Bgp route selection, Route selection rules, Route selection with bgp load balancing – H3C Technologies H3C S7500E Series Switches User Manual
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No_Export_Subconfed: After received, routes with this attribute cannot be advertised out the local
AS or other ASs in the local confederation.
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:
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Select the route with the highest Preferred_value
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Select the route with the highest LOCAL_PREF
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Select the route originated by the local router
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Select the route with the shortest AS-PATH
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Select the IGP, EGP, or Incomplete route in turn
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Select the route with the lowest MED value
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Select the route learned from eBGP, confederation, or iBGP in turn
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Select the route with the smallest next hop metric
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Select the route with the shortest CLUSTER_LIST
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Select the route with the smallest ORIGINATOR_ID
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Select the route advertised by the router with the smallest Router ID
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Select the route advertised by the peer with the lowest IP address
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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.
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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. In this case, 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.
Currently, the system supports BGP load balancing based on route recursion, namely, 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. Note that 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:
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IGP routing protocols such as RIP, 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.