Cisco NCS 6000 Series Configuration Manual page 49

Implementing BGP
If the paths have different origins, the path with the lower origin is selected. Interior Gateway Protocol
7.
(IGP) is considered lower than EGP, which is considered lower than INCOMPLETE.
If appropriate, the MED of the paths is compared. If they are unequal, the path with the lower MED is
8.
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:
Note
If no MED attribute was received with the path, then the MED is considered to be 0 unless the bgp bestpath
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.
If the paths have unequal IP cost communities, the path with the lower IP cost community is selected
11.
as the best path.
If all path parameters in Step 1 through Step 10 are the same, then the router IDs are compared. If the
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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.
Note
Where the originator is used as the router ID, it is possible to have two paths with the same router ID. It is
also possible to have two BGP sessions with the same peer router, and therefore receive two paths with the
same router ID.
• If a path has no AS path or the AS path starts with an AS_SET, then the path is considered to be
internal, and the MED is compared with other internal paths.
• If the AS path starts with an AS_SEQUENCE, then the neighbor AS is the first AS number in the
sequence, and the MED is compared with other paths that have the same neighbor AS.
• If the AS path contains only confederation segments or starts with confederation segments followed
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.
• If the AS path starts with confederation segments followed by an AS_SEQUENCE, then the
neighbor AS is the first AS number in the AS_SEQUENCE, and the MED is compared with other
paths that have the same neighbor AS.
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
Comparing Pairs of Paths
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