Cost Community Support For Aggregate Routes And Multipaths - Cisco ASR 9000 Series Routing Configuration Manual

BGP Cost Community
Applying the cost community attribute at the POI allows you to assign a value to a path originated or learned
by a peer in any part of the local autonomous system or confederation. The cost community can be used as a
"tie breaker" during the best-path selection process. Multiple instances of the cost community can be configured
for separate equal cost paths within the same autonomous system or confederation. For example, a lower cost
community value can be applied to a specific exit path in a network with multiple equal cost exit points, and
the specific exit path is preferred by the BGP best-path selection process. See the scenario described
inInfluencing Route Preference in a Multiexit IGP Network, on page
The cost community comparison in BGP is enabled by default. Use the bgp bestpath cost-community
Note
ignore command to disable the comparison.
SeeBGP Best Path Algorithm, on page 39

Cost Community Support for Aggregate Routes and Multipaths

The BGP cost community feature supports aggregate routes and multipaths. The cost community attribute
can be applied to either type of route. The cost community attribute is passed to the aggregate or multipath
route from component routes that carry the cost community attribute. Only unique IDs are passed, and only
the highest cost of any individual component route is applied to the aggregate for each ID. If multiple component
routes contain the same ID, the highest configured cost is applied to the route. For example, the following
two component routes are configured with the cost community attribute using an inbound route policy:
• 10.0.0.1
• 192.168.0.1
If these component routes are aggregated or configured as a multipath, the cost value 200 is advertised,
because it has the highest cost.
If one or more component routes do not carry the cost community attribute or the component routes are
configured with different IDs, then the default value (2147483647) is advertised for the aggregate or
multipath route. For example, the following three component routes are configured with the cost
community attribute using an inbound route policy. However, the component routes are configured with
two different IDs.
• 10.0.0.1
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
34
◦ POI=IGP
◦ cost community ID=1
◦ cost number=100
◦ POI=IGP
◦ cost community ID=1
◦ cost number=200
◦ POI=IGP
◦ cost community ID=1
◦ cost number=100
35.
for information on the BGP best-path selection process.
Implementing BGP