Posts Tagged ‘mpls l3vpn’


Update: Congrats to Mark, our winner of 100 rack rental tokens for the first correct answer, that XR2 is missing a BGP router-id.  In regular IOS, a router-id is chosen based on the highest Loopback interface.  If there is no Loopback interface the highest IP address of all up/up interfaces is chosen.  In the case of IOS XR however, the router-id will not be chosen from a physical link.  It will only be chosen from the highest Loopback interface, or from the manual router-id command.  Per the Cisco documentation:

BGP Router Identifier

For BGP sessions between neighbors to be established, BGP must be assigned a router ID. The router ID is sent to BGP peers in the OPEN message when a BGP session is established.

BGP attempts to obtain a router ID in the following ways (in order of preference):

  • By means of the address configured using the bgp router-id command in router configuration mode.
  • By using the highest IPv4 address on a loopback interface in the system if the router is booted with saved loopback address configuration.
  • By using the primary IPv4 address of the first loopback address that gets configured if there are not any in the saved configuration.

If none of these methods for obtaining a router ID succeeds, BGP does not have a router ID and cannot establish any peering sessions with BGP neighbors. In such an instance, an error message is entered in the system log, and the show bgp summary command displays a router ID of

After BGP has obtained a router ID, it continues to use it even if a better router ID becomes available. This usage avoids unnecessary flapping for all BGP sessions. However, if the router ID currently in use becomes invalid (because the interface goes down or its configuration is changed), BGP selects a new router ID (using the rules described) and all established peering sessions are reset.

Since XR2 in this case does not have a Loopback configured, the BGP process cannot initialize.  The kicker with this problem is that the documentation states that when this problem occurs you should see that “an error message is entered in the system log”, however in this case a Syslog was not generated about the error.  At least this is the last time this problem will bite me ;)


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