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Connections between BGP speakers of different ASs are referred to as External BGP (EBGP) connections. BGP enforces the rule that peer routers for EBGP connections need to be on a directly attached network. If the peer routers are multiple hops away from each other or if multiple links are between them, you can override this restriction by enabling the EBGP multihop feature. TCP connections between EBGP peers are tied to the addresses of the outgoing interfaces. Therefore, a single interface failure severs the session even if a viable path exists between the peers.
EBGP multihop support can provide redundancy so that an EBGP peer session persists even in the event of an interface failure. Using an address assigned to the loopback interface for the EBGP peering session ensures that the TCP connection stays up even if one of the links between them is down, provided the peer loopback address is reachable. In addition, you can use EBGP multihop support to balance the traffic among all links.
Use the TTL (Time to Live) parameter to limit the number of hops over which the External BGP (EBGP) multihop session is established. You can configure the TTL only if EBGP multihop is enabled. The default TTL is 64. When multihop is disabled the default TTL is 1.
When traffic comes from a router that is not directly connected and multihop is enabled, BGP uses that router as the next hop, irrespective of the advertised routes that it gets.
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Important - Enabling multihop BGP connections is dangerous because BGP speakers might establish a BGP connection through a third-party AS. This can violate policy considerations and introduce forwarding loops. |
