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.

Sometimes, parts of router configuration get lost during the reload process: although the configuration commands are saved in NVRAM, they are not processed after the reload and thus do not appear in the running configuration. Re-entering these commands manually solves the problem ... but it's obviously not a reliable solution.

Embedded Event Manager (EEM) solves this issue as well. You just configure an applet that triggers on syslog message SYS-5-RESTART and reapplies the necessary configuration commands.

TACACS+ protocol introduced with the IOS AAA architecture had great provisions for customizing the whole login process (user-defined banners, prompts ...). Unfortunately, it never really took off and most AAA solutions deployed today rely on RADIUS servers that cannot control the login process itself (the RADIUS server can only check the username/password pair for validity).

To change the login prompts when using RADIUS servers, use the aaa authentication [banner|fail-message|password-prompt|username-prompt] text configuration command.

Early EIGRP implementation treated changes in EIGRP summary address configuration (configured with the ip summary-address eigrp interface configuration command) very disruptively: all EIGRP sessions across the affected interface were cleared, sometimes resulting in a large number of routes entering active state, potentially leading to a stuck-in-active condition.

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.

I've almost started writing a Tcl procedure to display top-10 CPU-intensive processes on a router ... and then discovered the sorted option of the show processes cpu command. Even more, starting in IOS release 12.2T, the show processes cpu history command gives you a nice CPU utilization graph.

The Warm Reload functionality introduced in IOS release 12.3(2)T significantly reduces the reload time. In my test lab, the reload time of a Cisco 2800 router booting from flash was reduced from 135 to 54 seconds as measured by the %SYS-6-BOOTTIME: Time taken to reboot after reload ... syslog message.

The theory behind warm reload is simple: the router saves initial data (as stored in IOS image) in a separate memory region and reuses saved data together with IOS code already residing in RAM to restart IOS. Of course, the IOS code (depending on platform's memory management capabilities) or saved data could get corrupted, therefore the warm reload cannot be used continuously (and the router falls back to traditional reload if the router crashes before a specified time interval).

Warm reload is configured with the warm-reboot count number uptime minutes configuration commands. After it has been configured, a router reload (or power-up) is needed to initialize the saved data region. When the warm reboot is operational (as verified with the show warm-reboot command), you can use reload warm command to start it.

When pinging a directly-attached host (end-station) from a router, it's quite common to lose the first reply, as shown in the following example (the same symptom might occur when pinging a remote host that has been inactive).

a2#ping 10.0.0.10

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.0.10, timeout is 2 seconds:
.!!!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 1/1/4 ms

Actually, it's not the reply that was lost, the request was never sent out. Whenever a router has to send a packet to the next-hop (or directly attached destination) that has no entry in the ARP table, the ARP request is sent out, but the original packet is unconditionally dropped.

In a previous post, I’ve described how to execute a Tcl file with the tclsh command.

You can do even more than that: you can pass parameters to the executed file. Every word you enter after the file name in the tclsh command line is passed as a parameter to the Tcl code you execute. To get these parameters in Tcl, use Tcl commands similar to the code below: