The order of comparison determined by Part 2 is important because the comparison operation is not
transitive; that is, if three paths, A, B, and C exist, such that when A and B are compared, A is better, and
when B and C are compared, B is better, it is not necessarily the case that when A and C are compared,
A is better. This nontransitivity arises because the multi exit discriminator (MED) is compared only among
paths from the same neighboring autonomous system (AS) and not among all paths.
Note
Comparing Pairs of Paths
Perform the following steps to compare two paths and determine the better path:
1
If either path is invalid (for example, a path has the maximum possible MED value or it has an unreachable
next hop), then the other path is chosen (provided that the path is valid).
2
If the paths have unequal pre-bestpath cost communities, the path with the lower pre-bestpath cost
community is selected as the best path.
3
If the paths have unequal weights, the path with the highest weight is chosen.
The weight is entirely local to the router, and can be set with the weight command or using a routing
policy.
Note
4
If the paths have unequal local preferences, the path with the higher local preference is chosen.
If a local preference attribute was received with the path or was set by a routing policy, then that value is
used in this comparison. Otherwise, the default local preference value of 100 is used. The default value
can be changed using the bgp default local-preference command.
Note
5
If one of the paths is a redistributed path, which results from a redistribute or network command, then
it is chosen. Otherwise, if one of the paths is a locally generated aggregate, which results from an
aggregate-address command, it is chosen.
Step 1 through Step 4 implement the “Path Selection with BGP”of RFC 1268.
Note
6
If the paths have unequal AS path lengths, the path with the shorter AS path is chosen. This step is skipped
if bgp bestpath as-path ignore command is configured.
When calculating the length of the AS path, confederation segments are ignored, and AS sets count as 1.Note
eiBGP specifies internal and external BGP multipath peers. eiBGP allows simultaneous use of internal
and external paths.
Note
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
40
Implementing BGP
BGP Best Path Algorithm
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