Generating a Report of Routing Table Information

Problem

You need to extract the IP routing table from one of your routers.

Solution

The script in Example 2-2, rt.pl, uses SNMP to extract the routing table from a specified router, and displays this information to standard output (STDOUT). The script expects to find a hostname or IP address of a router on the command line.

Example 2-2. rt.pl

#!/usr/bin/perl
#
# rt.pl -- a script to extract the routing table
# from a router.
#
#Set behavior
$snmpro="ORARO";
#
$x=0;
$snmpwalk="/usr/local/bin/snmpwalk -v 1 -c $snmpro";
$snmpget="/usr/local/bin/snmpget -v 1 -c $snmpro";
chomp ($rtr=$ARGV[0]);
if ( $rtr eq "" ) {die "$0: Must specify a router
"};
print "Destination	Mask		Nexthop";
print "		 Proto	Interface
";
@iftable=Q$snmpwalk $rtr ifDescrQ;
for $ifnum (@iftable) {
 chomp (($intno, $intname) = split (/ = /, $ifnum));
 $intno=~s/.*ifDescr.//;
 $intname=~s/"//gi;
 $int{$intno}=$intname;
}
@ipRouteDest=Q$snmpwalk $rtr ipRouteDestQ;
@ipRouteMask=Q$snmpwalk $rtr ipRouteMaskQ;
@ipRouteNextHop=Q$snmpwalk $rtr ipRouteNextHopQ;
@ipRouteProto=Q$snmpwalk $rtr ipRouteProtoQ;
@ipRouteIfIndex=Q$snmpwalk $rtr ipRouteIfIndexQ;
#@ipRouteMetric1=Q$snmpwalk $rtr ipRouteMetric1Q;
for $intnum (@ipRouteIfIndex) {
 chomp (($foo, $int) = split (/= /, $intnum));
 chomp (($foo, $dest) = split (/: /, @ipRouteDest[$x]));
 chomp (($foo, $mask) = split (/: /, @ipRouteMask[$x]));
 chomp (($foo, $nhop) = split (/: /, @ipRouteNextHop[$x]));
 chomp (($foo, $prot) = split (/= /, @ipRouteProto[$x]));
 #chomp (($foo, $metr) = split (/= /, @ipRouteMetric1[$x]));
 $int1 = $int{$int};
 if ($int1 eq '') {$int1="Local"};
 $prot=~s/(.*//; $prot=~s/ciscoIgrp/(e)igrp/;
 printf ("%-15s %-15s %-15s %7s %-25s
",$dest, $mask, $nhop, $prot, $int1);
 $x++
}

Discussion

The rt.pl script is written in Perl and uses NET-SNMP to extract the routing table information via SNMP. It expects to find both Perl and NET-SNMP in the /usr/local/bin directory. For more information on Perl or NET-SNMP, please see Appendix A.

Before using the script, you must define the variable $snmpro to contain the SNMP read-only community string for the router:

Freebsd% ./rt.pl toronto
Destination Mask Nexthop Proto Interface
10.1.1.0 255.255.255.252 172.25.1.5 ospf Ethernet0 
10.2.2.2 255.255.255.255 172.25.1.5 ospf Ethernet0 
172.16.2.0 255.255.255.0 172.25.1.5 ospf Ethernet0 
172.20.0.0 255.255.0.0 172.25.1.5 local Local 
172.20.1.0 255.255.255.252 172.25.1.5 ospf Ethernet0 
172.20.10.0 255.255.255.0 172.25.1.5 ospf Ethernet0 
172.20.100.1 255.255.255.255 172.25.1.5 ospf Ethernet0 
172.22.0.0 255.255.0.0 172.25.1.5 (e)igrp Ethernet0 
172.22.1.0 255.255.255.0 172.25.1.5 ospf Ethernet0 
172.25.1.0 255.255.255.0 172.25.1.7 local Ethernet0 
172.25.2.0 255.255.255.252 172.25.1.5 (e)igrp Ethernet0 
172.25.25.1 255.255.255.255 172.25.1.5 (e)igrp Ethernet0 
172.25.25.6 255.255.255.255 172.25.25.6 local Loopback0 
172.25.26.4 255.255.255.252 172.25.1.5 (e)igrp Ethernet0 
172.25.26.5 255.255.255.255 172.25.1.5 ospf Ethernet0
Freebsd%

The output from the script is relatively straightforward, except for static routes and directly connected routes, which require a little explanation.

For static routes, the output shows a value of local in the protocol field, and an interface name of Local. Directly connected routes also appear as local protocol information, but the interface name is the real interface associated with this route.

For example, the route 172.20.0.0 255.255.0.0 is a static route:

172.20.0.0 255.255.0.0 172.25.1.5 local Local 

And 172.25.1.0 255.255.255.0 is a directly connected route:

172.25.1.0 255.255.255.0 172.25.1.7 local Ethernet0 

Since this script queries only open standard SNMP MIB values, you can use it to extract IP route information from most any SNMP-enabled device, including nonCisco equipment.

See Also

Appendix A