the SSE

The Silicon Switch Engine (SSE) acts as a programmable cache to speed the switching of packets. To enable or disable the SSE, perform one of the following task in interface configuration mode:
Task Command

Enable the SSE function.

source-bridge route-cache sse

Disable the SSE function.

no source-bridge route-cache sse

Establish Connection Timeout Interval

It may be necessary to adjust timeout intervals in a complex topology such as a large multihop WAN with virtual rings or satellite links. The timeout interval is used when a connection to a remote peer is attempted. If the timeout interval expires before a response is received, the connection attempt is aborted.

To set the connection timeout interval, perform the following task in global configuration mode:
Task Command

Set the connection timeout interval

source-bridge connection-timeout seconds

Optimize Explorer Processing

Efficient explorer processing is vital to the operation of SRB. The default configuration is satisfactory for most situations. However, there might be circumstances that create unexpected broadcast storms. You can optimize the handling of explorer frames, thus reducing processor overhead and increasing explorer packet throughput. Optimizing explorer processing enables the router to perform substantially better during explorer broadcast storms.

In networks with redundant topologies---two or more routers connected to the same set of Token Rings and doing source-route bridging---a station on one Token Ring trying to get to a station on another Token Ring may choose a less than optimal route through unnecessary routers, causing explorer storms due to excessive forwarding of explorer frames. For example, in the redundant topology example shown in Figure 53, if Station X on Token Ring 1 attempts to get to Station Z on Token Ring 4 by going through Router A, Token Ring 2, and Router B---a less than optimal route, excessive forwarding of explorer frames may cause explorer storms.


Figure 53: Controlling Explorer Storms in Redundant Network Topologies


The source-bridge explorer-dup-ARE-filter command can be used to reduce explorer traffic by filtering explorer frames.

To optimize explorer processing, perform one or more of the following tasks in global configuration mode:
Task Command

Set the maximum explorer queue depth.

source-bridge explorerq-depth depth

Prevent explorer storms in redundant network topologies by filtering explorers that have already been forwarded once.

source-bridge explorer-dup-ARE-filter

Set the maximum byte rate of explorers per ring.

source-bridge explorer-maxrate maxrate

You must also disable explorer fast-switching which is, by default, enabled. To disable explorer fast-switching, perform the following task in global configuration mode:
Task Command

Disable explorer fast switching.

no source-bridge explorer-fastswitch

To enable explorer fast-switching after it has been disabled, perform the following task in global configuration mode:
Task Command

Enable explorer fast switching.

source-bridge explorer-fastswitch

Configure Proxy Explorers

You can use the proxy explorers feature to limit the amount of explorer traffic propagating through the source-bridge network.

To configure proxy explorers, perform the following task in interface configuration mode:
Task Command

Enable the interface to respond to any explorer packets that meet certain conditions necessary for a proxy response to occur.

source-bridge proxy-explorer

The Cisco IOS software does not propagate proxy responses for a station. Instead, the software obtains the RIF path from the RIF cache, changes the explorer to a specific frame, and forwards this frame to the destination. If the Cisco IOS software does not receive a response before the validation timer expires, the RIF entry is marked as invalid. The invalid RIF entry is flushed from the cache table when another explorer for this station is received, and an explorer is forwarded to discover a path to this station.

Establish SRB Interoperability with Specific Token Ring Implementations

This section describes how you can establish interoperability between routers and specific Token Ring implementations. It includes the following sections:

Establish SRB Interoperability with IBM PC/3270 Emulation Software

You can establish interoperability with the IBM PC/3270 emulation program Version 3.0, even though it does not properly send packets over a source-route bridge.

Our implementation rewrites the RIF headers of the explorer packets that the PC/3270 emulation program sends to go beyond the local ring, thus confusing the IBM implementation into not looking beyond the local ring for the remote host.

To rewrite RIF headers, perform the following task in interface configuration mode:
Task Command

Rewrite the RIF headers of explorer packets send by the PC/3270 emulation program to go beyond the local ring.

source-bridge old-sna

Establish SRB Interoperability with TI MAC Firmware

You can use a workaround to establish interoperability with Texas Instruments (TI) MAC firmware.

There is a known defect in earlier versions of the TI Token Ring MAC firmware. This implementation is used by Proteon, Apollo, and IBM RTs. A host using a MAC address whose first two bytes are zeros (such as a Cisco router) will not properly communicate with hosts using that version of TI firmware.

There are two solutions. The first involves installing a static RIF entry for every faulty node with which the router communicates. If there are many such nodes on the ring, this may not be practical.

You also can set the MAC address of our Token Ring to a value that works around the problem. Resetting the MAC address forces the use of a different MAC address on the specified interface, thereby avoiding the TI MAC firmware problem. However, you must ensure that no other host on the network is using that MAC address.

To reset the MAC address, perform the following task in interface configuration mode:
Task Command

Reset the MAC address of the Token Ring interface to a value that provides a workaround to a problem in TI Token Ring MAC firmware.

mac-address ieee-address

Reporting Spurious Frame-Copied Errors

An IBM 3174 cluster controller can be configured to report frame-copied errors to IBM LAN Network Manager software. These errors indicate that another host is responding to the MAC address of the 3174 cluster controller. Both the 3174 cluster controller and the IBM LAN Network Manager software can be configured to ignore frame-copied errors.

Monitor and Maintain the SRB Network

You can display a variety of information about the SRB network. To display the information you require, perform one or more of the following tasks in EXEC mode.
Task Command

Display internal state information about the Token Ring interfaces in the system.

show controllers token

Provide high-level statistics about the state of source bridging for a particular interface.

show interfaces

Display all currently configured bridges and all parameters that are related to the bridge as a whole and not to one of its interfaces.

show lnm bridge

Display the logical (multiport bridge) configuration of the Cisco IOS software.

show lnm config

Display all LNM-relevant information about a specific interface.

show lnm interface [type number]

Display all LNM-relevant information about a specific ring number.

show lnm ring [ring-number]

Display all LNM-relevant information about a specific station or about all known stations on the ring.

show lnm station [address]

Show the current state of any current local acknowledgment for both LLC2 and SDLLC connections.

show local-ack

Display the contents of the NetBIOS cache.

show netbios-cache

Display the contents of the RIF cache.

show rif

Display the current source bridge configuration and miscellaneous statistics.

show source-bridge

Display the spanning-tree topology for the router.

show span

Display a summary of Silicon Switch Processor (SSP) statistics.

show sse summary

To maintain the SRB network, perform any of the following tasks in privileged EXEC mode:
Task Command

Clear the entries of all dynamically learned NetBIOS names.

clear netbios-cache

Clear the entire RIF cache.

clear rif-cache

Clear the SRB statistical counters.

clear source-bridge

Reinitialize the SSP on the Cisco 7000 series.

clear sse

In addition to the EXEC-mode tasks to maintain the SRB network, you can perform the following task in global configuration mode:
Task Command

Limit the size of the backup queue for RSRB to control the number of packets that can wait for transmission to a remote ring before they start being thrown away.

source-bridge tcp-queue-max number1

1This command is documented in the "Remote Source-Route Bridging Commands" chapter of the Bridging and IBM Networking Command Reference.

SRB Configuration Examples

The following sections provide SRB configuration examples:

Basic SRB with Spanning-Tree Explorers Example

Figure 54 illustrates a simple two-port bridge configuration. Token Rings 129 and 130 are connected through the router.


Figure 54: Dual Port Source-Route Bridge Configuration


The example that follows routes IP, but source-route bridges all other protocols using spanning-tree explorers:

interface tokenring 0
ip address 131.108.129.2 255.255.255.0
source-bridge 129 1 130
source-bridge spanning
multiring all
!
interface tokenring 1
ip address 131.108.130.2 255.255.255.0
source-bridge 130 1 129
source-bridge spanning
! use RIFs, as necessary, with IP routing software
multiring all

SRB with Automatic Spanning-Tree Function Configuration Example

The following example of a Cisco series 7000 router configuration illustrates how to enable the automatic spanning tree function on an SRB network.

source-bridge ring-group 100
interface tokenring 0/0
no ip address
ring-speed 16
multiring all
source-bridge active 1 10 100
source-bridge spanning 1
!
interface tokenring 0/1
no ip address
ring-speed 16
multiring all
source-bridge active 2 10 100
source-bridge spanning 1
!
bridge 1 protocol ibm

Optimized Explorer Processing Configuration Example

The following configuration example improves the handling of explorer frames, enabling the Cisco  IOS software to perform substantially better during explorer broadcast storms. In this configuration, the maximum byte rate of explorers is set to 100000.

source-bridge explorer-maxrate 100000
source-bridge explorerQ-depth 100
no source-bridge explorer-fastswitch

SRB-Only Example

The following example shows that all protocols are bridged, including IP. Because IP is being bridged, the system has only one IP address.

no ip routing
!
interface tokenring 0
ip address 131.108.129.2 255.255.255.0
source-bridge 129 1 130
source-bridge spanning
!
interface tokenring 1
ip address 131.108.129.2 255.255.255.0
source-bridge 130 1 129
source-bridge spanning
!
interface ethernet 0
ip address 131.108.129.2 255.255.255.0

SRB and Routing Certain Protocols Example

In the following configuration, IP, XNS, and IPX are routed, while all other protocols are bridged between rings. While not strictly necessary, the Novell IPX and XNS network numbers are set consistently with the IP subnetwork numbers. This makes the network easier to maintain.

xns routing 0000.0C00.02C3
!
novell routing 0000.0C00.02C3
!
interface tokenring 0
ip address 131.108.129.2 255.255.255.0
xns network 129
novell network 129
source-bridge 129 1 130
source-bridge spanning
multiring all
!
interface tokenring 1
ip address 131.108.130.2 255.255.255.0
xns network 130
novell network 130
source-bridge 130 1 129
source-bridge spanning
multiring all
!
interface ethernet 0
ip address 131.108.2.68 255.255.255.0
xns network 2
novell network 2

Multiport SRB Example

Figure 55 shows an example configuration of a four-port Token Ring source-route bridge. Rings  1000, 1001, 1002, and 1003 are all source-route bridged to each other across ring group 7.


Figure 55:
Four-Port Source-Route Bridge


The following is a sample configuration file:

source-bridge ring-group 7
!
interface tokenring 0
source-bridge 1000 1 7
source-bridge spanning
!
interface tokenring 1
source-bridge 1001 1 7
source-bridge spanning
!
interface tokenring 2
source-bridge 1002 1 7
source-bridge spanning
!
interface tokenring 3
source-bridge 1003 1 7
source-bridge spanning

SRB with Multiple Virtual Ring Groups Example

Two virtual ring groups can only be connected through an actual Token Ring. Figure 56 shows Virtual Rings 100 and 200 connected through Token Ring 3.


Figure 56: Two Virtual Rings Connected by an Actual Token Ring


Configuration for Router A
source-bridge ring-group 100
!
interface tokenring 0
source-bridge 3 4 100
source-bridge spanning
interface tokenring 1
source-bridge 1 4 100
source-bridge spanning
Configuration for Router B
source-bridge ring-group 200
!
interface tokenring 0
source-bridge 3 1 200
source-bridge spanning
interface tokenring 2
source-bridge 2 1 200
source-bridge spanning

FDDI SRB Configuration Example

The following configuration for a Cisco 7000 series router illustrates how to enable SRB over FDDI:

interface fddi 1/0
source-bridge 1 10 100
source-bridge spanning
source-bridge route-cache cbus

SRB/FDDI Fast-Switching Example

The following example enables SRB/FDDI fast-switching:

int fddi 2/0
source-bridge 1 10 2
source-bridge spanning
source-bridge route-cache
multiring ip

Adding a Static RIF Cache Entry Example

In the example configuration in Figure 57, the path between rings 8 and 9 connected via Bridge 1 is described by the route descriptor 0081.0090. A full RIF, including the route control field, would be 0630.0081.0090.


Figure 57: Assigning a RIF to a Source-Route Bridge


The static RIF entry would be submitted to the router on the left as follows:

rif 1000.5A12.3456 0630.0081.0090

Adding a Static RIF Cache Entry for a Two-Hop Path Example

In Figure 58, assume that a datagram was sent from a router on ring 21 (15 hexadecimal), across Bridge 5 to ring 256 (100 hexadecimal), and then across Bridge 10 (A hexadecimal) to ring  1365 (555 hexadecimal) for delivery to a destination host on that ring.


Figure 58:
Assigning a RIF to a Two-Hop Path


The RIF in the router on the left describing this two-hop path is 0830.0155.100a.5550 and is entered as follows:

rif 1000.5A01.0203 0830.0155.100a.5550

SR/TLB for a Simple Network Example

In the simple example illustrated in Figure 59, a four-port router with two Ethernets and two Token Rings is used to connect transparent bridging on the Ethernets to SRB on the Token Rings.


Figure 59: Example of a Simple SR/TLB Configuration


Assume that the following configuration for SRB and transparent bridging existed before you wanted to enable SR/TLB:

interface tokenring 0
source-bridge 1 1 2
!
interface tokenring 1
source-bridge 2 1 1
!
interface ethernet 0
bridge-group 1
!
interface ethernet 0
bridge-group 1
!
bridge 1 protocol dec

To enable SR/TLB, one aspect of this configuration must change immediately---a third ring must be configured. Before SR/TLB, the two Token Ring interfaces were communicating with two-port local source-route bridging; after SR/TLB, these two interfaces must be reconfigured to communicate through a virtual ring, as follows:

source-bridge ring-group 10
!
interface tokenring 0
source-bridge 1 1 10
!
interface tokenring 1
source-bridge 2 1 10
!
interface ethernet 0
bridge-group 1
!
interface ethernet 1
bridge-group 1
!
bridge 1 protocol dec

Now you are ready to determine two things:

Once you have determined the ring number and the bridge number, you can add the source-bridge transparent command to the file, including these two values as parameters for the command. The following partial configuration includes this source-bridge transparent entry:

!
source-bridge ring-group 10
source-bridge transparent 10 3 1 1
!
interface tokenring 0
source-bridge 1 1 10
!
interface tokenring 1
source-bridge 2 1 10
!
interface ethernet 0
bridge-group 1
!
interface ethernet 1
bridge-group 1
!
bridge 1 protocol dec

SR/TLB with Access Filtering Example

In the example shown in Figure 60, you want to connect only a single machine, Host E, on an Ethernet to a single machine, Host R, on the Token Ring.


Figure 60: Example of a Bit-Swapped Address


You want to allow only these two machines to communicate across the router. Therefore, you might create the following configuration to restrict the access. However, this configuration will not work, as explained in the paragraph following the sample configuration file.


Note For the sake of readability, the commands to control the bridging are not shown here, just the commands to control the filtering.
interface tokenring 0
access-expression output smac(701)
!
interface ethernet 0
bridge-group 1 input-address-list 701
!
access-list 701 permit 0110.2222.3333

The command for the Token Ring interface specifies that the access list 701 be applied on the source address of frames going out to the Token Ring, and the command for the Ethernet interface specifies that this access list be applied on the source address frames entering the interface from Ethernet. This would work if both interfaces used the same bit ordering, but Token Rings and Ethernets use opposite (swapped) bit orderings in their addresses in relationship to each other. Therefore, the address of Host E on the Token Ring is not 0110.2222.3333, but rather 8008.4444.cccc, resulting in the following configuration. The following configuration is better. This example shows that access lists for Token Ring and Ethernet should be kept completely separate from each other.

interface tokenring 0
source-bridge input-address-list 702
!
interface ethernet 0
bridge-group 1 input-address-list 701
!
access-list 701 permit 0110.2222.3333
!
access-list 702 permit 0110.1234.5678

NetBIOS Support with a Static NetBIOS Cache Entry Example

Figure 61 shows a NetBIOS client on a Token Ring connected through a cloud to a NetBIOS server on another Token Ring.


Figure 61: Specifying a Static Entry


In Figure 61, a static entry is created in the router attached to ring 1 on the client side of the ring group. The static entry is to the server DEF, which is reached through the router attached to ring 3. If server DEF has the MAC address 0110.2222.3333, the configuration for the static entry on the client side is as follows:

rif 0110.2222.3333 0630.0021.0030 ring-group 2
netbios name-cache 0110.2222.3333 DEF ring-group 2

LNM for a Simple Network Example

Figure 62 shows a router with two Token Rings configured as a local source-route bridge.


Figure 62:
Router with Two Token Rings Configured as a Local Source-Route Bridge


The associated configuration file follows:

interface tokenring 0
source-bridge 1 2 3
!
interface tokenring 1
source-bridge 3 2 1

The show lnm config command displays the logical configuration of this bridge, including the LNM configuration information that needs to be entered at the LNM Station. A sample show lnm config display follows:

Wayfarer# show lnm config
Bridge(s) currently configured:
From     ring 001, address 0000.3000.abc4
Across bridge 002
To       ring 003, address 0000.3000.5735

In this example, the MAC addresses 0000.3000.abc4 and 000.3000.5735 must be configured as Adapter Addresses at the LNM Station.

LNM for a More Complex Network Example

Figure 63 shows a router with three Token Rings configured as a multiport bridge, thus employing the concept of the virtual ring.


Figure 63: Router with Three Token Rings Configured as a Multiport Bridge


The associated configuration file follows.

source-bridge ring-group 8
!
interface tokenring 0
source-bridge 1 1 8
!
interface tokenring 1
source-bridge 2 2 8
!
interface tokenring 2
source-bridge 3 3 8

The show lnm config command displays the logical configuration of this bridge, including all the pertinent information for configuring this router into LNM:

Wayfarer# show lnm config
Bridge(s) currently configured:
            From     ring 001, address 0000.0028.abcd
            Across bridge 001
            To       ring 008, address 4000.0028.abcd
            From     ring 002, address 0000.3000.abc4
            Across bridge 002
            To       ring 008, address 4000.3000.abc4
            From     ring 003, address 0000.3000.5735
            Across bridge 003
            To       ring 008, address 4000.3000.5735

In this example, six station definitions must be entered at the LNM Station, one for each of the MAC addresses listed in this sample show lnm config display.

NetBIOS Access Filters Example

The following command permits packets that include the station name ABCD to pass through the router, but denies passage to packets that do not include the station name ABCD:

netbios access-list host marketing permit ABCD

The following command specifies a prefix where the pattern matches any name beginning with the characters DEFG. Note that the string DEFG itself is included in this condition.

netbios access-list host marketing deny DEFG*

The following command permits any station name with the letter W as the first character and the letter Y as the third character in the name. The second and fourth letters in the name can be any character. This example would allow stations named WXYZ and WAYB; however, stations named WY and WXY would not be included in this statement, because the question mark must match some specific character in the name.

netbios access-list host marketing permit W?Y?

The following command illustrates how to combine wildcard characters:

netbios access-list host marketing deny AC?*

The command specifies that the marketing list deny any name beginning with AC that is at least three characters in length (the question mark would match any third character). The string ACBD and ACB would match, but the string AC would not.

The following command removes the entire marketing NetBIOS access list.

no netbios access-list host marketing

To remove single entries from the list, use a command such as the following:

no netbios access-list host marketing deny AC?*

This example removes only the list that filters station names with the letters AC at the beginning of the name.

Keep in mind that the access lists are scanned in order. In the following example, the first list denies all entries beginning with the letters ABC, including one named ABCD. This voids the second command, because the entry permitting a name with ABCD comes after the entry denying it.

netbios access-list host marketing deny ABC*
netbios access-list host marketing permit ABCD

Filtering Bridged Token Ring Packets to IBM Machines Example

The example in Figure 64 disallows the bridging of Token Ring packets to all IBM workstations on Token Ring 1.


Figure 64: Router Filtering Bridged Token Ring Packets to IBM Machines


This example assumes that all hosts on Token Ring 1 have Token Ring addresses with the vendor code 1000.5A00.0000. The first line of the access list denies access to all IBM workstations, while the second line permits everything else. The access list is assigned to the input side of Token Ring 1.

! deny access to all IBM workstations
access-list 700 deny 1000.5A00.0000   8000.00FF.FFFF 
! permit all other traffic 
access-list 700 permit 0000.0000.0000   FFFF.FFFF.FFFF 
! 
interface token ring 1 
! apply access list 700 to the input side of Token Ring 1
source-bridge input-address-list 700

Administrative Access Filters---Filtering SNAP Frames on Output Example

Figure 65 shows a router connecting four Token Rings.


Figure 65: Router Filtering SNAP Frames on Output


The following example allows only AppleTalk Phase 2 packets to be source-route bridged between Token Rings 0 and 1, and allows Novell packets only to be source-route bridged between Token Rings 2 and 3.

source-bridge ring-group 5
!
interface tokenring 0
ip address 131.108.1.1 255.255.255.0
source-bridge 1000 1 5
source-bridge spanning
source-bridge input-type-list 202
!
interface tokenring 1
ip address 131.108.11.1 255.255.255.0
source-bridge 1001 1 5
source-bridge spanning
source-bridge input-type-list 202
!
interface tokenring 2
ip address 131.108.101.1 255.255.255.0
source-bridge 1002 1 5
source-bridge spanning
source-bridge input-lsap-list 203
!
interface tokenring 3
ip address 131.108.111.1 255.255.255.0
source-bridge 1003 1 5
source-bridge spanning
source-bridge input-lsap-list 203
!
! SNAP type code filtering
! permit ATp2 data (0x809B)
! permit ATp2 AARP (0x80F3)
access-list 202 permit 0x809B 0x0000
access-list 202 permit 0x80F3 0x0000
access-list 202 deny 0x0000 0xFFFF
!
! LSAP filtering
! permit IPX (0xE0E0)
access-list 203 permit 0xE0E0 0x0101
access-list 203 deny 0x0000 0xFFFF

Note that it is not necessary to check for an LSAP of 0xAAAA when filtering SNAP-encapsulated AppleTalk packets, because for source-route bridging, the use of type filters implies SNAP encapsulation.

Creating Access Expressions Example

In math, you have the following:

Similarly, the following access expressions would return TRUE if lsap(201) and dmac(701) returned TRUE or if smac(702) returned TRUE:

However, the following access expression would return TRUE only if lsap(201) returned TRUE and either of dmac(701) or smac(702) returned TRUE:

Referring to the earlier example, "An Example Using NetBIOS Access Filters," we had the phrase:

This phrase was converted to the simpler form of:

So, for the following configuration:

! Access list 201 passes NetBIOS frames (command or response)
access-list 201 permit 0xF0F0 0x0001 
!
access-list 202 permit 0x0404 0x0001 ! Permits SNA frames (command or response)
access-list 202 permit 0x0004 0x0001 ! Permits SNA Explorers with NULL DSAP
!
! Access list 701 will permit the FEP MAC address
! of 0110.2222.3333
access-list 701 permit 0110.2222.3333

The following access expression would result:

access-expression in lsap(201) | (lsap(202) & dmac(701))

Therefore, the full configuration example is as follows:

interface tokenring 0
access-expression in lsap(201 | (lsap(202) & dmac(701))
!
! Access list 201 passes NetBIOS frames (command or response
access-list 201 permit 0xF0F0 0x0001
!
access-list 202 permit 0x0404 0x0001 ! Permits SNA frames (command or response)
access-list 202 permit 0x0004 0x0001 ! Permits NSA Explorers with NULL DSAP
!
! Access list 701 will permit the FEP MAC address
! of 0110.2222.3333
access-list 701 permit 0110.2222.3333

Access Expressions Example

Figure 66 shows two routers connecting two Token Rings to an FDDI backbone.


Figure 66: Network Configuration Using NetBIOS Access Filters


Suppose you want to permit the IBM 3174 cluster controllers to access the FEP at address 0110.2222.3333, and also want the NetBIOS clients to access the NetBIOS server named FILESVR3. The following set of router configuration commands would meet this need:

netbios access-list host MIS permit FILESVR3
netbios access-list host MIS deny *
!
access-list 202 permit 0x0404 0x0001 ! Permits SNA frames (command or response)
access-list 202 permit 0x0004 0x0001 ! Permits SNA Explorers with NULL DSAP
!
access-list 701 permit 0110.2222.3333
!
interface tokenring 0
access-expression in (lsap(202) & dmac(701)) | netbios-host(MIS)

Fast-Switching Example

The following example disables fast switching between two Token Ring interfaces in the same router:

! global command establishing the ring group for the interface configuration commands 
source-bridge ring-group 2 
!
! commands that follow apply to interface token 0
interface tokenring 0
! enable srb between local ring 1, bridge 1, and target ring 2 
source-bridge 1 1 2
!disable source-route fast-switching cache on interface token 0
no source-bridge route-cache
!
interface token 1
! enable srb between local ring 2, bridge 1, and target ring 1 
source-bridge 2 1 1
no source-bridge route-cache

Frames entering Token Ring interfaces 0 or 1 will not be fast switched to the other interface.

Autonomous Switching Example

The following example enables use of autonomous switching between two ciscoBus Token Ring interfaces in the same router:

! global command to apply interface configuration commands to the ring group 
source-bridge ring-group 2 
!
! commands that follow apply to interface token 0
interface tokenring 0
! enable srb between local ring 1, bridge 1, and target ring 2 
source-bridge 1 1 2
! enable autonomous switching for interface token 0 
source-bridge route-cache cbus
!
interface tokenring 1
! enable srb between local ring 2, bridge 1, and target ring 1 
source-bridge 2 1 1
source-bridge route-cache cbus

Frames entering Token Ring interfaces 0 or 1 will be autonomously switched to the other interface.


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