BGP: Checks Performed Before Becoming Neighbors

BGP has certain checks that it runs before establishing a neighborship with a peering router.  The following list describes the checks BGP performs when forming neighbor relationships:

  • The router must receive a TCP connection request with a source address that the router finds under its BGP neighbor commands
  • A router’s ASN must match the neighboring routers reference to that ASN via the remote-as command.
  • The BGP RIDs of the two routers must not be the same
  • If configured MD5 authentication must pass

BGP uses a keepalive timer and a hold timer to determine the status of a neighbor.  If keep alive or hold timers are different, BGP neighbors will negotiate to the lowest value between the two.  Meaning that configurations for timers do not have to match in order for neighborships to become active.

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