IP Routing

Load Balancing

As per RFC 1771, BGP installs only one best path to a destination network. This scenario leaves little room to load-balance over multiple paths. However, it is possible to use an IGP (such as IGRP) to achieve load balancing between ASs.

In the network in Figure 7-12, ISP-A and ISP-B set up two links between each other over which traffic is to be load-balanced.

Figure 7-12. Load balancing

Both the peering routers set up loopback addresses (lines 41 and 46 in the following code blocks). BGP sessions between the peers are then established between the peers using these loopback addresses (lines 42 and 47 specify the loopback addresses of the neighbor; lines 43 and 48 say that the BGP TCP session should originate from the local loopback address). Since BGP normally expects its peers to be on a directly connected network, the ebgp-multihop command (lines 44 and 49) relaxes this restriction. None of this will work if each peer cannot route to the other’s loopback IP address. Lines 45 and 50 set up IGRP between ISP-A-1 and ISP-B-1, which permits the routers to share path information. Since IGRP will use both paths between the ASs to route to the peer’s loopback address, all traffic between the ASs will use both paths.

    hostname ISP-A-1
    !
41  interface Loopback0 ip address 98.2.0.1 255.255.0.0 ! interface Serial2 description * to ISP-B * ip address 100.1.1.1 255.255.255.0 ! interface ...