Category: Tcl

Jónatan Þór Jónasson took the time to implement Wake-on-LAN functionality using UDP support introduced in Cisco IOS Tcl in release 15.1(1)T. He found a TCL/TK example of a magic packet being sent, used that as a base, and with small modifications got it to work on his router. Here‘s his code (it’s obviously a proof-of-concept, but you need just a few more lines to get a working Tclsh script):

Unveiling of the Cisco IOS release 15.1(1)T was the extreme opposite of the CRS-3 and Catalyst 3750-X splashes; the next release of one of the foundations of Cisco’s core business deserved a modest two-paragraph mention in the What's New in Cisco Product Documentation page.

If you’re a voice guru, you’ll probably enjoy the list of 20+ voice-related new features, including the all-important Enhanced Music on Hold. For the rest of us, here’s what I found particularly interesting:

If you use Tcl to write Embedded Event Manager policies, you could use the action_syslog command to generate syslog messages. In all other Tcl-based environments (including tclsh), this API is not available, but you could use the syslog: file system to generate debugging messages.

Recently, on an IPSec-based customer network, we installed one of the brand new platforms introduced by Cisco Systems. The initial software release had memory leaks (no problem, we all know these things happen), so we upgraded the box to the latest software. It works perfectly … until you reload it. The software we’re forced to use cannot get IPSec to work if the startup configuration includes interface-level crypto-maps. Interestingly, you can configure crypto-maps manually and they work … until you save them into the startup configuration and reload the box.

What would you think if you’d receive three queries about the same (somewhat obscure) feature within six hours? It started with a nice e-mail from an engineer that I’ve corresponded with in the past. He wanted to send a Wake-on-LAN packet to a PC in a remote office. Usually you could use the ip directed-broadcast feature, but he wanted to use the remote office router to generate the packet.

Another undocumented (and thus very probably unsupported) Tcl-on-IOS detail: numerous Tcl packages are bundled with IOS and available in the tmpsys:lib/tcl directory (the tmpsys: is a virtual file system mapped to a part of the IOS image).

A few days ago, a reader sent me an e-mail titled “Telnet Automation from a Cisco Router” and complained that IOS Tcl does not support the expect commands (spawn, send and expect). Since Expect is a Tcl extension, not part of the core Tcl, it’s not included in Cisco IOS, which was the only answer I could give.

It all started with a innocuous question: can you detect voice traffic with EEM? Looks simple enough: create a QoS class-map that matches voice calls and read the cbQosClassMapStats table in the CISCO-CLASS-BASED-QOS-MIB. The first obstacle was finding the correct indexes, but a Tcl script quickly solved that; I was ready to create the EEM applet. The applet failed to work correctly and after lots of debugging I figured out the counters in the cbQosClassMapStats table change only every 10 seconds.

I couldn’t believe my eyes and simply had to test other MIB variables as well. As expected, the IF-MIB (standard interface MIB) counters increase in real-time, but obviously someone had the bright idea that we need to detect changes in traffic profile only every now and then. Although I've received numerous suggestions from my readers, none of them works on a Cisco 1800 or a Cisco 7200. Oh, well, Cisco developers from the days when I started working with routers would have known better…

Jared Valentine sent me a really interesting problem: he would like to detect voice traffic and start shaping TCP traffic for the duration of the voice call. The ideal solution would be an EEM applet reacting to the changes in the CISCO-CLASS-BASED-QOS-MIB; one of its tables contains the amount of traffic for each class configured in a service policy.

The MIB navigation looks simple: you just read the values from the cbQosClassMapStats table, indexed by policy ID and class ID. The real problem is finding the correct index values. I could walk the MIB manually with a MIB browser or snmp_getnext TCL calls, but this approach is obviously not scalable, so I wrote a script that walks through the cbQosServicePolicy, cbQosObjects, cbQosPolicyMapCfg and cbQosClassMapCfg tables and prints the index values you need.

Last week I've described how you can extend the exec-mode CLI commands with almost no knowledge of Tcl. A bit more work is required if your commands include Tcl special characters (quotes, braces or backslashes).

For example, to display all routes advertised by customers of AS X, you'd use the following show command: show ip bgp regexp _X_([0-9]+)(_\1)*$ (the regular expression is explained in the AS-path based filter of customer BGP routes post). This command cannot be entered as a Tcl string with variable substitution; Tcl would interpret the [ and \ characters. You could enter the whole command in curly braces, but then there would be no variable substitution that we need to insert command line parameters. To make Tcl happy, use the following Tcl commands: