Category: Command Line Interface

If you do a lot of tests in a router lab, you're probably getting upset when you have to retype the login and enable password whenever you log into a router. What I do in my labs is to disable VTY login, set the default privilege level to 15 and disable exec timeout (to stop the router from terminating my session).

line con 0
 exec-timeout 0 0
 privilege level 15
line vty 0 4
 exec-timeout 0 0
 privilege level 15
 no login

Obviously, this would not bring you additional points on the CCIE lab exam :)

Cisco IOS allows you to define aliases for the commands you commonly use with the alias global configuration command. The alias command accepts the CLI mode (exec, configuration ...) for the new command and the string that replaces the command name. If you specify additional parameters in the new command, they are appended to the alias string.

For example, if want to have the ipconfig command that displays interface IP configuration, you can configure alias exec ipconfig show ip interface. When you execute ipconfig ifname the alias is expanded into show ip interface ifname and displays the IP configuration of a single interface.

It's amazing how many options (most of them still undocumented) the show interfaces command accepts in IOS release 12.4T (I won't even start guessing when each one was introduced, if you're running old IOS releases, please feel free to comment):

• show interfaces description displays interface names, L1 and L2 status (line and line-protocol status) and interface description. Extremely handy if you want to check which interfaces are up/down.
• show interfaces counters protocol status displays the L3 protocols active on each interface.
• show interfaces summary displays the state of various interface queues and related drop counters in a nice tabular format.
• show interfaces accounting displays per-protocol in/out counters.

Here are a few sample printouts:

The flash memory available in newer router platforms (at the very minimum the ISR routers and 37xx series) is capable of being used as a regular disk drive (for example, to store system logging information), but it might be formatted as a traditional Low-End File System (LEFS) flash card (more likely if the router was not manufactured recently). To change the flash card format to disk-like FAT32 format, use the format flash: privileged-level command (and don't forget to store the IOS image to another location before formatting the flash). After the format process is complete, you can create subdirectories on the flash: memory and use it as a regular disk device.

When developing yet another Tcl script, I've stumbed across an interesting show command: the show ip access-list interface name introduced in IOS release 12.4(6)T displays the contents of the inbound and outbound IP access-list applied to the specified interface. The really nice part is that the ACL statistics (number of matches displayed next to the ACL lines) are kept and displayed per-interface.

Contrary to what the regular reload does, the warm reload does not change the configuration register value (obviously that's done by ROMMON, which is not involved in the warm reload process). If you just did a password recovery and changed the configuration register back to a normal value, you'd thus be unpleasantly surprised when the NVRAM would be ignored (yet again) after a warm reload (I stumbled across this as I was trying a new IOS release with the reload warm file URL command).

In his comment to my post about re-enabling debugging after router reload, Mike pointed out an interesting IOS feature: you can execute the do command from a configuration file, not just from the user interface. To make his tip even more useful, you can store the do command(s) in an external file on a TFTP server, not in the startup configuration (which would have to be edited manually). With the boot host URL configuration command you'd then ensure that these commands are executed after the router reload.

Pete Vickers sent me a very interesting configuration sample:

To get an IOS device to upload it’s configuration periodically to an external FTP server:

ip ftp source-interface loopback 0
ip ftp username ftp_username
ip ftp password ftp_password
file prompt quiet
!
kron policy-list backup
 cli copy running-config ftp://10.20.30.40
!
kron occurrence daily-backup at 0:30 recurring
 policy-list backup

The beauty of this example is that you can use it on platforms that don't support Embedded Event Manager (which has a very similar cron functionality) as the kron commands were introduced in 12.2T and 12.3 IOS releases.

The comment to one of my previous posts reminded me of a cool feature that's been available in Cisco IOS for a number of years - you can change how the IP addresses and routes are displayed in various show printouts (but not in the router configuration) with the terminal ip netmask-format bit-count|decimal exec-level command. You can even make the change permanent by configuring ip netmask-format format on console and VTY lines.

Similar to the show processes cpu sorted command, the show processes memory sorted printout displays the top memory consumers (see example below).