Blade ICE G8124 Manuale Utente

BMD00220, October 2010

Chapter 19: Border Gateway Protocol  

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BGP selects only one path as the best path. It does not rely on metric 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 G8124 uses the following 
criteria to select a path when the same route is received from multiple peers.

Local fixed and static routes are preferred over learned routes.

With iBGP peers, routes with higher local preference values are selected.

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 traversed).

In the case of equal weight and routes learned from peers that reside in the same AS, the lower 
metric is selected.

A route with a metric is preferred over a route without a metric.

The lower cost to the next hop of routes is selected.

In the case of equal cost, the eBGP route is preferred over iBGP.

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.