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Implementing BGP on Cisco IOS XR Software
Information About Implementing BGP on Cisco IOS XR Software
RC-24
Cisco IOS XR Routing Configuration Guide
OL-14356-01
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 in the “Influencing Route Preference in a Multiexit IGP
Network” section on page RC-25.
Note The cost community comparison in BGP is enabled by default. Use the bgp bestpath cost-community
ignore command to disable the comparison.
See the “BGP Best Path Algorithm” section on page RC-27 for information on the BGP best-path
selection process.
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
–
POI=IGP
–
cost community ID=1
–
cost number=100
• 192.168.0.1
–
POI=IGP
–
cost community ID=1
–
cost number=200
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
–
POI=IGP
–
cost community ID=1
–
cost number=100
• 172.16.0.1
–
POI=IGP
–
cost community ID=2
–
cost number=100