BGP Peering

BGP mandates that each external router establish an internal BGP connection with each other external router in the same AS. These external peered routers will be linked by a fully meshed graph of route paths. The internal interconnections are used to propagate external routing information, independently of whatever Interior Gateway Protocol (IGP) might be running, such as OSPF or EIGRP.

These internal interconnections are also used among BGP routers to reach an agreement on the best route to an external destination network. This ensures that only the router that is managing the best path inserts information into the AS internal routing tables through the IGP in use.

Two types of BGP peers exist:

• Internal— BGP peers that are connected within the same AS (with intra-AS links)

• External— BGP peers that are interconnected between two ASs (with inter-AS links)

Figure 25-7 illustrates internal and external BGP peers, with respective inter- and intra-AS links.

Figure 25-7. Internal and External BGP Peers

Figure 25-7. Internal and External BGP Peers

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