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| 9.9 |  | Redundancy, Symmetry, and Load Balancing |
| 9.9.5 |  | Load balancing |

Load balancing is the capability to divide data traffic over multiple connections. A BGP speaker may learn two identical EBGP paths for a prefix from a neighboring AS. If this happens, it will choose the path with the lowest route ID as the best path. This best path is installed in the IP routing table. If BGP multipath support is enabled and the EBGP paths are learned from the same neighboring AS, the best path may not be chosen. Instead, multiple paths are installed in the IP routing table.

To enable BGP load balancing over equal cost paths, use the

maximum-paths command, which has the following syntax:

> Router(config-router)# maximum-paths number

BGP supports a maximum of six paths per destination, but only if they are sourced from the same AS. By default, BGP will install only one path to the IP routing table.

Figure illustrates how inbound and outbound traffic behaves. The path for outbound traffic to reach NetA depends on where NetA is learned. Because NetA is received from both San Francisco and New York, outbound traffic toward NetA can go by way of San Francisco or New York.

On the other hand, the path for inbound traffic to reach the local networks, NetB and NetC, depends on how these networks are advertised. If NetC is advertised over the New York link only, then incoming traffic toward NetC will take the New York link. Similarly, if NetB is advertised over the San Francisco link only, traffic toward NetB will take the San Francisco link.

| Web Links BGP ECourse- Load Balancing in BGP http://www.cisco.com/warp/public/615/ tac_elearn/ecourse/BGP__1/ html/loframe16778560.htm