Operation Manual – IPv4 RoutingH3C S5500-EI Series Ethernet Switches Chapter 5 BGP Configuration5-10z No_Advertise: After received, routes with this attribute cannot be advertised toother BGP peers.z No_Export_Subconfed: After received, routes with this attribute cannot beadvertised out the local AS or other ASs in the local confederation.5.1.3 BGP Route SelectionI. Route selection rulesBGP supports the following route selection rules:z Discard routes with unreachable NEXT_HOP firstz Select the route with the highest Preferred_valuez Select the route with the highest LOCAL_PREFz Select the route originated by the local routerz Select the route with the shortest AS-PATHz Select IGP, EGP, Incomplete routes in turnz Select the route with the lowest MED valuez Select routes learned from EBGP, confederation, IBGP in turnz Select the route with the smallest next hop costz Select the route with the shortest CLUSTER_LISTz Select the route with the smallest ORIGINATOR_IDz Select the route advertised by the router with the smallest Router IDNote:z CLUSTER_IDs of route reflectors form a CLUSTER_LIST. If a route reflectorreceives a route that contains its own CLUSTER ID in the CLUSTER_LIST, therouter discards the route to avoid routing loops.z If load balancing is configured, the system selects available routes to implementload balancing.II. Route selection with BGP load balancingThe next hop of a BGP route may not be a directly connected neighbor. One of thereasons is next hops in routing information exchanged between IBGPs are not modified.In this case, a router finds the direct route via IGP route entries to reach the next hop.The direct route is called the reliable route. The process of finding a reliable route toreach a next hop is route recursion.Currently, the switch supports BGP load balancing based on route recursion, namely ifreliable routes are load balanced (suppose three next hop addresses), BGP generatesthe same number of next hops to forward packets. Note that BGP load balancing based
