834 CHAPTER 56: BGP CONFIGURATIONFigure 240 Network diagram for BGP load balancingIn the above figure, Router D and Router E are IBGP peers of Router C. Router Aand Router B both advertise a route destined for the same destination to Router C.If load balancing is configured and the two routes have the same AS_PATHattribute, ORIGIN attribute, LOCAL_PREF and MED, Router C adds both the tworoutes to its route table for load balancing. After that, Router C forwards routes toRouter D and Router E only once, with AS_PATH unchanged, NEXT_HOP changedto Router C’s address. Other BGP transitive attributes apply according to routeselection rules.BGP route advertisement ruleThe current BGP implementation supports the following route advertisement rules:■ When multiple available routes exist, a BGP speaker advertises only the bestroute to its peers.■ A BGP speaker advertises only routes used by itself.■ A BGP speaker advertises routes learned from EBGPs to all BGP peers, includingboth EBGP and IBGP peers.■ A BGP speaker does not advertise routes learned from IBGPs to IBGP peers.■ A BGP speaker advertises routes learned from IBGPs to EBGP peers. Note that ifinformation synchronization is disabled between BGP and IGP, IBGP routes areadvertised to EBGP peers. If enabled, only IGP advertises the IBGP routes canthese routes be advertised to EBGP peers.■ A BGP speaker advertises all routes to a newly connected peer.IBGP and IGPInformationSynchronizationThe routing Information synchronization between IBGP and IGP is for avoidance ofgiving wrong directions to routers outside of the local AS.If a non-BGP router works in an AS, a packet forwarded via the router may bediscarded due to unreachable destination. As shown in Figure 241, Router Elearned a route 8.0.0.0/8 from Router D via BGP. Then Router E sends a packet toRouter A through Router D, which finds from its routing table that Router B is thenext hop (configured using the peer next-hop-local command). Since Router Dlearned the route to Router B via IGP, it forwards the packet to Router C usingRouter CRouter E Router DRouter A Router BAS 100AS 200
