F3215-HP Load Balancing Module Appendix Protocol Reference-6PW101
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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. Highest Preferred_value
2. Highest LOCAL_PREF
3. Summary route
4. Shortest AS_PATH
5. IGP, EGP, or INCOMPLETE route in turn
6. Lowest MED value
7. Learned from EBGP, confederation, or IBGP in turn
8. Smallest next hop metric
9. Shortest CLUSTER_LIST
10. Smallest ORIGINATOR_ID
11. Advertised by the router with the smallest router ID
12. 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.
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.
• 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.
• A BGP speaker advertises routes learned from an IBGP peer to EBGP peer, 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 routes to the peer.
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.
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