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BGP bases its decision process on the attribute values. When faced
with multiple routes to the same destination, BGP chooses the best
route for routing traffic toward the destination. The following
process summarizes how BGP chooses the best route:
- If the next hop is inaccessible, the route is ignored. This is
why it is important to have an IGP route to the next hop.
- The BGP router will prefer the path with the largest weight.
Weight is a Cisco proprietary parameter.
- If the weights are the same, the BGP router will prefer the
route with the largest local preference.
- If the routes have the same local preference, the BGP router
will prefer the route that was locally originated by
this router.
- If the local preference is the same, the BGP router will prefer
the route with the shortest AS_Path.
- If the AS_Path length is the same, the BGP router will prefer
the route with the lowest origin type. This is when IGP is lower
than EGP and EGP is lower than Incomplete.
- If the origin type is the same, the BGP router will prefer the
route with the lowest MED.
- If the routes have the same MED, the BGP router will prefer the
route in the following manner. External (EBGP) is better than
confederation external, which is better than IBGP. BGP
confederations are not covered in this course. For more information,
consult Cisco's web site, at
http://www.cisco.com/univercd/home/home.htm.
- If all the preceding scenarios are identical, the BGP router
will prefer the route that can be reached by way of the closest IGP
neighbor. That means taking the shortest internal path inside the AS
to reach the destination by following the shortest path to the BGP
Next Hop.
- If the internal path is the same, the BGP router ID will be a
tiebreaker. The BGP router will prefer the route coming from the BGP
router with the lowest router ID. The router ID is usually the
highest IP address on the router or the loopback address.
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