HP 6125G HP 6125G & 6125G/XG Blade Switches Layer 3 - IP Routing Confi - Page 134

BGP route selection Route selection rules BGP discards routes with unreachable NEXT_HOPs. If multiple routes to the same destination are available, BGP selects the best route in the following sequence: 1. The route with the highest Preferred_value 2. The route with the highest LOCAL_PREF 3. The route originated by the local router 4. The route with the shortest AS-PATH 5. The IGP, EGP, or INCOMPLETE route in turn 6. The route with the lowest MED value 7. The route learned from EBGP, confederation, or IBGP in turn 8. The route with the smallest next hop metric 9. The route with the shortest CLUSTER_LIST 10. The route with the smallest ORIGINATOR_ID 11. The route advertised by the router with the smallest router ID 12. The route advertised by the peer with the lowest IP address CLUSTER_IDs of route reflectors form a CLUSTER_LIST. If a route reflector receives a route that contains its own CLUSTER ID in the CLUSTER_LIST, the router discards the route to avoid routing loops. If load balancing is configured, the system selects available routes to implement load balancing. Route selection with BGP load balancing The next hop of a BGP route may not be directly connected. One of the reasons is next hops in routing information exchanged between IBGPs are not modified. The BGP router needs to find the directly connected next hop via IGP. The matching route with the direct next hop is called the "recursive route". The process of finding a recursive route is route recursion. The system supports BGP load balancing based on route recursion. If multiple recursive routes to the same destination are load balanced (suppose three direct next hop addresses), BGP generates the same number of next hops to forward packets. BGP load balancing based on route recursion is always enabled by the system rather than configured using commands. BGP differs from IGP in the implementation of load balancing in the following ways: • IGP routing protocols such as RIP and OSPF compute metrics of routes, and then implement load balancing over routes with the same metric and to the same destination. The route selection criterion is metric. • BGP has no route computation algorithm, so it cannot implement load balancing according to metrics of routes. However, BGP has abundant route selection rules, through which, it selects available routes for load balancing and adds load balancing to route selection rules. BGP implements load balancing only on routes that have the same AS_PATH, ORIGIN, LOCAL_PREF, and MED. BGP load balancing is applicable between EBGP peers, between IBGP peers, and between confederations. If multiple routes to the same destination are available, BGP selects a configurable number of routes for load balancing. 124