Cisco ASR 9000 Series Routing Configuration Manual page 71

Implementing BGP
7 If the paths have different origins, the path with the lower origin is selected. Interior Gateway Protocol
(IGP) is considered lower than EGP, which is considered lower than INCOMPLETE.
8 If appropriate, the MED of the paths is compared. If they are unequal, the path with the lower MED is
chosen.
A number of configuration options exist that affect whether or not this step is performed. In general, the
MED is compared if both paths were received from neighbors in the same AS; otherwise the MED
comparison is skipped. However, this behavior is modified by certain configuration options, and there are
also some corner cases to consider.
If the bgp bestpath med always command is configured, then the MED comparison is always performed,
regardless of neighbor AS in the paths. Otherwise, MED comparison depends on the AS paths of the two
paths being compared, as follows:
• If a path has no AS path or the AS path starts with an AS_SET, then the path is considered to be
• If the AS path starts with an AS_SEQUENCE, then the neighbor AS is the first AS number in the
• If the AS path contains only confederation segments or starts with confederation segments followed
• If the AS path starts with confederation segments followed by an AS_SEQUENCE, then the neighbor
If no MED attribute was received with the path, then the MED is considered to be 0 unless the bgp bestpath
Note
med missing-as-worst command is configured. In that case, if no MED attribute was received, the MED
is considered to be the highest possible value.
9 If one path is received from an external peer and the other is received from an internal (or confederation)
peer, the path from the external peer is chosen.
10 If the paths have different IGP metrics to their next hops, the path with the lower IGP metric is chosen.
11 If the paths have unequal IP cost communities, the path with the lower IP cost community is selected as
the best path.
12 If all path parameters in Step 1 through Step 10 are the same, then the router IDs are compared. If the path
was received with an originator attribute, then that is used as the router ID to compare; otherwise, the
router ID of the neighbor from which the path was received is used. If the paths have different router IDs,
the path with the lower router ID is chosen.
Where the originator is used as the router ID, it is possible to have two paths with the same router ID. It
Note
is also possible to have two BGP sessions with the same peer router, and therefore receive two paths with
the same router ID.
13 If the paths have different cluster lengths, the path with the shorter cluster length is selected. If a path was
not received with a cluster list attribute, it is considered to have a cluster length of 0.
internal, and the MED is compared with other internal paths.
sequence, and the MED is compared with other paths that have the same neighbor AS.
by an AS_SET, then the MED is not compared with any other path unless the bgp bestpath med
confed command is configured. In that case, the path is considered internal and the MED is compared
with other internal paths.
AS is the first AS number in the AS_SEQUENCE, and the MED is compared with other paths that
have the same neighbor AS.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
BGP Best Path Algorithm
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