Category: command line interface

Debugging time-based configurations could be a nightmare, as you have to switch router's time back and forth trying to debug your configuration and wait for the desired event to occur. When I was debugging my EEM-based solution to time-based BGP policy routing, I simply defined two aliases that would set the clock to 30 seconds before the event I wanted to test:

alias exec 859 clock set 08:59:30
alias exec 900 clock set 09:00:30

Obviously, these tests are best done in a lab setup … and you have to turn off NTP or any other form of time synchronization.

Someone has sent me an interesting question a while ago: he's changed the configuration of a single VTY line and got three blocks of VTY configuration commands, similar to this:

line vty 0 2
 login
line vty 3
 password secret
 login
line vty 4
 login

He wanted to merge the three configuration blocks back into a single one but somehow didn't know how to do it.

To realize what's going on, you have to understand how the IOS generates line configurations. It takes the first line (VTY 0, for example) and generates its configuration. If the next line (VTY 1) has exactly the same configuration, the range of numbers is expanded (becoming VTY 0 1) and so forth until the pool of similar lines is exhausted or a line is found that has at least one parameter different from the starting one, in which case a new block is started. That's why the sample configuration has three blocks (0-2, 3 and 4) even though the first and the third block are identical.

However, if you change the offending parameter, the VTY lines will have identical configurations and will be automatically merged. If you want to be on the safe side, you should change the parameter for all lines, for example:

line vty 0 4
 login
 password secret

Note: This article is part of You've asked for it series.

Terry Slattery took time (after 15 years) and wrote a short history of Cisco CLI. I've been involved with Cisco's software (it was remarketed as IOS in mid-nineties) for a few years and for me the CLI as we know it today was one of the best features introduced in IOS release 9.21 (I was ecstatic when I've got my hands on the first code during the beta tests). So now that I know who's responsible, I can only say “Thanks, Terry!”

If you do a lot of telnetting in your lab, you could set up an internal DNS server to be able to use router names instead of IP addresses.

Were you ever in a situation where a file that would have to be on the router was sitting on your laptop, but you couldn't store it into the router's flash across the Telnet session or through the console port?

If the file in question is a text file, and the router supports Tcl shell, danshtr documented an interesting trick: you create the file in Tclsh interpreter, cut-and-paste the text through the telnet session into a Tcl string and write the string to the file. If you want to have a more cryptic solution here it is:

• Start tclsh;
• Enter puts [open "flash:filename" w+] {. Do not hit the ENTER key at the end of the line
• Copy-paste the file contents. The contents should not include unmatched curly right brackets (every curly right bracket has to be preceded by a matching curly left bracket).
• After the file contents have been pasted, enter } and press ENTER.
• End the tclsh session with tclquit.

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.

Notes:

• The router expects a newline character at the end of the configuration file. The best way to ensure it's always there is to add a comment line at the end of the file
• The configuration file load usually fails immediately after the reboot, as the interfaces and IP routing processes are not yet fully operational. You might thus miss the first few seconds of the router's operations (unless you store the extra configuration file Flash or NVRAM).

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.

Note: You have to use the file prompt quiet configuration command as the commands executed by kron cannot supply any user input

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.

And the funniest part of the whole story is that I was utterly impressed with the feature when it was introduced ... and now almost started to reinvent the wheel and implement the same functionality in Tcl