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#tclsh
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
1
rtr(tcl)#puts $result
14886

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"