Category: Command Line Interface

Numerous comments to the "terminal exec prompt" post told me that it might be good to review the line/terminal configuration rules:

• If you want to configure a permanent line characteristic (for example, international), you should do so in the VTY configuration (see also how the VTY configurations are merged);
• If you want a temporary change in the characteristic of your current line (VTY or console), use terminal characteristic to enable it or terminal no characteristic to disable it.

Xavier has mentioned an interesting undocumented command in his comment to the “Continuous display of top CPU processes” post: after you execute terminal exec prompt timestamp, every show command displays current time and CPU utilization before the requested printout.

The brief display of the state of IPv6 interfaces in the router (show ipv6 interface brief) is significantly different from the well-known show ip interface brief display as the IPv6 address might not fit in the same line as all the other data. To filter the printout and display only the operational interfaces, you have to replace the include filter with the section filter, which displays all the lines matching the regular expression as well as associated follow-up lines.

PE-A#show ipv6 interface brief | section up
Serial1/0                  [up/up]
    unassigned
Serial1/1                  [up/up]
    FE80::C800:CFF:FEA7:0
Loopback0                  [up/up]
    unassigned

The definition of the associated follow-up lines depends on the printout. Usually the indented lines are assumed to belong to a section, but you might be surprised.

I’m probably getting old … I keep forgetting the exact names (and capitalization) of route-maps I’ve configured on the router. The show route-maps command is way too verbose when I’m simply looking for the exact name of the route-map I want to use, so I wrote a Tcl script that displays the names of the route-maps configured on the router. If you add a -d switch, it also displays their descriptions (to be more precise, the first description configured in the route-map).

In a previous post, I've described how you can use Tcl shell to upload text content into router's flash if the router has no connectivity to a suitable file server (or you don't have FTP or TFTP server handy).

The trick works flawlessly, but typing the same obscure Tcl commands gets tedious after a while, so the first time I had to use this solution to develop a Tcl script, I've quickly written another script that takes file name as the parameter and hides all the other murky details.

To use it, transfer the contents of storeFile.tcl (available from my web site) to the router's flash (using the previously described trick), follow the installation instructions in the source and you're ready to go.

Note: You can adapt the Tcl script to your needs; for example, you could add instructions to re-register EEM Tcl policy every time you upload the new code.

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.