External BGPInternal BGP
NeighborBetween differnet ASWithin the same AS
Route updatesRoutes are send to eBGP peers by defaultRoutes are not send to BGP peers by default
AS path additionyesno
Administrative distance20200
TopologyDoes not require a full meshFull mesh or Route reflectors or Confedoration
Loop Preventionas-pathBGP split horizon


  • 3 steps to get best routes;
    • Form a neighbor relationship.
      • router bgp 1000
      • neighbor remote-as 2000
    • Exchange topology information.
    • Run a best-path algorithm
  • Assumed to be directly connected (apart from multihop)

eBGP neighborship requirements

  • Peer must be reachable via an IGP route.
  • BGP router IDs cannot be the same.
  • Authentication must match.
  • Router must complete a TCP connection (tcp 3 way handshake) with the BGP peer.
  • Source adress must match the other side’s BGP neighbor command.
R1#(config)router bgp 10
R1#(config-router)neighbor remote-as 20

R2#(config)router bgp 20
R2#(config-router)neighbor remote-as 10

iBGP neighborship requirements

  • Must be in the same AS as the iBGP peer.

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