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Interface Configuration Overview

Interface Configuration Overview

Use the information in this chapter to understand the types of interfaces supported on Cisco routers and access servers and to locate configuration information for various types of interfaces.

For a complete description of the interface commands used in this and other chapters that describe interface configuration, refer to the "Interface Commands" chapter of the Cisco IOS Interface Command Reference. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.

This chapter contains general information that applies to all interface types; it includes these  sections:

For examples of configuration commands shown in this chapter, see "Interface Configuration Examples" at the end of this chapter.

For information about a specific type of interface, refer to the chapter or publication indicated here:
For this interface type . . . And these tasks . . . See this chapter or publication    . . .

Dialed interfaces

  • Configure Channelized E1, Channelized T1, or Channelized T1 on the Cisco AS5200

  • Configure a Dialer Interface

  • Configure an ISDN BRI, MBRI, or PRI Interface

Dial Solutions Configuration Guide and Dial Solutions Command Reference

LAN interfaces

  • Configure an Ethernet or Fast Ethernet Interface

  • Configure Fast EtherChannel

  • Configure a Fiber Distributed Data Interface

  • Configure a Hub Interface

  • Configure a LAN Extender Interface

  • Configure a Token Ring Interface

"Configuring LAN Interfaces" chapter in this publication

Serial interfaces

  • Configure a High Speed Serial Interface

  • Configure a Synchronous Serial Interface

  • Configure a Channelized T3 Interface Processor

  • Configure PA-E3 and PA-2E3 Serial Port Adapter

  • Configure PA-T3 and PA-2T3 Serial Port Adapter

  • Configure a Packet OC-3 Interface

  • Configure Automatic Protection Switching of Packet-over-Sonet Circuits

  • Configure Serial Interfaces for CSU/DSU Service Modules

  • Configure Low-Speed Serial Interfaces

"Configuring Serial Interfaces" chapter in this publication

Logical or virtual interfaces

  • Configure a Loopback Interface

  • Configure a Null Interface

  • Configure a Tunnel Interface

"Configuring Logical Interfaces" chapter in this publication

Channel Interface Processor (CIP)

Configure a Channel Interface Processor

"Configuring IBM Channel Attach" in the Bridging and IBM Networking Configuration Guide.

Interface Types Supported on Cisco Routers

Two types of interfaces are supported: physical and virtual interfaces. The types of physical interfaces on a device depend on its interface processors or port adapters. The virtual interfaces that Cisco routers and access servers support include subinterfaces and IP tunnels.

Cisco routers and access servers support the following types of interfaces:

In addition, the Cisco IOS software supports subinterfaces. See the Wide-Area Networking Configuration Guide and the protocol chapters in the Cisco IOS software configuration guides for specific information on how to configure a subinterface for a particular protocol.

For hardware technical descriptions and information about installing interfaces, refer to the hardware installation and maintenance publication for your product. For command descriptions and usage information, refer to the "Interface Commands" chapter of the Cisco IOS Interface Command Reference.

Understand Interface Configuration

These general instructions apply to all interface configuration processes. Begin interface configuration in global configuration mode. To configure an interface, follow these  steps:

    1. Use the configure EXEC command at the privileged EXEC prompt to enter global configuration mode.

    2. Once in the global configuration mode, start configuring the interface by using the interface command. Identify the interface type followed by the number of the connector or interface card. These numbers are assigned at the factory at the time of installation or when cards are added to a system and can be displayed with the show interfaces EXEC command. A report is provided for each interface that the device supports, as seen in the following partial sample display:

Use the show hardware EXEC command to see a list of the system software and hardware.

To begin configuring serial interface 0, you add the following line to the configuration file:


Note It is not necessary to add a space between the interface type and interface number. For example, in the preceding line you can specify either serial 0 or serial0. The command will work either way.

    3. Follow each interface command with the interface configuration commands your particular interface requires. The commands you use define the protocols and applications that will run on the interface. The commands are collected and applied to the interface command until you use another interface command, a command that is not an interface configuration command, or you type the Ctrl-Z sequence to get out of configuration mode and return to privileged EXEC mode.

    4. Once an interface is configured, you can check its status by using the EXEC show commands described after the command tables that follow.


Note Configuring channelized T1 and E1 interfaces requires additional steps. When you configure channelized T1or channelized E1, you must first define the channels and the time slots that comprise the channels by using the controller t1 and the channel-group controller configuration commands. Then configure the virtual serial interfaces using the interface serial global configuration commands. Refer to the Cisco IOS Dial Solutions Configuration Guide for instructions on configuring channelized E1 or channelized T1 interfaces.

Understand Subinterfaces

Configuring multiple virtual interfaces, or subinterfaces, on a single physical interface allows greater flexibility and connectivity on the network. A subinterface is a mechanism that allows a single physical interface to support multiple logical interfaces or networks. That is, several logical interfaces or networks can be associated with a single hardware interface. Subinterfaces are implemented in various WAN and LAN protocols, including ATM, Frame Relay, SMDS, X.25, and Novell IPX. For more information about using subinterfaces, refer to the appropriate protocol chapter.


Note The Cisco IOS software can support a maximum of 300 interfaces and
subinterfaces.

Configure Features Available on any Interface

The following sections describe optional tasks that you can perform on any type of interface:

Add a Description for an Interface

You can add a description about an interface to help you remember what is attached to it. This description is meant solely as a comment to help identify what the interface is being used for. The description will appear in the output of the following commands: show configuration, show system:running-config, and show interfaces. When you add a description for a T1 controller interface, it will appear in the output of the show controllers t1 and show system:running-config commands.

To add a description for any interface except a T1 or E1 controller interface, use the following command in interface configuration mode. To add a description for a T1 or E1 controller in a Cisco  4500 series, or Cisco 7200 series, or Cisco 7500 series routers, use the following command in controller configuration mode:
Command Purpose

description string

Add a description for an interface.

For examples of adding interface descriptions, see the section "Interface Description Examples" at the end of this chapter.

Configure MOP

You can enable Maintenance Operation Protocol (MOP) on an interface by using the following command in interface configuration mode:
Command Purpose

mop enabled

Enable MOP.

You can enable an interface to send out periodic MOP system identification messages on an interface by using the following command in interface configuration mode:
Command Purpose

mop sysid

Enable MOP message support.

Control Interface Hold-Queue Limits

Each interface has a hold-queue limit. This limit is the number of data packets that the interface can store in its hold queue before rejecting new packets. When the interface empties one or more packets from the hold queue, it can accept new packets again. You can specify the hold-queue limit of an interface in interface configuration mode by using the following command:
Command Purpose

hold-queue length {in | out}

Specify the maximum number of packets allowed in the hold queue.

Set Bandwidth

Higher-level protocols use bandwidth information to make operating decisions. For example, IGRP uses the minimum path bandwidth to determine a routing metric. TCP adjusts initial retransmission parameters based on the apparent bandwidth of the outgoing interface. Use the following command in interface configuration mode to set a bandwidth value for an interface:
Command Purpose

bandwidth kilobits

Set a bandwidth value.

The bandwidth setting is a routing parameter only; it does not affect the physical interface.

Set Interface Delay

Higher-level protocols might use delay information to make operating decisions. For example, IGRP can use delay information to differentiate between a satellite link and a land link. To set a delay value for an interface, use the following command in interface configuration mode:
Command Purpose

delay tens-of-microseconds

Set a delay value for an interface.

Setting the delay value sets an informational parameter only; you cannot adjust the actual delay of an interface with this configuration command.

Adjust Timers

To adjust the frequency of update messages, use the following command in interface configuration mode:
Command Purpose

keepalive [seconds]

Adjust the frequency with which the Cisco IOS software sends messages to itself (Ethernet and Token Ring) or to the other end (HDLC-serial and PPP-serial links) to ensure that a network interface is alive for a specified interface.

You also can configure the keepalive interval, the frequency at which the Cisco IOS software sends messages to itself (Ethernet and Token Ring) or to the other end (HDLC-serial, PPP-serial), to ensure that a network interface is alive. The interval in some previous software versions was 10 seconds; it is now adjustable in one-second increments down to one second. An interface is declared down after three update intervals have passed without receiving a keepalive packet.

When adjusting the keepalive timer for a very low bandwidth serial interface, large packets can delay the smaller keepalive packets long enough to cause the line protocol to go down. You might need to experiment to determine the best value.

Limit Transmit Queue Size

You can control the size of the transmit queue available to a specified interface on the MCI and SCI cards. To limit the size, use the following command in interface configuration mode:
Command Purpose

tx-queue-limit number

Limit the size of the transmit queue.

Adjust Maximum Packet Size or MTU Size

Each interface has a default maximum packet size or maximum transmission unit (MTU) size. This number generally defaults to 1500 bytes. On serial interfaces, the MTU size varies, but cannot be set smaller than 64 bytes. To adjust the maximum packet size, use the following command in interface configuration mode:
Command Purpose

mtu bytes

Adjust the maximum packet size or MTU size.

Caution Changing an MTU size on a Cisco 7500 series router results in recarving of buffers and resetting of all interfaces. The following message is displayed:

%RSP-3-Restart:cbus complex
.

Understand OIR

The online insertion and removal (OIR) feature---supported on the Cisco 7200 series and the Cisco  7500 series routers only---allows you to remove and replace interface processors while the system is online. You can shut down the interface processor before removal and restart it after insertion without causing other software or interfaces to shut down.


Note Do not remove or install more than one interface processor at one time. After a removal or installation, observe the LEDs before continuing.

You do not need to notify the software that you are going to remove or install an interface processor. When the route processor is notified by the system that an interface processor has been removed or installed, it stops routing and scans the system for a configuration change. All interface processors are initialized, and each interface type is verified against the system configuration; then the system runs diagnostics on the new interface. There is no disruption to normal operation during interface processor insertion or removal.

Only an interface of a type that has been configured previously will be brought on line; others require configuration. If a newly installed interface processor does not match the system configuration, the interface is left in an administratively down state until the system operator configures the system with the new interfaces.

Hardware (MAC-level) addresses for all interfaces on the Cisco 7500 series routers are stored on an electronically erasable programmable read-only memory (EEPROM) component in the Route Processor (RP) instead of on the individual interface boards. On the Cisco 7500 series routers, an address allocator in the EEPROM contains a sequential block of 40 addresses (5 interface slots times a maximum of 8 possible ports per slot; each address is assigned to a specific slot and port address in the chassis, regardless of how the interfaces are configured. On the Cisco 7200 series, hardware addresses are stored in a midplane EEPROM that supports 1024 addresses per box.

Storage of hardware addresses in EEPROM allows interfaces to be replaced online without requiring the system to update switching tables and data structures. Regardless of the types of interfaces installed, the hardware addresses do not change unless you replace the system RP. If you do replace the RP, the hardware addresses of all ports change to those specified in the address allocator on the new  RP.

Understand Fast Switching Support

Switching is the process by which packets are forwarded. The Cisco IOS software supports multiple methods of switching. Cisco routers fast switch Layer 2 Forwarding (L2F) traffic. In stack group environments in which some L2F traffic is offloaded to a powerful router, fast switching provides improved scalability.

For information about switching features, refer to the Cisco IOS Switching Services Configuration Guide. For documentation of commands used to configure switching features, refer to the Cisco IOS Switching Services Command Reference.

Monitor and Maintain the Interface

You can perform the tasks in the following sections to monitor and maintain the interfaces:

Monitor Interface and Controller Status

The software contains commands that you can use at the EXEC prompt to display information about the interface including the version of the software and the hardware, the controller status, and statistics about the interfaces. The following table lists some of the interface monitoring commands. (You can display the full list of show commands by using the show ? command at the EXEC prompt.) These commands are fully described in the Cisco IOS Interface Command Reference.

Use the following commands in EXEC mode:
Command Purpose

show async status
show interfaces async

Display the status of the asynchronous interface.

show compress

Display compression statistics on a serial interface.

show controllers [bri | cbus | fddi | lance | mci | serial | token]  

Display current internal status information for the interface controller cards.

show controllers cbus

Display information about the Switch Processor (SP) controller on the Cisco  7500 series routers.

show controllers [e1 | ethernet | fastethernet | fddi | serial | t1 | token]

Display current internal status information for the interface controller cards.

show controllers [ethernet | fastethernet | fddi | serial | token]

Display current internal status information for the interface controller cards on the Cisco 7200 series and Cisco 7500 series routers,

show diagbus [slot]

Display diagnostic information about the controller, interface processor, and port adapters associated with a specified slot of a Cisco  7200 series, or Cisco 7500 series router.

show interfaces [type number] [first] [last] [accounting]

show interfaces [type slot/port] [accounting]

show interfaces [type slot/port-adapter/port] [accounting]

If accounting is configured, display the number of packets of each protocol type that have been sent through the interface.

For Cisco 7500 series with a Packet over SONET Interface Processor.

For the Cisco 7500 series with VIP or VIP2 cards.

show interfaces posi [slot/port]

Display information about the Cisco 7500 series with a Packet over SONET Interface Processor.

show interfaces async [number] [accounting]

Display the number of packets of each protocol type that have been sent through the asynchronous serial line.

show system:running-config

Display the currently running configuration in RAM.

show rif

Display the current contents of the routing information field (RIF) cache.

show protocols

Display the global (system-wide) and interface-specific status of any configured Level 3 protocol.

show version

Display the hardware configuration, software version, the names and sources of configuration files, and the boot images.

Monitor the T1 or E1 Controller

This section applies to channelized T1 or E1 interfaces. Because the T1 or E1 link itself is viewed as the controller, use the following commands in EXEC mode to display information about activity on the T1 or E1 line.
Command Purpose

show controller t1

Display information about the T1 link.

show controller e1

Display information about the E1 link.

Alarms, line conditions, and other errors are displayed. The data is updated every 10 seconds. Every 15  minutes, the cumulative data is stored and retained for 24 hours. This means at any one time, up to  96 15-minute accumulations are counted in the data display.

Monitor and Maintain CSU/DSU Service Modules

This section describes how to monitor and maintain service modules. Tasks involved to monitor and maintain service modules are described in these sections:

Perform a Self-Test

To perform a self-test on the integrated CSU/DSU, use the following command in privileged EXEC mode:
Command Purpose

test service-module interface

Perform a self test. Specify the interface type and number.

This command cannot be used if a DTE, line, or remote loopback is in progress. A series of tests are performed on the CSU/DSU, which include a ROM checksum test, RAM test, EEPROM checksum test, flash checksum test, and a DTE loopback with an internal pattern test. This self-test is also performed at power on.

Data transmission is interrupted for five seconds when you issue this command. To view the output of the most recent self-test, enable the show service-module command.

Display a Performance Report

To display the performance report for an integrated CSU/DSU, use one of the following commands in privileged EXEC mode:
Command Purpose

show service-module interface

Display a performance report. Choose either serial interface 1 or 0.

show service-module interface performance-statistics [interval-range]

Display the CSU/DSU performance statistics for the past 24 hours. This command applies only to the FT1/T1 module.

The interval-range value specifies the number of 15-minute intervals displayed in the report. You can choose a range from 1 to 96, where each value represents the CSU/DSU activity performed in that 15-minute interval. For example, a range of 2-3 displays the performance statistics for the intervals two and three.

Perform Loopback Tests

You can loop packets back to the network from the integrated CSU/DSU and loop packets through a local CSU/DSU to a remote CSU/DSU.

Perform Loopback Line Test

To loop data received from the line at the integrated CSU/DSU and loop packets back to the line, use the following commands in interface configuration mode:
Step Command Purpose

1 . 

loopback line

Perform loopback at a point close to the network to CSU/DSU interface.

2 . 

loopback line payload

Perform loopback at a point close to the interface between the CSU/DSU and the router.

Packets are looped from an incoming network transmission back into the network at a CSU or DSU loopback point.

When the loopback line command is configured on the 2-wire, 56-kbps CSU/DSU module or the 4-wire, 56/64-kbps CSU/DSU modules installed on a Cisco 2524 or Cisco 2525 router, the network data loops back at the CSU and the router data loops back at the DSU. If the CSU/DSU is configured for switched mode, you must have an established connection to perform a payload-line loopback. When the loopback line payload command is configured, the CSU/DSU module loops the data through the DSU portion of the module. Data is not looped back to the serial interface.

If you enable the loopback line command on the fractional T1/T1 module, the CSU/DSU performs a full-bandwidth loopback through the CSU portion of the module and data transmission through the serial interface is interrupted for the duration of the loopback. No reframing or corrections of bipolar violation errors or cyclic redundancy check (CRC) errors are performed. When you configure the line loopback payload command on the FT1/T1 module, the CSU/DSU performs a loopback through the DSU portion of the module. The line loopback payload command reframes the data link, regenerates the signal, and corrects bipolar violations and Extended Super Frame CRC errors.

When performing a T1-line loopback with Extended Super Framing, communication over the facilities data link is interrupted but performance statistics are still updated. To show interfaces currently in loopback operation, use the show service-module EXEC command.

Perform Loopback DTE

To loop packets back to DTE from within the local CSU/DSU, use the following command in interface configuration mode:
Command Purpose

loopback dte

Loop packets to DTE.

Packets are looped from within the CSU/DSU back to the serial interface of the router. Send a test ping to see if the packets successfully looped back. To cancel the loopback test, use the no  loopback  dte command.

When using the 4-wire, 56/64-kbps CSU/DSU module, an out-of-service signal is transmitted to the remote CSU/DSU.

Perform a Remote Loopback Test Using the FT1/T1 CSU/DSU Module

This command applies only when the remote CSU/DSU device is configured for this function. It is used for testing the data communication channels along with or without remote CSU/DSU circuitry. The loopback is usually performed at the line port, rather than the DTE port, of the remote CSU/DSU.

On the integrated FT1/T1 CSU/DSU module installed on a Cisco 2524 and Cisco 2525 router, the loopback remote full command sends the loopup code to the remote CSU/DSU. The remote CSU/DSU should perform a full-bandwidth loopback through the CSU portion of the module. The loopback remote payload command sends the loopup code on the configured timeslots, while maintaining the D4-ExtendedSuper Framing. The remote CSU/DSU performs the equivalent of a loopback line payload request. The remote CSU/DSU loops back only those timeslots that are configured of the remote end. This loopback reframes the data link, regenerates the signal, and corrects bipolar violations and Extended Super Frame CRC errors. The loopback remote smart-jack command sends a loopup code to the remote smart jack. You cannot put the local smart jack into loopback.

To loop packets on the integrated FT1/T1 CSU/DSU module, use the following command in interface configuration mode:
Command Purpose

loopback remote {full | payload | smart-jack} [0in1 | 1in1 | 1in2 | 1in5 | 1in8 | 3in24 | qrw | user-pattern 24bit-binary value]

Loop packets at a remote CSU/DSU using the fractional T1/T1 CSU/DSU module.

Failure to loop up or initiate a remote loopback request could be caused by enabling the no  service-module t1 remote-loopback command or having an alternate remote-loopback code configured on the remote end. When the loopback is terminated, the result of the pattern test is displayed.


Note If the FT1/T1 CSU/DSU module is configured to provide internal clocking, the module ceases to generate clocking when it is placed into loopback.
2- and 4-Wire, 56/64-kbps CSU/DSU Modules

This command applies only when the remote CSU/DSU device is configured for this function. It is used for testing the data communication channels along with or without remote CSU/DSU circuitry. The loopback is usually performed at the line port, rather than the DTE port, of the remote CSU/DSU.

On the 2- and 4-wire, 56/64-kbps CSU/DSU modules, an active connection is required before a loopup can be initiated while in switched mode. When transmitting V.54 loopbacks, the remote device is commanded into loopback using V.54 messages. Failure to loop up or initiate a remote loopback request could be caused by enabling the no service-module 56k remote-loopback command.

To loop packets at the remote CSU/DSU, use the following command in interface configuration mode:
Command Purpose

loopback remote [2047 | 511 | stress-pattern pattern number]

Loop packets at a remote CSU/DSU using the 2- and 4-wire, 56/64-kbps CSU/DSU modules.

To show interfaces currently in loopback operation, use the show interfaces loopback EXEC command.

Reset the CSU/DSU

To reset the CSU/DSU, use the following command in privileged EXEC mode:
Command Purpose

clear service-module interface

Reset the CSU/DSU. Specify the interface type and number.

Use this command only in severe circumstances (for example, when the router is not responding to a CSU/DSU configuration command).

This command terminates all DTE and line loopbacks that are locally or remotely configured. It also interrupts data transmission through the router for up to 15 seconds. The software performs an automatic software reset in case of two consecutive configuration failures.

The CSU/DSU module is not reset with the clear interface command.

Caution If you experience technical difficulties with your router and intend to contact customer support, refrain from using this command. The command erases the router's past CSU/DSU performance statistics. To clear only the CSU/DSU performance statistics, issue the clear counters command.

Monitor the LAN Extender Interface

To monitor the LAN Extender interface, the Ethernet interface that resides on the LAN Extender, the serial interface that resides on the LAN Extender, or the serial interface connected to the LAN Extender, use one or more of the following commands at the EXEC prompt:
Command Purpose

show controllers lex [number]
or

show controllers lex [slot/port]

Display hardware and software information about the LAN Extender.

Display information on the Cisco 7500 series routers.

show interfaces lex number [ethernet | serial]

Display statistics about the LAN Extender interface.

show interfaces serial number [accounting]

or

show interfaces serial slot/port [accounting]

Display statistics about the serial interface on the host router that is physically connected to the LAN Extender.

Display statistics on the Cisco 7500 series.

For more complete network troubleshooting information, refer to the Troubleshooting Internetworking Systems publication.

Monitor and Maintain a Hub

You can perform the tasks in the following sections to monitor and maintain the hub:

Shut Down the Hub Port

To shut down or disable a hub port, use the following commands beginning in global configuration mode:
Step Command Purpose

1 . 

hub ethernet number port [end-port]

Specify the hub number and the hub port (or range of hub ports) and place you in hub configuration mode.

2 . 

shutdown

Shut down the hub port.

See the examples of shutting down a hub port at the end of the "Configuring LAN Interfaces" chapter in "Hub Configuration Examples."

Reset the Hub or Clear the Hub Counters

To reset the hub or clear the hub counters, use one of the following commands in EXEC mode:
Command Purpose

clear hub ethernet number

Reset and reinitialize the hub hardware.

clear hub counters [ethernet number [port [end-port]]]

Clear the hub counters displayed by the show hub command.

Monitor the Hub

To display hub information, use the following command in EXEC mode:
Command Purpose

show hub [ethernet number [port [end-port]]]

Display hub statistics.

Monitor Tunnels

Use any of the following commands in EXEC mode to monitor the IP tunnels you have configured:
Command Purpose

show interfaces tunnel unit [accounting]

List tunnel interface information.

show protocol route

List the routes that go through the tunnel.

show ip route

List the route to the tunnel destination.

Clear and Reset the Interface

To clear the interface counters shown with the show interfaces command, use any of the following commands at the EXEC prompt:
Command Purpose

clear counters [type number] [ethernet | serial]

Clear the interface counters.

clear counters fastethernet number

Clear interface counters for the FastEthernet NIM on the Cisco 4000 series or Cisco 4500 series routers.

clear counters [type slot/port]

Clear interface counters for the Cisco 7200 series routers.

clear counters [type slot/port-adaptor]

Clear interface counters for the Cisco 7500 series with VIP or VIP2 Interface Processors.

The command clears all the current interface counters from the interface unless the optional arguments are specified to clear only a specific interface type from a specific slot and port number.


Note This command will not clear counters retrieved using SNMP, but only those seen with the EXEC show interfaces command.

Use the following commands in EXEC mode to clear and reset interfaces. Under normal circumstances, you do not need to clear the hardware logic on interfaces.
Step Command Purpose

1 . 

clear interface type number

Reset the hardware logic on an interface.

2 . 

clear line [number]

Reset the hardware logic on an asynchronous serial line.

3 . 

clear rif-cache

Clear the entire Token Ring RIF cache.

Shut Down and Restart the Interface

You can disable an interface. Doing so disables all functions on the specified interface and marks the interface as unavailable on all monitoring command displays. This information is communicated to other network servers through all dynamic routing protocols. The interface will not be mentioned in any routing updates. On serial interfaces, shutting down an interface causes the DTR signal to be dropped. On Token Ring interfaces, shutting down an interface causes the interface to deinsert from the ring. On FDDIs, shutting down an interface causes the optical bypass switch, if present, to go into bypass mode.

To shut down an interface and then restart it, use the following commands in interface configuration mode:
Step Command Purpose

1 . 

shutdown

Shut down an interface.

2 . 

no shutdown

Reenable an interface.

To check whether an interface is disabled, use the EXEC command show interfaces. An interface that has been shut down is shown as administratively down in the show interfaces command display. See examples in the section "Interface Shutdown Examples" at the end of this chapter.

One reason to shut down an interface is if you want to change the electrical interface type or mode of a Cisco 7500 series port online. You replace the serial adapter cable and use software commands to restart the interface, and if necessary, reconfigure the port for the new interface. At system startup or restart, the FSIP polls the interfaces and determines the electrical interface type of each port (according to the type of port adapter cable attached). However, it does not necessarily repoll an interface when you change the adapter cable online. To ensure that the system recognizes the new interface type, shut down using the shutdown command, and reenable the interface after changing the cable. Refer to your hardware documentation for more details.

Configure Loopback Detection

When an interface has a backup interface configured, it is often desirable that the backup interface be enabled when the primary interface is either down or in loopback. By default, the backup is only enabled if the primary interface is down. By using the down-when-looped command, the backup interface will also be enabled if the primary interface is in loopback. To achieve this condition, use the following command in interface configuration mode:
Command Purpose

down-when-looped

Configure an interface to tell the system it is down when loopback is detected.

If testing an interface with the loopback command, you should not have loopback detection configured, or packets will not be transmitted out the interface that is being tested.

Run Interface Loopback Diagnostics

You can use a loopback test on lines to detect and distinguish equipment malfunctions between line and modem or Channel Service Unit/Digital Service Unit (CSU/DSU) problems on the network server. If correct data transmission is not possible when an interface is in loopback mode, the interface is the source of the problem. The DSU might have similar loopback functions you can use to isolate the problem if the interface loopback test passes. If the device does not support local loopback, this function will have no effect.

You can specify hardware loopback tests on the Ethernet and synchronous serial interfaces, and all Token Ring interfaces that are attached to CSU/DSUs and that support the local loopback signal. The CSU/DSU acts as a Data Communications Equipment (DCE) device; the router or access server acts as a Data Terminal Equipment (DTE) device. The local loopback test generates a CSU loop---a signal that goes through the CSU/DSU to the line, then back through the CSU/DSU to the router or access server. The ping command can also be useful during loopback operation.

The loopback tests are available on the following interfaces:

The following sections describe each test.


Note Loopback does not work on an X.21 DTE because the X.21 interface definition does not include a loopback definition.

Enable Loopback Testing on the HSSI

The HSSI allows you to perform the tasks described in these sections:

These tests apply only when the device supports them and are used to check the data communications channels. The tests are usually performed at the line port rather than the DTE port of the remote CSU/DSU.

The internal loopback concepts are illustrated in Figure 2.


Figure 2: HSSI Loopback Testing


Enable Loopback Test to the DTE

You can loop packets to DTE within the CSU/DSU at the DTE interface, when the device supports this function. Doing so is useful for testing the DTE-to-DCE cable. To loop the packets to DTE, use the following command in interface configuration mode:
Command Purpose

loopback dte

Loop packets to DTE internally.

Enable Loopback Test through the CSU/DSU

You can loop packets completely through the CSU/DSU to configure a CSU loop, when the device supports this feature. Doing so is useful for testing the DCE device (CSU/DSU) itself. To configure a CSU loop, use the following command in interface configuration mode:
Command Purpose

loopback line

Loop packets completely through the CSU/DSU.

Enable Loopback Test over Remote DS-3 Link

You can loop packets through the CSU/DSU, over the Digital signal level 3 (DS-3) link, and to the remote CSU/DSU and back. To do so, use the following command in interface configuration mode:
Command Purpose

loopback remote

Loop packets through the CSU/DSU to a remote CSU/DSU over the DS-3 link.

This command applies only when the device supports the remote function. It is used for testing the data communication channels. The loopback usually is performed at the line port, rather than the DTE port, of the remote CSU/DSU.

Configure the Ethernet Loopback Server

The router software provides an Ethernet loopback server that supports Digital Equipment Corporation (Digital), Intel, and Xerox systems specified by the "blue book," a joint specification written by Digital, Intel, and Xerox that defines the Ethernet protocol. The loopback server responds to forward data loopback messages sent either to the server's MAC address or to the broadcast address. Currently, the Ethernet loopback server does not respond to the loopback assistance multicast address.

Use the Ethernet loopback server to test communications between your internetworking products and Digital systems that do not support the IP ping command, such as DECnet-only VMS systems.

To originate a loop test on your VMS system with a Cisco server, use the Digital Network Control Program (NCP) command loop circuit. For more information about the loop circuit command, consult the DECnet VAX documentation. Cisco network servers support all options that can be specified by the VMS hosts.

Enable Loopback on Token Ring Cards

You can place all of the Token Ring interface cards into loopback mode by using the following command in interface configuration mode:
Command Purpose

loopback

Enable loopback to verify that the Token Ring interface receives back every packet it sends.

Enable Loopback Testing of Fractional T1/T1

For information, see the "Perform Loopback Tests" in the "Monitor and Maintain CSU/DSU Service Modules" section earlier in this chapter.

Interface Configuration Examples

Examples are included in this section to illustrate configuration tasks described in this chapter. These configuration examples are provided:

Enable Interface Configuration Examples

The following example illustrates how to begin interface configuration on a serial interface. It assigns Point-to-Point (PPP) encapsulation to serial interface 0.

interface serial 0
encapsulation ppp

The same example on a Cisco 7500 series routers requires the following commands:

interface serial 1/0
encapsulation ppp

Configure Specific IP Addresses for an Interface Example

This example shows how to configure the access server so that it will use the default address pool on all interfaces except interface 7, on which it will use an address pool called lass:

ip address-pool local
ip local-pool lass 172.30.0.1
  async interface
  interface 7
  peer default ip address lass

Interface Description Examples

The following example illustrates how to add a description about an interface that will appear in configuration files and monitoring command displays:

interface ethernet 0
description First Ethernet in network 1
ip address 101.13.15.78 255.255.255.0

The following example for a Cisco 7500 series routers describes an administration network attached to the Ethernet processor in slot 2, port 4:

interface ethernet 2/4
description 2nd floor administration net

Interface Shutdown Examples

The following example turns off the Ethernet interface in slot 2 at port 4:

interface ethernet 2/4
shutdown

The following example turns the interface back on:

interface ethernet 2/4
no shutdown

The following example shuts down a Token Ring interface:

interface tokenring 0
shutdown

The following example shuts down a T1 circuit number 23 running on a Cisco 7500 series routers:

interface serial 4/0:23
shutdown

The following example shuts down the entire T1 line physically connected to a Cisco 7500 series routers:

controller t1 4/0
shutdown


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