Cisco NCS 5500 Series Configuration Manual page 141

Implementing BGP
The weight is entirely local to the router, and can be set with the weight command or using a routing
Note
policy.
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
Note
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.
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.
Note When calculating the length of the AS path, confederation segments are ignored, and AS sets count as 1.
eiBGP specifies internal and external BGP multipath peers. eiBGP allows simultaneous use of internal
Note
and external paths.
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
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.
BGP Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.2.x
Information about Implementing BGP
131