After working with MPLS Traffic Engineering lab for a few days and interpreting IP addresses from various traceroute outputs, I finally had enough and wrote a simple Perl script (below) that parses router configurations and produces ip host configuration commands for every interface IP address it encounters. When you paste the ip host commands into the configuration of the edge router from which you do the tests, the meaningless numbers finally make sense.

Compare the “traditional” output produced with MPLS-enabled traceroute…

PE-A#traceroute PE-C

Type escape sequence to abort.
Tracing the route to PE-C (10.0.1.5)

  1 10.0.7.6 [MPLS: Label 1017 Exp 0] 16 msec 136 msec 52 msec
  2 10.0.7.26 [MPLS: Label 3018 Exp 0] 8 msec 20 msec 8 msec
  3 10.0.7.30 [MPLS: Label 4018 Exp 0] 16 msec 20 msec 56 msec
  4 10.0.7.33 [MPLS: Label 2017 Exp 0] 20 msec 44 msec 20 msec
  5 10.0.7.17 24 msec *  24 msec

… with this one:

PE-A#traceroute PE-C

Type escape sequence to abort.
Tracing the route to PE-C (10.0.1.5)

  1 Serial1-0.C1 (10.0.7.6) [MPLS: Label 1017 Exp 0] 56 msec
  2 Serial1-2.C3 (10.0.7.26) [MPLS: Label 3018 Exp 0] 12 msec
  3 Serial1-1.C4 (10.0.7.30) [MPLS: Label 4018 Exp 0] 48 msec
  4 Serial1-2.C2 (10.0.7.33) [MPLS: Label 2017 Exp 0] 52 msec
  5 Serial1-0.PE-C (10.0.7.17) 16 msec *  64 msec

Perl script

#!/usr/bin/perl

use strict;
use Getopt::Long;

our ($host,$domain,$FQDN,$loop,$ifname,$inIfMode);
our ($opt_dbg);

GetOptions("debug" => \$opt_dbg);

while (<>) {
  if (/^hostname\s+(.*)/) { 
    $host = $1; 
    print STDERR "processing router $host\n" if $opt_dbg; 
    $FQDN = $host; $loop = 0; 
  }
  if (/^ip\s+domain-name\s+(.*)/) { 
    $domain = $1; 
    print STDERR "router $host in domain $domain" if $opt_dbg; 
    $FQDN = "$host.$domain"; }
  if (/^interface\s+(.*)/) { 
    $ifname = $1; $ifname =~ s/[:\/.]/-/gi; $ifname = "$ifname.";
    print "interface $ifname\n" if $opt_dbg; 
    if ($ifname =~ /loopback/i) { $ifname = "" unless $loop++; }
    $inIfMode = 1 ;
  } elsif (/^[a-z]/i) { $inIfMode = 0; }
    elsif (/^\s+ip address ([0-9]+\.[0-9]+\.[0-9]+\.[0-9]+)/i && $inIfMode) { 
      print "ip host $ifname$FQDN $1\n"; 
  }
}