Bgp Route Selection - H3C S5800 Series Configuration Manual

BGP Route Selection

Route selection rules
The current BGP implementation supports the following route selection sequence:
Discard routes with unreachable NEXT_HOPs first
Select the route with the highest Preferred_value
Select the route with the highest LOCAL_PREF
Select the route originated by the local router
Select the route with the shortest AS-PATH
Select IGP, EGP, Incomplete routes in turn
Select the route with the lowest MED value
Select routes learned from eBGP, confederation, iBGP in turn
Select the route with the smallest next hop cost
Select the route with the shortest CLUSTER_LIST
Select the route with the smallest ORIGINATOR_ID
Select the route advertised by the router with the smallest Router ID
Select the route 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. 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:
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.
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.
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