Operation Manual – IPv4 RoutingH3C S3610&S5510 Series Ethernet Switches Chapter 5 BGP Configuration5-11BGP differs from IGP in the implementation of load balancing in the following:z IGP routing protocols such as RIP, OSPF compute metrics of routes, and thenimplement load balancing on routes with the same metric and to the samedestination. The route selection criterion is metric.z BGP has no route computation algorithm, so it cannot implement load balancingaccording to metrics of routes. However, BGP has abundant route selection rules,through which, it selects available routes for load balancing and adds loadbalancing to route selection rules.Note:z BGP implements load balancing only on routes that have the same AS_PATH,ORIGIN, LOCAL_PREF and MED.z BGP load balancing is applicable between EBGPs, between IBGPs and betweenconfederations.z If multiple routes to the same destination are available, BGP selects routes for loadbalancing according to the configured maximum number of load balanced routes.Figure 5-10 Network diagram for BGP load balancingIn the above figure, Router D and Router E are IBGP peers of Router C. Router A andRouter B both advertise a route destined for the same destination to Router C. If loadbalancing is configured and the two routes have the same AS_PATH attribute, ORIGINattribute, LOCAL_PREF and MED, Router C installs both the two routes to its routetable for load balancing. After that, Router C forwards routes to Router D and Router Eonly once, with AS_PATH unchanged, NEXT_HOP changed to Router C’s address.Other BGP transitive attributes apply according to route selection rules.
