Category: Command Line Interface

It's a bit embarrassing: I've posted a hint on how to execute commands after a router reload and Aamer figured out something I've wanted to have for years: the ability to automatically start debugging after the router reload.

After you've configured the Warm Reload, you can also perform warm IOS upgrade/downgrade (assuming that you already run at least the IOS release 12.3(11)T or 12.4). The Warm Upgrade functionality loads the new IOS image into the main memory, decompresses it and starts it, significantly reducing the downtime (in my case, a 2800 router reloaded in 62 seconds as compared to 415 seconds it took to load the image from a locally-attached server).

Apart from the downtime reduction, the warm upgrade (requested with the reload warm file url command) has a number of other benefits:

• The new image does not have to be stored in flash
• You don't have to change the boot image with the boot system command
• If the new image crashes, the router will revert to the original IOS image stored in flash

I've recently got an intriguiging question: how can I change the color of the terminal emulator font from the CLI by sending the telnet program an escape sequence?

For those of you that haven't worked with escape sequences before - you can control a lot of parameters in you terminal emulation program by sending it a special sequence starting with [ ( begin character code 27). These sequences work even in the simplest telnet clients on Windows and Linux thanks to built-in operating system support or ANSI.SYS driver (on Windows); you can get an in-depth description and the list of all supported escape sequences from Wikipedia.

If you want to execute IOS show commands periodically (for example, to monitor router status or take snapshots of routing tables), you can combine new output redirection features introduced in IOS release 12.2T in an Embedded Event Manager (EEM) applet. For example, to store the brief interface status into a file on an FTP server, use the following EEM applet:

event manager applet SaveInterfaceStatus
 event timer watchdog name SaveIfStat time 60
 action 1.0 cli command "show ip interface brief | redirect ftp://username@password:host/path"
 action 2.0 syslog msg "Interface status saved"

Notes:

The Split DNS functionality introduced in IOS release 12.4(9)T has survived a single maintenance cycle before being broken. While you can still configure the DNS views in 12.4(11)T2 (and they still work), the view names are missing from the router-generated configuration (show running, for example), making the configuration syntactically incorrect. The router will thus reboot without DNS views after you've saved the running configuration to NVRAM.

During network troubleshooting or proof-of-concept testing, I often change the state of a loopback interface (to insert or remove an IP prefix from the routing protocols) or flap it to test the impact on network stability. The traditional approach to this procedure should be known to everyone:

IOS release 12.2(13)T (integrated in IOS release 12.3) has added the capability to redirect output of an IOS show command to a file. This feature uses Unix-style pipes (similar to the include, exclude and section keywords) and adds append, redirect and tee (redirect + print) keywords.

The show output can be redirected to a local filename (in flash, on usb token or even in NVRAM) or sent to a remote server (currently only FTP and TFTP servers are supported). For example, the show ip interface brief | redirect ftp://student:lab@192.168.0.10/ifstatus command will store the current interface status to an FTP server.

I've always wanted a short summary display of DLCIs configured on my Frame Relay boxes (or whatever your favorite WAN technology is), but the only printout I would get from the router would be the lengthy show frame pvc printout. Fortunately, a judicious use of output filters can get you a summary printout from almost anything Cisco IOS produces.

A lot of people arriving to my blog ask about continuous ping performed from a router. Well, you cannot generate never-ending ping from a command line interface, but you can get pretty close with a very large repeat count:

• Before starting the ping, set the line escape character to something you can generate from your keyboard (otherwise you won't be able to stop end the command). For example, terminal escape 3 will set the escape character to Ctrl-C.
• Start the ping with the ping ip destination repeat very-large-value command. For example, ping ip 10.0.0.1 repeat 1000000 will ping the target host longer than you'll be willing to wait.

Sometimes you'd like to automate execution of command sequences or create a command alias that would trigger a series of commands. One way of achieving this is by creating an EEM applet. For example, to clear IP routing table and reset BGP neighbors, define the following EEM applet:

event manager applet ClearAll
 event none
 action 1.0 cli command "clear ip route *"
 action 2.0 cli command "clear ip bgp *"

You can trigger this applet with the event manager run ClearAll command or you could configure a command alias, for example alias exec cleanup event manager run ClearAll.