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Wide-Area Networking Overview

Wide-Area Networking Overview

Cisco IOS software provides a range of wide-area networking capabilities to fit almost every network environment need. Cisco offers cell relay via the Switched Multimegabit Data Service (SMDS), circuit switching via Integrated Services Digital Network (ISDN), packet switching via Frame Relay, and the benefits of both circuit and packet switching via Asynchronous Transfer Mode (ATM). LAN emulation (LANE) provides connectivity between ATM and other LAN types. Refer to the Dial Solutions Configuration Guide for further information on configuring ISDN. Refer to the Cisco IOS Switching Services Configuration Guide for information on configuring LANE.

Document Objectives

The Wide-Area Networking Configuration Guide presents a set of general guidelines for configuring the following software components:

This overview chapter gives a high-level description of each technology. For specific configuration information, refer to the appropriate chapter in this module.

Document Organization

This document includes the following chapters:

ATM

ATM is a cell-switching and multiplexing technology designed to combine the benefits of circuit switching (constant transmission delay and guaranteed capacity) with those of packet switching (flexibility and efficiency for intermittent traffic).

Cisco provides ATM access in the following ways, depending on the hardware available in the router:

In routers that do not support the hardware listed above, a serial interface can be configured for multiprotocol encapsulation over the Asynchronous Transfer Mode-Data Exchange Interface (ATM-DXI), as specified by RFC 1483. This standard describes two methods for transporting multiprotocol connectionless network interconnect traffic over an ATM network. One method allows multiplexing of multiple protocols over a single permanent virtual circuit (PVC). The other method uses different virtual circuits to carry different protocols. Our implementation supports transport of AppleTalk, Banyan VINES, Internet Protocol (IP), and Novell Internetwork Packet Exchange protocol (IPX) traffic.

If you configure ATM access over a serial interface, an ATM data service unit (ADSU) is required to do the following:

On the Cisco  7500 series routers, network interfaces reside on modular interface processors, which provide a direct connection between the high-speed Cisco Extended Bus (CxBus) and the external networks. Each AIP provides a single ATM network interface; the maximum number of AIPs that the Cisco  7500 series supports depends on the bandwidth configured. The total bandwidth through all the AIPs in the system should be limited to 200 Mbps full-duplex (two TAXI interfaces, or one SONET and one E3, or one SONET and one lightly used SONET, five E3s, or four T3s). For a complete description of the Cisco  7500 series routers and AIP, refer to the Hardware Installation and Maintenance publication for your specific router.


Note Beginning in Cisco  IOS Release  11.3, all commands supported on the Cisco  7500 series routers are also supported on Cisco  7000 series routers equipped with RSP7000.

The ATM port adapter and enhanced ATM port adapter are available on Cisco 7200 series routers and on the second-generation Versatile Interface Processor (VIP2) in Cisco 7500 series routers. The ATM-CES port adapter is available on the 7200 series routers only. For a complete description of these ATM port adapters, refer to the PA-A1 ATM Port Adapter Installation and Configuration, PA-A3 Enhanced ATM Port Adapter Installation and Configuration, and PA-A2 ATM-CES Port Adapter Installation and Configuration publications, respectively.

Cisco  4500 and Cisco  4700 routers support one OC-3c NPM or up to two slower E3/DS3 NPMs. Physical Layer Interface Modules (PLIMs) that support Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) 155 Mbps are available for both single-mode and multimode fiber. For a complete description of the Cisco  4500 and Cisco  4700 routers and the NPM, refer to the Cisco  4000 Hardware Installation and Maintenance manual. For information about installing the NPM, see the document called "Installing Network Processing Modules in the Cisco 4000 Series" (online, it is in the Cisco 4000 Series Configuration Notes).

The 1-port ATM-25 network module is available on the Cisco  2600 series and Cisco  3600 series routers. For complete information about installing this network module, refer to the 1-Port ATM-25 Network Module Configuration Note.

Cisco IOS ATM software supports a subset of the specification in AToM MIB (RFC 1695) for Cisco  IOS Release  11.2 software or later. Cisco  IOS Release  11.3 software or later supports the proprietary Cisco  AAL5  MIB that is an extension to RFC  1695.

ATM Environment

ATM is a connection-oriented environment. All traffic to or from an ATM network is prefaced with a virtual path identifier (VPI) and virtual channel identifier (VCI). A VPI-VCI pair is considered a single virtual circuit. Each virtual circuit is a private connection to another node on the ATM network. Each virtual circuit is treated as a point-to-point mechanism to another router or host and is capable of supporting bidirectional traffic.

Each ATM node is required to establish a separate connection to every other node in the ATM network that it needs to communicate with. All such connections are established by means of a PVC or a switched virtual circuit (SVC) with an ATM signaling mechanism. This signaling is based on the ATM Forum User-Network Interface (UNI) Specification V3.0.

Each virtual circuit is considered a complete and separate link to a destination node. Users can encapsulate data as needed across the connection. The ATM network disregards the contents of the data. The only requirement is that data be sent to the router's ATM processor card in a manner that follows the specific ATM adaptation layer (AAL) format.

An AAL defines the conversion of user information into cells. An AAL segments upper-layer information into cells at the transmitter and reassembles the cells at the receiver. AAL1 and AAL2 handle isochronous traffic, such as voice and video, and are not relevant to the router. AAL3/4 and AAL5 support data communications; that is, they segment and reassemble packets.

An ATM connection is simply used to transfer raw bits of information to a destination router or host. The ATM router takes the common part convergence sublayer (CPCS) frame, carves it up into 53-byte cells, and sends these cells to the destination router or host for reassembly. In AAL5 format, 48 bytes of each cell are used for the CPCS data; the remaining 5 bytes are used for cell routing. The 5-byte cell header contains the destination VPI-VCI pair, payload type, cell loss priority (CLP), and header error control.

The ATM network is considered a LAN with high bandwidth availability. Each end node in the ATM network is a host on a specific subnet. All end nodes needing to communicate with one another must be within the same subnet in the network.

Unlike a LAN, which is connectionless, ATM requires certain features to provide a LAN environment to the users. One such feature is broadcast capability. Protocols wishing to broadcast packets to all stations in a subnet must be allowed to do so with a single call to Layer 2. To support broadcasting, the router allows the user to specify particular virtual circuits as broadcast virtual circuits. When the protocol passes a packet with a broadcast address to the drivers, the packet is duplicated and sent to each virtual circuit marked as a broadcast virtual circuit. This method is known as pseudobroadcasting.


Note Effective with Cisco  IOS Release  11.0, point-to-multipoint signaling allows pseudobroadcasting to be eliminated. On routers with point-to-multipoint signaling, the router can set up calls between itself and multiple destinations; drivers no longer need to duplicate broadcast packets. A single packet can be sent to the ATM switch, which replicates it to multiple ATM hosts.

Classical IP and ARP

Cisco implements classical IP and Address Resolution Protocol (ARP) over ATM as described in RFC 1577. RFC 1577 defines an application of classical IP and ARP in an ATM environment configured as a logical IP subnetwork (LIS). It also describes the functions of an ATM ARP server and ATM ARP clients in requesting and providing destination IP addresses and ATM addresses in situations when one or both are unknown. Our routers can be configured to act as an ARP client, or to act as a combined ARP client and ARP server.

The ATM ARP server functionality allows classical IP networks to be constructed with ATM as the connection medium. Without this functionality, you must configure both the IP network address and the ATM address of each end device with which the router needs to communicate. This static configuration task takes administrative time and makes moves and changes more difficult.

Cisco's implementation of the ATM ARP server functionality provides a robust environment in which network changes can be made more easily and more quickly than in a pure ATM environment. Cisco's ATM ARP client works with any ARP server that is fully compliant with RFC 1577.

Cisco ATM Features

This section provides an overview of the ATM features, interfaces, virtual circuits, and microcode available on the AIP, ATM port adapter, ATM-CES port adapter, Enhanced ATM port adapter, NPM, and 1-port ATM-25 network module. These features are available on all of these interface cards, unless otherwise indicated.

The Cisco IOS software for ATM supports the following features:

Support for up to 2047 virtual circuits on the ATM-CES port adapter.
Support for up to 4096 virtual circuits on the enhanced ATM port adapter.
Support for up to 1023 virtual circuits on the NPM.
Support for up to 2048 virtual circuits on the 1-port ATM-25 network module.
An ATM adaptation layer (AAL) defines the conversion of user information into cells by segmenting upper-layer information into cells at the transmitter and reassembling them at the receiver. AAL1 and AAL2 handle isochronous traffic, such as voice and video, and are not relevant to the router. AAL3/4 and AAL5 support data communications by segmenting and reassembling packets. On the Cisco 4500 and 4700 routers, Cisco supports both AAL3/4 (except at OC-3 rates) and AAL5.
Fast-switched transparent bridging over ATM supports AAL5-SNAP encapsulated packets only. All bridged AAL5-SNAP encapsulated packets are fast switched. Fast-switched transparent bridging supports Ethernet, Fiber Distributed Data Interface (FDDI), and Token Ring packets sent in AAL5-SNAP encapsulation over ATM. You can enable fast-switched bridging for AAL5-SNAP as described later in this chapter.

ATM Interface Types

This section describes the following interface types that are available for ATM:

AIP Interface Types

All ATM interfaces are full duplex. You must use the appropriate ATM interface cable to connect the AIP with an external ATM network. Refer to the document called "ATM Interface Processor" (online, it is in the Interface Processor Installation and Configuration Guide) for descriptions of ATM connectors.

The AIP provides an interface to ATM switching fabrics for transmitting and receiving data at rates of up to 155 Mbps bidirectionally; the actual rate is determined by the physical layer interface module (PLIM). The PLIM contains the interface to the ATM cable. The AIP can support PLIMs that connect to the following physical layers:

For wide-area networking, ATM is currently being standardized for use in Broadband Integrated Services Digital Networks (BISDNs) by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) and the American National Standards Institute (ANSI). BISDN supports rates from E3 (34 Mbps) to multiple gigabits per second (Gbps).


Note The ITU-T carries out the functions of the former Consultative Committee for International Telegraph and Telephone (CCITT).

ATM Port Adapter Interface Types

The ATM port adapter provides a single SONET/SDH OC-3 full-duplex interface (either multimode or single-mode intermediate reach) and supports data rates of up to 155 Mbps bidirectionally. The ATM port adapter connects to a SONET/SDH multimode or SONET/STC-3C single-mode optical fiber cable (STS-3C or STM-1 physical layer) to connect the router to an external DSU (an ATM network).

ATM-CES Port Adapter Interface Types

The ATM-CES port adapters (PA-A2-4T1C-OC3SM, PA-A2-4T1C-T3ATM, PA-A2-4E1XC- OC3SM, PA-A2-4E1XC-E3ATM, PA-A2-4E1YC-OC3SM, and PA-A2-4E1YC-E3ATM) are available on Cisco  7200 series routers. The ATM-CES has four T1 (1.544  Mbps) or four E1 (2.048  Mbps) ports (75- or 120-ohm) that can support both structured (N x 64 kbps) and unstructured ATM Forum-compliant circuit emulation services (CES), and one port that supports an OC-3 (155  Mbps) single-mode intermediate reach interface or a T3 (45  Mbps) or E3 (34  Mbps) standards-based ATM interface.

Enhanced ATM Port Adapter Interface Types

The enhanced ATM port adapters (PA-A3-T3, PA-A3-E3, PA-A3-OC3MM, PA-A3-OC3SMI, and PA-A3-OC3SML) are available on the Cisco  7200 and 7500 series routers. They include five hardware versions that support the following standards-based physical interfaces:

NPM Interface Types

All ATM interfaces are full duplex. You must use the appropriate ATM interface cable to connect the NPM with an external ATM network. Refer to the Cisco 4000 Series Hardware Installation and Maintenance manual and the Installing NPMs in the Cisco 4000 Series manual for descriptions of ATM connectors.

The NPM provides an interface to ATM switching fabrics for transmitting and receiving data at rates of up to 155 Mbps bidirectionally; the actual rate is determined by the physical layer interface module (PLIM). The PLIM contains the interface to the ATM cable. The NPM can support PLIMs that connect to the following physical layers:

1-Port ATM-25 Network Module Interface Types

The 1-port ATM-25 network module has a single RJ-45 connector with signals compliant with the ATM Forum recommendation for the 25.6  Mbps ATM physical layer.

Virtual Circuits

A virtual circuit is a connection between remote hosts and routers. A virtual circuit is established for each ATM end node with which the router communicates. The characteristics of the virtual circuit that are established when the virtual circuit is created include the following:

Each virtual circuit supports the following router functions:

AIP Microcode

The AIP microcode is a software image that provides card-specific software instructions. An onboard read-only memory (ROM) component contains the default AIP microcode. The Cisco 7500 series supports downloadable microcode, which enables you to upgrade microcode versions by loading new microcode images onto the Route Processor (RP), storing them in Flash memory, and instructing the AIP to load an image from Flash memory instead of the default ROM image. You can store multiple images for an interface type and instruct the system to load any one of them or the default ROM image with a configuration command. All processor modules of the same type will load the same microcode image from either the default ROM image or from a single image stored in Flash memory.

Although multiple microcode versions for a specific interface type can be stored concurrently in Flash memory, only one image can load at startup. The show controller cxbus command displays the currently loaded and running microcode version for the Switch Processor (SP) and for each IP. The show running-config command shows the current system instructions for loading microcode at startup.

For a complete description of microcode and downloading procedures, refer to the document called "ATM Interface Processor" (online, it is in the Interface Processor Installation and Configuration Guide) and the Configuration Fundamentals Configuration Guide.

Frame Relay

Cisco's Frame Relay implementation currently supports routing on IP, DECnet, AppleTalk, Xerox Network Service (XNS), Novell IPX, International Organization for Standards (ISO) Connectionless Network Service (CLNS), Banyan VINES, and transparent bridging.

Although Frame Relay access was originally restricted to leased lines, dial-up access is now supported. For more information, see the "Configure DDR over Frame Relay" section for dialer profiles or for legacy DDR in the "Configuring DDR" chapter.

To install software on a new router or access server by downloading software from a central server over an interface that supports Frame Relay, see the "Loading Images and Configuration Files" chapter in the Configuration Fundamentals Configuration Guide.

To configure access between Systems Network Architecture (SNA) devices over a Frame Relay network, see the "Configuring SNA Frame Relay Access Support" chapter in the Bridging and IBM Networking Configuration Guide.

The Frame Relay software provides the following capabilities:

Switched virtual circuits (SVCs) allow access through a Frame Relay network by setting up a path to the destination endpoints only when the need arises and tearing down the path when it is no longer needed.
Frame Relay switching is used when all traffic arriving on one DLCI can be sent out on another DLCI to the same next hop address. In such cases, the Cisco IOS software does not have to examine the frames individually to discover the destination address, and as a result, the processing load on the router decreases.

SMDS

Cisco's implementation of the SMDS protocol is based on cell relay technology as defined in the Bellcore Technical advisories, which are based on the IEEE 802.6 standard. We provide an interface to an SMDS network using DS-1 or DS-3 high-speed transmission facilities. Connection to the network is made through a device called an SDSU---an SMDS channel service unit/digital service unit (CSU/DSU) developed jointly by Cisco Systems and Kentrox. The SDSU attaches to a Cisco router or access server through a serial port. On the other side, the SDSU terminates the line.

Cisco's implementation of SMDS supports the IP, DECnet, AppleTalk, XNS, Novell IPX,
Banyan VINES, and OSI internetworking protocols, and transparent
bridging.

Cisco's implementation of SMDS also supports SMDS encapsulation over an Asynchronous Transfer Mode (ATM) interface. For more information and for configuration tasks, see the configuring ATM chapters in this publication.

Routing of AppleTalk, DECnet, IP, IPX, and ISO CLNS is fully dynamic; that is, the routing tables are determined and updated dynamically. Routing of the other supported protocols requires that you establish a static routing table of SMDS neighbors in a user group. Once this table is set up, all interconnected routers and access servers provide dynamic routing.


Note 
When configuring IP routing over SMDS, you may need to make adjustments to accommodate split horizon effects. Refer to the "Configuring IP Enhanced IGRP" chapter in the Network Protocols Configuration Guide, Part 1 for information about how our software handles possible split horizon conflicts. By default, split horizon is disabled for SMDS networks.

Cisco's SMDS implementation includes multiple logical IP subnetworks support as defined by
RFC 1209. This RFC describes routing IP over an SMDS cloud in which each connection is considered a host on one specific private network, and points to cases where traffic must transit from network to network.

Cisco's implementation of SMDS also provides the Data Exchange Interface (DXI) Version 3.2 with heartbeat. The heartbeat mechanism periodically generates a heartbeat poll frame.

When a multicast address is not available to a destination, pseudobroadcasting can be enabled to broadcast packets to those destinations using a unicast address.

LAPB and X.25

X.25 is one of a group of specifications published by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T); these specifications are international standards that are formally called Recommendations. The ITU-T Recommendation X.25 defines how connections between data terminal equipment (DTE) and data communications equipment (DCE) are maintained for remote terminal access and computer communications. The X.25 specification defines protocols for two layers of the Open Systems Interconnection (OSI) reference model. The data link layer protocol defined is Link Access Procedure, Balanced (LAPB). The network layer is sometimes called the packet level protocol (PLP), but is commonly (although less correctly) referred to as the X.25 protocol.

The ITU-T updates its Recommendations periodically. The specifications dated 1980 and 1984 are the most common versions currently in use. Additionally, the International Standards Organization (ISO) has published ISO 7776:1986 as an equivalent to the LAPB standard, and ISO 8208:1989 as an equivalent to the ITU-T 1984 X.25 Recommendation packet layer. Cisco's X.25 software follows the ITU-T 1984 X.25 Recommendation, except for its Defense Data Network (DDN) and Blacker Front End (BFE) operation, which follow the ITU-T 1980 X.25 Recommendation.


Note The ITU-T carries out the functions of the former Consultative Committee for International Telegraph and Telephone (CCITT). The 1988 X.25 standard was the last published as a CCITT Recommendation. The first ITU-T Recommendation is the 1993 revision.

In addition to providing remote terminal access, Cisco's X.25 software provides transport for LAN protocols---IP, DECnet, XNS, ISO CLNS, AppleTalk, Novell IPX, Banyan VINES, and Apollo Domain---and bridging. For information about these protocols, refer to the Network Protocols Configuration Guide, Part 1, Network Protocols Configuration Guide, Part 2, and Network Protocols Configuration Guide, Part 3.

Briefly, Cisco IOS X.25 software provides the following capabilities:

Cisco's X.25 implementation does not support fast switching.


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