BGP: Contents of Update Messages

NLRIs are determined before being placed into Update messages.  There are rules for NLRIs to be included in an update.  Here are the rules about which routes are NOT included in updates to neighbors:

  • Routes considered not ‘best’
  • Routes matched by a deny clause in an outbound BGP filter
  • iBGP learned routes, except for network configurations utilizing route reflectors or confederations

By default when sending an update to an eBGP peer, the NEXT HOP is changed to an IP address on the advertising router specifically the same IP address the router used as the source IP address of the update message for each neighbor.

When sending to an iBGP peer the default action is to leave the NEXT HOP PA unchanged.

These behaviors can be altered via configuration.

 

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