BGP Multihoming

BGP's popularity stems from its ability to add redundancy in support of critical networks and their connections for two-way traffic (inbound and outbound). BGP also can be used to load balance, or load share, traffic across multiple links. Load balancing of BGP traffic is possible when a BGP AS speaker (router) learns of two EBGP paths for an IP prefix from a neighboring autonomous system. By default, the BGP routing process will select the path with the lowest router ID to enter into its routing table. BGP multipath must be enabled for BGP to take advantage of multiple paths to a destination.

BGP can support up to eight paths to a destination, either load balancing or load sharing across each path.

NOTE

Load balancing is an equal distribution of the traffic load across all available links. Load sharing is the shared distribution of traffic load across all available links, but not necessarily equal. Load balanced traffic is load shared; load shared traffic is not always load balanced.

For example, a customer might have Internet connections with two (or more) ISPs for network connectivity redundancy. Rather than have one line carry all the traffic and the other(s) sit idle, active only when the primary connection has failed, load sharing enables the customer to spread his Internet traffic across all his ISP connections.