Bgp Cost Community - Cisco NCS 6000 Series Configuration Manual

BGP Cost Community

For the best optimization of BGP update group generation, we recommend that the network operator keeps
outbound routing policy the same for neighbors that have similar outbound policies. This feature contains
commands for monitoring BGP update groups.
BGP Cost Community
The BGP cost community is a nontransitive extended community attribute that is passed to internal BGP
(iBGP) and confederation peers but not to external BGP (eBGP) peers. The cost community feature allows
you to customize the local route preference and influence the best-path selection process by assigning cost
values to specific routes. The extended community format defines generic points of insertion (POI) that
influence the best-path decision at different points in the best-path algorithm.
The cost community attribute is applied to internal routes by configuring the set extcommunity cost command
in a route policy. See the Routing Policy Language Commands on Cisco IOS XR Software module of Cisco
IOS XR Routing Command Reference for information on the set extcommunity cost command. The cost
community set clause is configured with a cost community ID number (0–255) and cost community number
(0–4294967295). The cost community number determines the preference for the path. The path with the lowest
cost community number is preferred. Paths that are not specifically configured with the cost community
number are assigned a default cost community number of 2147483647 (the midpoint between 0 and
4294967295) and evaluated by the best-path selection process accordingly. When two paths have been
configured with the same cost community number, the path selection process prefers the path with the lowest
cost community ID. The cost-extended community attribute is propagated to iBGP peers when extended
community exchange is enabled.
The following commands include the route-policy keyword, which you can use to apply a route policy that
is configured with the cost community set clause:
• aggregate-address
• redistribute
• network
How BGP Cost Community Influences the Best Path Selection Process
The cost community attribute influences the BGP best-path selection process at the point of insertion (POI).
By default, the POI follows the Interior Gateway Protocol (IGP) metric comparison. When BGP receives
multiple paths to the same destination, it uses the best-path selection process to determine which path is the
best path. BGP automatically makes the decision and installs the best path in the routing table. The POI allows
you to assign a preference to a specific path when multiple equal cost paths are available. If the POI is not
valid for local best-path selection, the cost community attribute is silently ignored.
Cost communities are sorted first by POI then by community ID. Multiple paths can be configured with the
cost community attribute for the same POI. The path with the lowest cost community ID is considered first.
In other words, all cost community paths for a specific POI are considered, starting with the one with the
lowest cost community. Paths that do not contain the cost community cost (for the POI and community ID
being evaluated) are assigned the default community cost value (2147483647). If the cost community values
are equal, then cost community comparison proceeds to the next lowest community ID for this POI.
To select the path with the lower cost community, simultaneously walk through the cost communities of both
paths. This is done by maintaining two pointers to the cost community chain, one for each path, and advancing
both pointers to the next applicable cost community at each step of the walk for the given POI, in order of
community ID, and stop when a best path is chosen or the comparison is a tie. At each step of the walk, the
following checks are done:
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
22
Implementing BGP