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Category: Tcl

Predefine your own Tcl functions

If you want to have your own Tcl functions available when you start tclsh, you could use the scripting tcl init file configuration command that I've briefly mentioned in one of the previous posts. This command specifies a source file that is executed every time you start Tcl shell. The source file can contain function definitions, package declarations or any other Tcl code.

If you need to, you can specify multiple initialization files.

For example, if you'd like to implement a comfortable Tcl-based pinger (similar to the one Ethan Banks found in the Sadikhov forums, store the following Tcl code into the file flash:pinger.tcl

proc pinger { iplist } {
  foreach ip $iplist {
    if { [regexp "(!!!)" [exec "ping $ip timeout 1" ]] } {
      puts "$ip"
    } else { puts "$ip **** failed ***" }
… and configure scripting tcl init flash:pinger.tcl. Now you can ping a number of hosts in a single operation:
R1(tcl)#pinger { } **** failed *** **** failed ***

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Display the names of the configured route-maps

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).

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Phase 2: Upload text files through a Telnet session

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.

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Implement “wc -l” in Cisco IOS

Sometimes it would be nice to have the full complement of Unix utilities available on Cisco IOS. That's not going to happen for a while, but we can use Tcl to make our life simpler in the meantime. Xavier Brouckaert, a regular contributor to my blog, has sent me the Tcl implementation of line counting utility (equivalent to wc -l on Unix).

First you have to define the wc Tcl procedure:
proc wc { cmd } { llength [split [exec $cmd] "\n" ] }
You can define the procedure interactively in Tclsh (but then you have to do it every time you start Tclsh) or you could store the code in a flash file and execute the file every time the Tclsh is started with the scripting tcl init filename global configuration command.

Once the wc procedure is defined, execute wc { IOS command } in Tclsh and you'll get the line count. For example, to get the number of directly connected routes use
wc { show ip route ¦ include ^C }

The include ^C filter includes all lines that start with letter C; in our case all directly connected routes

Obviously you could turn this idea into a full-blown Tclsh script that would accept CLI arguments … but I'll leave this as an exercise for the readers (you can probably tell I've been reading some academic literature lately :). However, if you find the time to write a more complete wc implementation on IOS, please do post the URL here.

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Tabular Display of OSPF External Routes

I was testing OSPF external routes recently and wanted to have a comprehensive display of OSPF type-5 LSAs (not the too-verbose information IOS generates), so I created the following Tcl script to displays type-5 (external) LSAs from the OSPF database in a tabular format.

Here is a sample printout from one of my lab routers:

External OSPF routes for OSPF process ID 1
  Prefix                Cost   Tag            ASBR Forward addr
>          10 E1     1        2000 E1     1
>           5 E2     2         200 E2     2
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Copy the text files into router's flash through a Telnet session

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.
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Can I combine EEM applets with Tcl shell?

When I’ve been describing the limitations of kron, someone quickly asked an interesting question:

As I cannot insert extra input keystrokes with EEM applet, can I run a Tcl script from it with the action sequence cli command “tclsh script" command and use the typeahead function call to get around the limitation?”

The only answer I could give at that time was “maybe” … and obviously it was time for a more thorough test. The short result is: YES, you can do it (at least in IOS release 12.4(15)T1).

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Persistent EEM variables

Someone has asked me a while ago whether it's possible to retain variable values between invocations of an EEM policy. Since a new copy of Tcl interpreter is started for each event, global variables obviously won't work; they are lost as soon as the Tcl policy is finished. A potential solution is to modify the router's configuration and save the values you wish to preserve in event manager environment, but that's a time-consuming process that interferes with whatever router configuration management process you have.

The real solution is based on the appl_setinfo and appl_reqinfo calls. They work, but like many other Tcl-related IOS features they are … well … weird.
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Tabular display of interface MTUs

When I started exploring the details of MTU handling in Cisco IOS, I quickly got tired of analyzing various long printouts to extract the MTU sizes, so I wrote a Tcl script that display hardware, IP and MPLS MTUs in a tabular format. To install it on your router:
  1. Download it from my web site and copy it to your router's flash or NVRAM.
  2. Define an alias, for example alias exec mtu tclsh flash:displayMTU.tcl.

The script recognizes two parameters: the ip parameter displays only the interfaces that have IP configured and the mpls parameter displays only the MPLS-enabled interfaces.

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SNMP with Tcl

Looking from the outside, it looks like Tcl SNMP routines in Cisco IOS were designed by a commitee or came straight from Dilbert. The snmp_getone function that reads a single SNMP value does not return an array or a list (as one would expect), but a string representation of something that looks like an XML object (but is not, since its attributes are not properly quoted). As Tcl on Cisco IOS has no built-in XML support, parsing the return values is a pure joy (and a nice exercise in writing regular expressions).

The following excerpt of a telnet session shows how to extract a single SNMP value in Tcl (I've used extra steps and an interactive tclsh session for illustration purposes). The SNMP community has to be configured in advance with the snmp-server community test ro configuration command.
rtr(tcl)#set value [snmp_getone test system.3.0]
{<obj oid='sysUpTime.0' val='14886'/>}
rtr(tcl)#regexp -inline {oid='(.*)'.*val='(.*)'} $value
{oid='sysUpTime.0' val='14886'} sysUpTime.0 14886
rtr(tcl)#regexp {oid='(.*)'.*val='(.*)'} $value ignore oid result
rtr(tcl)#puts $result
And now for a complete example: the following script prints the router uptime.
# Simple Tcl script to print system uptime
set value [snmp_getone test system.3.0]
regexp {oid='(.*)'.*val='(.*)'} $value ignore oid result
set result [expr $result / 100]
puts "Router uptime is $result seconds"
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Display IP packet filters attached to router's interfaces

A few days ago, Jeremy Stretch asked me whether there's a command to display packet lists attached to router's interfaces. While he got pretty far with the output filters, he would like to have a nice tabular format as well as the contents of the access lists displayed next to the interfaces. The show ip access-list interface name command comes pretty close, but it displays the information only for a single interface, so it was time to write another Tcl script. To install it on your router:
  1. Download it from my web site and copy it to your router's flash or NVRAM.
  2. Define an alias, for example alias exec filters tclsh flash:packetFilters.tcl.

The script recognizes two parameters: the all parameter displays all interfaces, including ones with no access lists and the verbose parameter displays the contents of the access list after the interface name.

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Display OSPF neighbor sorted by OSPF process ID

I had two issues with the show ip ospf neighbors command:
  • It is not sorted by the OSPF process ID, so you get a mess if you have more than one OSPF process and don't specify the process ID in the show command
  • It does not display the OSPF area the neighbor belongs to
To fix both problems, I wrote a Tcl script that displays OSPF neighbors sorted by process ID and includes the fields I wanted to have. To install it on your router:
  1. Download it from my web site.
  2. Copy the ospfNeighbors.tcl file to your router's flash (or NVRAM).
  3. Define an alias, for example alias exec ospf tclsh flash:ospfNeighbors.tcl.
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DNS resolver package for IOS Tcl

I've ported the dns package of the Tcl standard library to Cisco IOS. You can download it from my web site and install it on your router in just a few steps:
  1. Extract all the files from the ZIP archive and copy the Tcl files into a subdirectory on your router's flash (I would recommend you use flash:tcllib/dns).
  2. Configure the package initialization script with the scripting tcl init flash:tcllib/dns/pkgIndex.tcl global configuration command
To test the successful installation, start the Tcl shell from the command prompt and try to load the DNS package:
router(tcl)#package require dns
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