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 Cisco 4500 series, Cisco 7200 series, or Cisco 7500 series routers, use the following command in controller configuration mode:

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

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

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:

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:

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:

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

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:

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

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

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 or 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.

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:

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:

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 Chapter 4, "Configuring Interface Commands."

Use the following commands in EXEC mode:

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:

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 that, at any one time, up to 96 15-minute accumulations are counted in the data display.

This section describes how to monitor and maintain channel service unit/digital service unit (CSU/DSU) modules. Tasks involved to monitor and maintain service modules are described in these sections:

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:

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

You can use any of the following commands in EXEC mode to monitor the IP tunnels you have configured:

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

The clear counters 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.

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.

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:

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" later in 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, 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.

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:

You should not have loopback detection configured when testing an interface with the loopback command because packets will not be transmitted out of the interface that is being tested.

You can use a loopback test on lines to detect and distinguish equipment malfunctions between line and modem or 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 on 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 and 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.

Enabling Loopback Testing on the HSSI

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

• Enabling the Loopback Test to the DTE

• Enabling the Loopback Test through the CSU/DSU

• Enabling the Loopback Test over a Remote DS3 Link

These tests apply only when the device supports them and they 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 I-1.

Figure I-1. HSSI Loopback Testing

For information, see the section"Performing Loopback Tests" in the "Monitoring and Maintaining CSU/DSU Service Modules" section earlier in this chapter.

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

interface serial 0
encapsulation ppp

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

interface serial 1/0
encapsulation ppp

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

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 router describes an administration network attached to the Ethernet processor in slot 2 at port 4:

interface ethernet 2/4
description 2nd floor administration net

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 router:

interface serial 4/0:23
shutdown

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

controller t1 4/0
shutdown

The Cisco IOS 12.0 Reference Library consists of 11 books, including this one. Each book contains technology-specific configuration chapters with corresponding command reference chapters. Each configuration chapter describes Cisco's implementation of protocols, technologies, and related configuration tasks and contains comprehensive configuration examples. Each command reference chapter complements the organization of its corresponding configuration chapter and provides complete command syntax information.

The Cisco IOS documentation set uses the following conventions:

Examples use the following conventions:

The following conventions are used to attract the reader's attention:

Within the Cisco IOS12.0 Reference Library, the term router is used to refer to both access servers and routers. When a feature is supported on the access server only, the term access server is used.

Within examples, routers and access servers are alternately shown. These products are used only for example purposes; that is, an example that shows one product does not indicate that the other product is not supported.

Command descriptions use the following conventions:

Cisco Connection Online (CCO) is Cisco Systems's primary, real-time support channel. Maintenance customers and partners can self-register on CCO to obtain additional information and services.

Available 24 hours a day, 7 days a week, CCO provides a wealth of standard and value-added services to Cisco's customers and business partners. CCO services include product information, product documentation, software updates, release notes, technical tips, the Bug Navigator, configuration notes, brochures, descriptions of service offerings, and download access to public and authorized files.

CCO serves a wide variety of users through two interfaces that are updated and enhanced simultaneously: a character-based version and a multimedia version that resides on the World Wide Web (WWW). The character-based CCO supports Zmodem, Kermit, Xmodem, FTP, and Internet e-mail, and it is excellent for quick access to information over lower bandwidths. The WWW version of CCO provides richly formatted documents with photographs, figures, graphics, and video, as well as hyperlinks to related information.

You can access CCO in the following ways:

Entering a question mark (?) at the system prompt displays a list of commands available for each command mode. You can also get a list of any command's associated keywords and arguments with the context-sensitive help feature.

To get help specific to a command mode, a command, a keyword, or an argument, use one of the following commands:

Router> enable
Password: <password>
Router#

Router# config terminal
Enter configuration commands, one per
line. End with Ctrl-Z.
Router(config)#

Router(config)# controller t1 ?

  <0-3>  Controller unit number
Router(config)# controller t1 1
Router(config-controller)#

Router(config-controller)# ?
Controller configuration commands:
  cablelengthSpecify the cable length
for a DS1 link
  cas-groupConfigure the specified
timeslots for CAS (Channel Associate
Signals)
  channel-groupSpecify the timeslots to
channel-group mapping for an interface
  clockSpecify the clock source for a
DS1 link
  defaultSet a command to its defaults
  descriptionController specific
description
  ds0ds0 commands
  exitExit from controller
configuration mode
  fdlSpecify the FDL standard for a DS1
data link
  framingSpecify the type of Framing on
a DS1 link
  helpDescription of the interactive
help system
  linecodeSpecify the line encoding
method for a DS1 link
  loopbackPut the entire T1 line into
loopback
  noNegate a command or set its defaults
  pri-groupConfigure the specified
timeslots for PRI
  shutdownShut down a DS1 link (send
Blue Alarm)
Router(config-controller)#

Router(config-controller)# cas-group ?
  <0-23>Channel number
Router(config-controller)# cas-group

Router(config-controller)# cas-group 1
?
  timeslotsList of timeslots in the cas-
group
Router(config-controller)# cas-group 1 

Router(config-controller)# cas-group 1

timeslots ?
  <1-24>List of timeslots which comprise
the cas-group
Router(config-controller)# cas-group 1
timeslots

Router(config-controller)# cas-group 1
timeslots 1-24 ?
  serviceSpecify the type of service 
  typeSpecify the type of signaling
Router(config-controller)# cas-group 1
timeslots 1-24

Router(config-controller)# cas-group 1
timeslots 1-24 type ?
  e&m-fgbE & M Type II FGB
  e&m-fgdE & M Type IIFGD
  e&m-immediate-startE & M Immediate
Start
  fxs-ground-startFXS Ground Start
  fxs-loop-startFXS Loop Start
  sas-ground-startSAS Ground Start
  sas-loop-startSAS Loop Start
Router(config-controller)# cas-group 1
timeslots 1-24 type

Router(config-controller)# cas-group 1

timeslots 1-24 type e&m-fgb ?
  dtmfDTMF tone signaling
  mfMF tone signaling
  serviceSpecify the type of service
  <cr>
Router(config-controller)# cas-group 1
timeslots 1-24 type e&m-fgb

Router(config-controller)# cas-group 1
timeslots 1-24 type e&m-fgb dtmf ?
  dnisDNIS addr info provisioned
  serviceSpecify the type of service
  <cr>

Router(config-controller)# cas-group 1
timeslots 1-24 type e&m-fgb dtmf

Router(config-controller)# cas-group 1
timeslots 1-24 type e&m-fgb dtmf
Router(config-controller)#

The Cisco IOS user interface is divided into many different modes. The commands available to you at any given time depend on which mode you are currently in. Entering a question mark (?) at the system prompt allows you to obtain a list of commands available for each command mode.

When you start a session on the router, you begin in user mode, often called EXEC mode. Only a limited subset of the commands is available in EXEC mode. In order to have access to all commands, you must enter privileged EXEC mode. Normally, you must enter a password to enter privileged EXEC mode. From privileged mode, you can enter any EXEC command or enter global configuration mode. Most of the EXEC commands are one-time commands, such as show commands, which show the current status of something, and clear commands, which clear counters or interfaces. The EXEC commands are not saved across reboots of the router.

The configuration modes allow you to make changes to the running configuration. If you later save the configuration, these commands are stored across router reboots. In order to get to the various configuration modes, you must start at global configuration mode. From global configuration mode, you can enter interface configuration mode, subinterface configuration mode, and a variety of protocol-specific modes.

ROM Monitor mode is a separate mode used when the router cannot boot properly. If your router or access server does not find a valid system image when it is booting, or if its configuration file is corrupted at startup, the system might enter read-only memory (ROM) monitor mode.

Almost every configuration command also has a no form. In general, use the no form to disable a function. Use the command without the keyword no to re-enable a disabled function or to enable a function that is disabled by default. For example, IP routing is enabled by default. To disable IP routing, specify the noiprouting command and specify ip routing to re-enable it. The Cisco IOS software command references provide the complete syntax for the configuration commands and describes what the no form of a command does.

Configuration commands can also have a default form. The default form of a command returns the command setting to its default. Most commands are disabled by default, so the default form is the same as the no form. However, some commands are enabled by default and have variables set to certain default values. In these cases, the default command enables the command and sets variables to their default values. The Cisco IOS softwarecommand referencesdescribe what the default form of a command does if the command is not the same as the no form.

Enter the copy system:running-config nvram:startup-config command to save your configuration changes to your startup configuration so that they will not be lost if there is a system reload or power outage. For example:

Router# copy system:running-config nvram:startup-config
Building configuration…

It might take a minute or two to save the configuration. After the configuration has been saved, the following output appears:

[OK]
Router#

On most platforms, this step saves the configuration to nonvolatile random-access memory (NVRAM). On the Class A Flash file system platforms, this step saves the configuration to the location specified by the CONFIG_FILE environment variable. The CONFIG_FILE variable defaults to NVRAM.