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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"


  1. Hi,

    it's very nice and look like pretty easy, but is any chance make it for current interface traffic load per 30 sec. For example i have router like 1800 and i would like check current interface load i'll use "show interfaces | inc rate" but i want to see periodicly?



  2. Hi Karl!

    Tcl isn't the best tool to use if you want to see a longer-term series of a value (for example, interface load). I would use a free network management graphing tool to do it (MRTG immediately springs to mind).


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