H3C Technologies H3C S12500-X Series Switches - Manual (Page 193)

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BGP route selection
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 generated by the network command, the route redistributed by the import-route
command, or the summary route in turn.
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 EBGP, confederation IBGP, 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.
The 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.
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BGP route advertisement rules
BGP follow these rules for route advertisement:
• When multiple feasible routes to a destination exist, BGP advertises only the best route to its peers.
If the advertise-rib-active command is configured, BGP advertises the best route in the IP routing
table; if not, BGP advertise the best route in the BGP routing table.
• BGP advertises only routes that it uses.
• BGP advertises routes learned from an EBGP peer to all BGP peers, including both EBGP and IBGP
peers.
• BGP advertises routes learned from an IBGP peer to EBGP peers, rather than other IBGP peers.
• After establishing a session with a new BGP peer, BGP advertises all the routes matching the above
rules to the peer. After that, BGP advertises only incremental updates to the peer.
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BGP load balancing
BGP implements load balancing through route recursion and route selection.
• BGP load balancing through route recursion.
The next hop of a BGP route might not be directly connected. One of the reasons is next hops in
routing information exchanged between IBGP peers are not modified. The BGP router must find the
directly-connected next hop through IGP. The matching route with the direct next hop is called the
"recursive route." The process of finding a recursive route is route recursion.