Cisco IP Routing Protocols: Trouble Shooting Techniques (Charles River Media Networking/Security)

To discuss the troubleshooting of BGP multihoming and loadsharing scenarios, consider the example depicted in Figure 11.11. In the figure, AS6666 is multihomed to AS7777 and AS8888. Router A1 is connected via serial links to routers B1 and C1. Both C1 and A1 are advertising the default route to A1.

Figure 11.11: A BGP network depicting multihoming and loadsharing problems.

In Figure 11.11, you see that load balancing between the two outbound paths is not happening. In outbound load balancing:

• If the default route is advertised by both B1 and C1, the weight attribute of updates received from both the ISPs can be set to the same value so that load balancing occurs.

• If, in AS6666, the links to the ISPs corresponding to AS7777 and AS8888 are terminated in different routers and not in the same router, the local preference attribute instead of the weight attribute can be used to achieve loadsharing.

In order to achieve inbound load balancing:

• Different prefixes are advertised to the BGP peers belonging to different ASs to which the subject AS is multihomed. In this case, mutual backup can be configured by use of AS path prepend.

• If there are multiple links to the same ISP, terminate them in the same router on both sides. Configure peering between the loopback addresses on both sides. The route to the loopback address on either side is statically configured pointing to the physical links. The type of load balancing in this case depends on the type of switching enabled in the router. In the case of process switching, per packet load balancing is achieved, fast switching results in per destination load balancing. In case of CEF, loadsharing can be achieved per source/destination pair. In this case, load balancing can be achieved in both the inbound and outbound direction.