In well designed networks, you can achieve a surprising amount of load balancing even when you use BGP as one of your core routing protocols.
It’s always possible to load-balance between a pair of edge routers; if you don’t have two equal-cost paths between them across your network, you can engineer them with MPLS Traffic Engineering. Similarly, you can always load-balance traffic across parallel links between two edge routers in adjacent autonomous systems.
Furthermore, you can configure EBGP load balancing on edge routers that have multiple links connected to neighbors in the same adjacent autonomous system, as long as routes received from all neighbors have the same AS path and MED (and they have the same local preference and weight if you’ve set these attributes with an inbound route-map). Load balancing between EBGP and IBGP routes is not possible, as it might lead to forwarding loops.
You can also configure IBGP load-balancing, allowing you to split traffic between a number of edge routers (up to 16 with IOS release 12.3T or 12.4); as long as all the BGP paths received from them have identical path-selection attributes and the IGP cost to the BGP next-hop is the same. With the support of link bandwidth extended community introduced in IOS release 12.3, you can split the traffic proportionally to the interface bandwidth of the inter-AS links.
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