• Prefer the route with the highest LOCAL_PREF value.
• Prefer the path that was locally originated via a network or through redistribution from an IGP.
• Prefer the route with the shortest path, excluding confederation segments.
• Prefer the route with the “best” ORIGIN. IGP is better than EGP, which is better than Incomplete.
• If bgp always-compare-med is not configured, prefer any routes that do not have an inferior MED. If bgp
always-compare-med has been configured, prefer the route with the lowest MED.
• Prefer the route with the lowest IGP cost to the BGP next hop. IGP cost is determined by comparing the
preference, then the weight, then the metric, and finally the metric2 of the two resolving routes.
• If “ip load-sharing” is enabled, BGP inserts up to n most recently received paths in the IP routing table. This
allows eBGP multipath load sharing. The maximum value of n is currently 4. The default value of n, when “ip
load-sharing” is disabled, is 1. The oldest received path is marked as the best path in the output of
show ip
bgp
prefix/len
.
• Prefer routes received from external peers.
• If
bgp tie-break-on-age
has been specified, prefer the older route.
• If
bgp bestpath compare-router-id
has been specified, prefer the route learned with the lowest router
ID. The router ID is taken from the Open message of the peering session over which the route was received,
unless
bgp bestpath compare-originator-id
has been specified, and the route was received with an
ORIGIN_ID. In the latter case, the ORIGIN_ID is used instead of the router ID from the Open message.
• If
bgp bestpath compare-cluster-list-length
has been specified, prefer the route with the lowest
CLUSTER_LIST length.
• Prefer the route with the lowest neighbor address.
NOTE:
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.
Recursive route in iBGP
The nexthop of an iBGP route may not always 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.
Route selection with BGP load sharing
BGP differs from IGP in the implementation of load balancing in the following:
• IGP routing protocols such as RIP and OSPF compute metrics of routes, and then implement load sharing
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 sharing according to metrics of routes.
However, BGP has abundant route selection rules, through which it selects available routes for load sharing
and adds load sharing to route selection rules.
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Aruba 3810 / 5400R Multicast and Routing Guide for ArubaOS-
Switch 16.08
