Home > Articles > Cisco Certification > CCIE > Configuring the Catalyst

Configuring the Catalyst

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Feb 1, 2002.

Chapter Description

This sample chapter from Cisco Press compares differences between the router CLI and the Catalyst 4000/5000/6000 family. It also describes the command line interface including aspects like command line recall, command editing, uploading and downloading code images and configuration files, and finally provides an overview of the menu driven configuration for the other Catalysts.

Catalyst 5000 Configuration Methods

When you attempt to log in to the Catalyst, the Catalyst presents you with a password prompt. If you enter the correct password, you enter the Catalyst's NORMAL mode. Normal mode equates to a router's User EXEC mode allowing you to view most Catalyst parameters, but not authorizing any configuration changes. To make changes, you must enter the Catalyst's PRIVILEGED mode. The privileged mode functionally equates to the router's PRIVILEGED EXEC mode. In the privileged mode, you can view configuration files and make configuration changes. You enter the Catalyst's privileged mode with the enable command. The Catalyst then prompts for a password.

You can access the Catalyst CLI through one of three methods: through the console interface, Telnet, or Trivial File Transfer Protocol (TFTP). The following sections detail each access method.

Like a router, commands are additive—adding configuration statements to an existing file does not completely overwrite the existing configuration. Suppose you have an existing configuration that assigns ports 2/1-10 to VLAN 5. If you add a configuration that assigns ports 2/1-5 to VLAN 5, but you do nothing to ports 2/6-10, 2/6-10 remain in VLAN 5. The absence of these ports in the new VLAN assignment does not remove them from VLAN 5. If, however, the additional configuration includes a line that assigns ports 2/6-10 to VLAN 20, they move VLAN membership.

A foolproof way of ensuring that a new configuration completely overwrites an existing file is to enter clear config all (see Example 4-1). If you clear the configuration from Telnet or TFTP, you do not see this output. You only see this when directly attached to the console. This CLI command returns the Catalyst Supervisor module to its default configuration where all ports belong to VLAN 1, there is no VTP domain (explained in Chapter 12, "VLAN Trunking Protocol"), and all Spanning Tree parameters back to default values. NOTE also that entering this command clears the console's IP address too. You can clear the configuration with any of the access methods, but if you do so while you access the Catalyst through Telnet, you disconnect from the Catalyst because the Catalyst no longer has an IP address either.

Example 4-1 clear config all Output

Console> (enable) clear config

Usage: clear config all

clear config <mod_num>

clear config rmon

Console> (enable) clear config all

This command will clear all configuration in NVRAM.

This command will cause ifIndex to be reassigned on the next system startup.

Do you want to continue (y/n) [n]? y






System configuration cleared.

Use 'session' command to clear ATM or Router specific configurations.

Console> (enable)

From the Supervisor console, or via Telnet, you can clear the Catalyst configuration with the clear config all command. clear config all in Example 4-1 resets the Supervisor module to its defaults. NOTE that this command does not clear the files for the ATM LANE module, nor for the RSM (or MSM in a Catalyst 6000). This only affects the modules directly configured from the Supervisor module. To clear the configurations on the ATM or router modules, you need to access the modules with the session module_number command. This command performs the equivalent of an internal Telnet to the module so that you can make configuration changes. The ATM and router modules use IOS commands to change, save, and clear configurations.


Configuring the Catalyst through the console and through Telnet allows you to enter commands in real time, but only one at a time. Unlike Cisco routers, the Catalyst immediately stores commands in nonvolatile random-access memory (NVRAM) and does not require you to perform a copy run start like a router. Any command you type in a Catalyst is immediately remembered, even through a power cycle. This presents a challenge when reversing a series of commands. On a router, you can reverse a series of commands with reload, as long as you didn't write the running configuration into NVRAM.

Before making serious changes to a Catalyst, copy the configuration to an electronic NOTE pad. On the Catalyst, use the command set length 0 to terminate the more function, enable screen capture on your device, and enter show config to capture the current configuration. Then if you do not like the changes you made and cannot easily reverse them, clear config all and replay the captured configuration file to locally restore the starting configuration.

Console Configuration

The Catalyst 5000 series Supervisor module has one physical console connection. For a Supervisor I or a Supervisor II, the connection is an EIA-232 25-pin connection. For a Supervisor III module, the connection is an RJ-45 connector. Make sure that you know which kind of Supervisor module you are working with to ensure that you can attach to the console.

The console has an interesting feature in that it can operate in one of two modes: either as a console or slip interface. When used as a console, you can attach a terminal or terminal emulation device such as a PC with appropriate software to the interface. This provides direct access to the CLI regardless of the configuration. You use this access method when you have no IP addresses configured in the Catalyst; without an IP address, you cannot Telnet to the Catalyst over the network. You also use this method whenever you need to do password recovery. (Password recovery is discussed in a later section.) And, you will probably elect to access the Catalyst with this method whenever you are local to the Catalyst with an available terminal.

You can enable the console port as a SLIP interface. (SLIP [serial line interface protocol] is the precursor to PPP.) When used in the slip mode, you can Telnet directly to the console port. In a likely setup, you attach a modem to the console port enabling you to Telnet directly to the Catalyst without having to traverse the network. This can be useful when troubleshooting the Catalyst from a remote location when you cannot access it over the network. When used as a slip interface, the interface designator is sl0 [SL0]. You can use the interface as a direct console attachment or a slip interface, but not both. It can only operate as one or the other. By default, it operates as a console interface. To configure the console as a slip interface, you need to assign an IP address to sl0 using the set interface command.

Lastly, you can access the CLI through Telnet over the network. The Catalyst has an internal logical interface, sc0, that you can assign an IP address to. This address becomes the source address when generating traffic in the Catalyst, or the destination address when you attempt to reach the Catalyst. Assigning an address to this logical interface causes the Catalyst to act like an IP end station on the network. You can use the address to perform Telnet, TFTP, BOOTP, RARP, ICMP, trace, and a host of other end station functions.

By default, the sc0 has no IP address and belongs to VLAN 1. If you want to change any of these parameters, use the set interface command. You can modify sc0's IP address and VLAN assignment in one statement. For example, set int sc0 10 assigns sc0 to VLAN 10 and configures an IP address of with a "Class C" IP mask.

Telnet Configuration

Before you can Telnet to a Catalyst, you need to assign an IP address to the sc0 interface on the Supervisor module. The previous section, "Console Configuration," demonstrated how to do this. You can Telnet to a Catalyst as long as your Telnet device can reach the VLAN and IP network that the sc0 interface belongs to. Telnetting to the Catalyst allows you to perform any command as if you were directly attached to the Catalyst console. You do, however, need to know the normal mode and privileged EXEC passwords to gain access.

It was also mentioned earlier that if you enter clear config all from a remote location, you effectively cut yourself off from communicating with the Catalyst through the network. Changing the IP address or VLAN assignment on sc0 can do the same thing. Therefore, be sure to thoroughly review the results of changing the Catalyst IP address or VLAN assignment remotely.

A Catalyst security feature allows you to specify an access list of authorized stations that can access the Catalyst through Telnet or Simple Network Management Protocol (SNMP). You can specify up to 10 entries through the set ip permit command. To enable the access list, you need to specify the set of authorized stations and then turn on the IP permit filter.

To specify the list of allowed stations, use the command syntax set ip permit ip_address [mask]. The optional mask allows you to specify wildcards. For example, if you type set ip permit, you authorize all stations in subnet to access the console interface. If you enter the command set ip permit with no mask, the implied mask is used, which specifies a specific host.

You can specify up to 10 entries this way. To activate the permit list, use the command set ip permit enable.

Activating the list does not affect any other transit or locally originated IP processes such as trace route and ICMP Echo Requests/Replies. The IP permit list only controls inbound Telnet and SNMP traffic addressed to the Catalyst. If the source IP address does not fall within the permitted range, the Catalyst refuses the connection.


If you apply a permit list from a remote Telnet connection, ensure that you include yourself in the permit list. Otherwise, you disconnect yourself from the Catalyst you are configuring.

You can also secure the Catalyst using Terminal Access Controller Access Control System Plus (TACACS+) authentication. TACACS+ enables a communication protocol between your Catalyst and a TACACS+ server. The server authenticates a user based upon the username and password for each individual you want to access the Catalyst console. Normally, the Catalyst authenticates using local parameters, the exec and enable passwords. If the user accessing the Catalyst knows these passwords, the user is authenticated to enter the corresponding mode.

TACACS+ allows you to demand not just a password, but also a username. The user attempting to log in must have a valid username/password combination to gain access.

If a user attempts to log in to the Catalyst when TACACS+ is enabled, the Catalyst sends a request to the TACACS+ server for authentication information. The server replies and the user is either authenticated or rejected.

To enable TACACS+, you need to have a functional and accessible TACACS+ server in your network. The specifics of configuring TACACS+ is beyond the scope of this book. See the Catalyst documentation for configuration details.


If you configure TACACS+ on a Catalyst and the TACACS+ server becomes unavailable for some reason, the locally configured normal and privileged passwords can be used as a "backdoor" (be sure to set these passwords to something other than the default of no password).

TFTP Configuration

The Catalyst has a TFTP client allowing you to retrieve and send configuration files from/to a TFTP server. The actual syntax to do TFTP configuration file transfers depends upon the version of Supervisor module installed in the Catalyst.

If you plan for the Catalyst to obtain the new configuration over the network after a clear config all, you either need to restore a valid IP address and default gateway setting or you need to have the configuration file on an accessible TFTP server. Details for using TFTP are described in the section, "Catalyst Configuration File Management."

Table 4-2 compares various access methods for configuring the Catalyst.

Table 4-2 Comparing the Usage of Catalyst Access Methods

Access Method


When to Use


Direct Supervisor module attachment

Use when network attachments not available, such as Telnet, TFTP, or SNMP. Also used for local access, initial configuration, and password recovery.



Use for Telnet, TFTP, or SNMP configurations to the built-in logical interface. Accessible through network connections.


Supervisor module attachment

Use this for remote access to the Supervisor module when network access not available.



Use this to download a configuration to the Catalysts sc0 or sl0 interface.



Used to make configuration changes from a network management station.

3. Using the Catalyst 5000/6000 Command-Line Interface | Next Section Previous Section

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