iBGP:

  • Packets default to TTL 255
    • Neighbors do not have to be connected as long as IGP reachability exists.
  • Peers typically peer via Loopbacks
    • ‘neighbor x.x.x.x update-source l0’
    • Allows rerouting around failed paths via IGP.
    • Required for things like MPLS L3VPN.
  • Loop Prevention
    • iBGP learned routes cannot be advertised on to another iBGP neighbor.
    • BGP requires the following:
      • Full Mesh
      • Route Reflectors
      • Confederation
  • Next Hop Processing
    • Outbound iBGP updates do not modify next-hop regardless of iBGP type.
    • Modifying next hop:
      • ‘neighbor next-hop-self’
      • Route map
        • action – set next-hop
      • IOS 15.1(1)SY – next-hop-self ALL

EBGP:

  • Packets default to TTL 1
    • Can be modified if needed.
    • ‘neighbor ebgp-multihop <ttl>’
    • ‘neighbor ttl-security hops <ttl>’
  • Single hop peers must be directly connected by default.
    • Can be modified if directly connected neighbors peer via Loopbacks.
    • neighbor disable-connected-check.
  • Loop Prevention
    • AS-Path
      • Local ASN is prepended to outbound updates.
      • Inbound updates containing local ASN are discarded.
      • Can be modified with the following:
        • ‘neighbor allowas-in’
        • ‘as-override’
  • Next Hop Processing
    • Outbound EBGP updates have local update-source set to next-hop by default.
    • Modification
      • Route map
        • action – set next-hop
      • ‘neighbor next-hop-unchanged’

General:

  • BGP next hop controls IGP route recursion.
    • BGP knows the next hop but not the outgoing interface.
      • BGP is not a routing protocol by itself.
    • IGP must be able to perform recursion otherwise route cannot be used.
    • Result of failed recursion means route does not get installed into RIB.

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