H3C Technologies H3C S12500 Series Switches - Manual (Page 213)

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You can select an extended community attribute type as needed to implement route filtering and
control. This simplifies configuration and management.
The device supports the Route-Target for VPN and Source of Origin (SOO) attributes. For more
information, see MPLS Configuration Guide.
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 summary route
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.
BGP route advertisement rules
The current BGP implementation supports the following route advertisement rules:
• When multiple feasible routes to a destination exist, the BGP speaker advertises only the best route
to its peers.
• A BGP speaker advertises only routes that it uses.
• A BGP speaker advertises routes learned through EBGP to all BGP peers, including both EBGP and
IBGP peers.
• A BGP speaker advertises routes learned through IBGP to EBGP peers, rather than IBGP peers.
• After a BGP speaker establishes a session with a newly connected BGP peer, it advertises all routes
to the peer according to the advertisement rules. After that, the BGP speaker advertises only route
updates to the peer.
BGP load balancing
BGP implements load balancing by using route recursion and modifying route selection rules.
• Implement BGP load balancing by using route recursion.