Border Gateway Protocol - CCIE Routing and Switching Exam Certification Guide - page 446

To select the best path to a destination, Cisco routers running BGP use the following algorithm:

If the specified next hop is inaccessible, drop the path. 1

If the path is internal, synchronization is enabled, and the path is not in the IGP; drop
the path.

2

Prefer the path with the largest weight. (This step is Cisco specific and weight is localized
to the router.)

3

If the weights are the same, prefer the path with the largest local preference. Used by iBGP
only to reach the preferred external BGP router.

4

If the local preferences are the same, prefer the path that was originated by BGP running
on this router. These are routes originated by the network command, aggregate-address
command, or redistribution. (This step is Cisco specific.)

5

If no route was originated, prefer the route that has the shortest AS path. (This step is Cisco
specific.)

6

If all paths have the same AS path length, prefer the path with the lowest origin type. Paths
with an origin type of IGP are (lower) preferred over paths that originated from exterior
gateway protocol (EGP), and EGP origin is preferred over a route with an incomplete
origin. (This step is Cisco specific.)

7

If the origin codes are the same, prefer the path with the lowest MED attribute. Used by an
eBGP peer to select a best path to AS. (This step is a tiebreaker, as described in the RFC.)

8

If the paths have the same MED, prefer the eBGP over the iBGP path. (This step is Cisco
specific.)

9

If the paths are still the same, prefer the path through the closest IGP neighbor. (This step
is a tiebreaker, as described in the RFC.)

10

Prefer the path with the BGP neighbor with the lowest RID (described in the RFC). 11

After BGP has decided on a best path, it marks it with a > sign in the show ip bgp table and
adds it to the IP routing table.

BGP Route Dampening, Peer Groups, Route Reflectors,
and Confederations
This section covers route dampening, which is a method to reduce the network instability that
results from oscillating routes. Also, peer groups are covered, which apply a common set of
policies to a group of BGP peers. Route reflectors and confederations are covered, which reduce
the amount of BGP neighbor meshing that is required in iBGP networks.