26 June 2012

MPLS Routing Loop

Certain destination IP addresses are not routable within certain parts of the network, and appear to loop.  This has been narrowed down to the label swap on a Cisco 6500 for the next hop of a GRE tunnel [point2point]. 

As per the LFIB, the Cisco 6500 is appending label 133 for network and sending it out tun0, but appears to be snatching the packet back again, as it has a local label of 133 for the network

##Traceroute from test router (simplified for readability) and are c6500 interfaces, the latter being the GRE interface.

[admin@rtr3] > /tool trace
 # ADDRESS                                    STATUS                                 
 1      <MPLS:L=873,E=0>     **local-label for                     
 2      <MPLS:L=71,E=0>      **local-label for 
                                                                                                         **appends label 133 as per LFIB
 3          <MPLS:L=133,E=0>     **takes the packet back as per LIB
 4      <MPLS:L=218,E=0>     **appends label for                      
 5      <MPLS:L=422,E=0>     **pops label                   
 6                            **reads original ip header and appends
                                                                                                            label for                     
 7       <MPLS:L=1032,E=0>    **                      
 8      <MPLS:L=835,E=0>     **                      
 9      <MPLS:L=71,E=0>      ** and carries on until the TTL expires

#show mpls ldp bindings local-label 71
  lib entry:, rev 1474
        local binding:  label: 71

#show mpls ldp bindings local-label 133
  lib entry:, rev 636
        local binding:  label: 133

#show mpls forwarding-table labels 71
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop
Label      Label      or Tunnel Id     Switched      interface
71         133  22346897      Tu0        point2point

No routing loops occur when the router preceeding the c6500 pops the label. This is obviously due to using the routing table.

Possible solutions perhaps specify next hop IP for the label rather than the p2p interface, somehow preventing local labels from having the same labels as the remote gre end point.

I believe if I rebuild the LFIB, this problem will go away... until next time. I would like to find a solution, whether it be a bug fix or best practice, or perhaps something that I am missing or haven't RTFM'd properly.

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