Cost Community Support For Aggregate Routes And Multipaths - Cisco NCS 6000 Series Configuration Manual

Implementing BGP
If neither pointer refers to a cost community,
Note Paths that are not configured with the cost community attribute are considered by the best-path selection
process to have the default cost value (half of the maximum value [4294967295] or 2147483647).
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
Note The cost community comparison in BGP is enabled by default. Use the bgp bestpath cost-community ignore
command to disable the comparison.
SeeBGP Best Path Algorithm, on page 25

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
Declare a tie;
Elseif a cost community is found for one path but not for the other,
Choose the path with cost community as best path;
Elseif the Community ID from one path is less than the other,
Choose the path with the lesser Community ID as best path;
Elseif the Cost from one path is less than the other,
Choose the path with the lesser Cost as best path;
Else Continue.
• POI=IGP
• cost community ID=1
• cost number=100
• POI=IGP
• cost community ID=1
• cost number=200
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
Cost Community Support for Aggregate Routes and Multipaths
24.
for information on the BGP best-path selection process.
23