Operation Manual – IPv4 RoutingH3C S5500-EI Series Ethernet Switches Chapter 5 BGP Configuration5-12only once, with AS_PATH unchanged, NEXT_HOP changed to Router C’s address.Other BGP transitive attributes apply according to route selection rules.III. BGP route advertisement rulesBGP supports the following route advertisement rules:z When multiple feasible routes exist, a BGP speaker advertises only the best routeto its peers.z A BGP speaker advertises only routes used by itself.z A BGP speaker advertises routes learned through EBGP to all BGP peers,including both EBGP and IBGP peers.z A BGP speaker does not advertise IBGP routes to IBGP peers.z A BGP speaker advertises IBGP routes to EBGP peers. Note that if BGP and IGPsynchronization is disabled, IBGP routes are advertised to EBGP peers directly. Ifthe feature is enabled, only IGP advertises the IBGP routes can BGP advertisethese routes to EBGP peers.z A BGP speaker advertises all routes to a newly connected peer.5.1.4 IBGP and IGP SynchronizationThe 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 an unreachable destination. As shown in Figure 5-11, Router Elearned a route of 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 the nexthop (configured using the peer next-hop-local command). Since Router D learned theroute to Router B via IGP, it forwards the packet to Router C using route recursion.Router C has no idea about the route 8.0.0.0/8, so it discards the packet.Figure 5-11 IBGP and IGP synchronizationIf synchronization is configured in this example, the IBGP router (Router D) checks thelearned IBGP route from its IGP routing table first. Only the route is available in the IGP
