Selecting Route Paths In Bgp - Nortel Alteon OS 42C4911 Application Manual

Selecting Route Paths in BGP

BGP selects only one path as the best path. It does not rely on metrics attributes to determine
the best path. When the same network is learned via more than one BGP peer, BGP uses its
policy for selecting the best route to that network. The BGP implementation on the GbE
Switch Module uses the following criteria to select a path when the same route is received
from multiple peers.
1. Local fixed and static routes are preferred over learned routes.
2. With iBGP peers, routes with higher local preference values are selected.
3. In the case of multiple routes of equal preference, the route with lower AS path weight is
selected.
AS path weight = 128 x AS path length (number of autonomous systems transversed).
4. In the case of equal weight and routes learned from peers that reside in the same AS, the
lower metric is selected.
N – A route with a metric is preferred over a route without a metric.
OTE
5. The lower cost to the next hop of routes is selected.
6. In the case of equal cost, the eBGP route is preferred over iBGP.
7. If all routes are from eBGP, the route with the lower router ID is selected.
When the path is selected, BGP puts the selected path in its routing table and propagates the
path to its neighbors.
42C4911, January 2007
Alteon OS Application Guide
Chapter 11: Border Gateway Protocol 181