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:

One of my readers has asked an interesting question: can you reload a router when pinging a specific IP address from it fails? While there are other ways of dealing with stuck interfaces or routing processes, sometimes such a drastic measure is the only workaround, so here's how you do it:

By default, IOS routers use enable passwords to authenticate incoming HTTP (web) requests. You could also use local usernames as the authentication mechanism, or you could deploy full-blown AAA-based solution.

The Tcl procedures used to execute IOS commands in Embedded Event Manager (cli_open, cli_write …) don’t work when you start Tcl shell from command line interface. To execute IOS commands in this context, use:

• exec command to execute an exec-level command, for example exec “show ip route”
• ios_config mode command to configure the router

If the first parameter of the ios_config command is a global configuration command, you shall omit the second parameter (for example, ios_config “hostname router”). To configure a parameter in one of the sub-configuration modes (for example, interface state), use the first parameter to specify the configuration mode and the second parameter as the actual configuration command (for example, ios_config “interface loop 0” “no shutdown”).

TL&DR: Don’t.

EIGRP computes its composite metric from five parameters, one of them being interface load, therefore raising the theoretical possibility of having route metrics that include interface load. However, tweaking EIGRP K-values with the metric weights command to include interface load in metric calculations is highly discouraged – every change in interface load could lead to network instability.

The more command display the specified file as a hex dump if the contents don't look like a text file. In my example, it didn't like the CR/LF pairs in the Autorun.inf file written on an USB token by a Windows PC, but you could also dump an IOS image or a tar archive used by SDM (or other web-based applications). To force the display format, use the /ascii, /binary or /ebcdic (for IBM/SNA gurus) parameters. Cool feature ... IOS is obviously full of hidden gems :)

I've always wanted to be able to execute a show command while configuring a router (I'm never good at remembering subinterface numbers). A while ago Cisco introduced the do configuration command that allows you to execute any exec-level command (including telnetting to another device) without leaving the current configuration mode.

Cisco routers preconfigured for SDM have default username/password cisco/cisco. As many users forget to disable or change the default username after configuring their router with SDM, they could end up with an exposed router.

Cisco has patched this vulnerability in IOS release 12.4(11)T that includes the one-time password/secret option of the username command, allowing you to define a username/password combination that can be used only once.

You might have wondered what happens with output produced by Tcl procedures (for example, with the puts command) when you use Tcl in Embedded Event Manager (EEM) or Embedded Syslog Manager (ESM). If the Tcl procedure executes in context of a line (console or virtual terminal), the output is sent straight to the attached line, otherwise it's processed by the logging manager (resulting in a syslog message).

There are two scenarios where Tcl would execute in context of a line: if you start a Tcl procedure with the tclsh command or if it's an EEM policy registered with the event_register_cli with sync parameter set to yes.