Adding Routes To The Routing Information Base - Cisco NCS 6000 Series Configuration Manual

Implementing BGP
Both paths are considered to be equal by BGP. If multipath loadsharing is configured, both paths to the routing
table are installed and are used to balance the load of traffic. If multipath load balancing is not configured,
the BGP selects the path that was learned first as the best path and installs this path to the routing table. This
behavior may not be desirable under some conditions. For example, the path is learned from ISP1 PE2 first,
but the link between ISP1 PE2 and ASBR1 is a low-speed link.
The configuration of the cost community attribute can be used to influence the BGP best-path selection process
by applying a lower-cost community value to the path learned by ASBR2. For example, the following
configuration is applied to ASBR2:
RP/0/RP0/CPU0:router(config)# route-policy ISP2_PE1
RP/0/RP0/CPU0:router(config-rpl)# set extcommunity cost (1:1)
The preceding route policy applies a cost community number of 1 to the 10.8.0.0 route. By default, the path
learned from ASBR1 is assigned a cost community number of 2147483647. Because the path learned from
ASBR2 has a lower-cost community number, the path is preferred.

Adding Routes to the Routing Information Base

If a nonsourced path becomes the best path after the best-path calculation, BGP adds the route to the Routing
Information Base (RIB) and passes the cost communities along with the other IGP extended communities.
When a route with paths is added to the RIB by a protocol, RIB checks the current best paths for the route
and the added paths for cost extended communities. If cost-extended communities are found, the RIB compares
the set of cost communities. If the comparison does not result in a tie, the appropriate best path is chosen. If
the comparison results in a tie, the RIB proceeds with the remaining steps of the best-path algorithm. If a cost
community is not present in either the current best paths or added paths, then the RIB continues with the
remaining steps of the best-path algorithm. See
BGP best-path algorithm.
BGP Best Path Algorithm
BGP routers typically receive multiple paths to the same destination. The BGP best-path algorithm determines
the best path to install in the IP routing table and to use for forwarding traffic. This section describes the Cisco
IOS XR software implementation of BGP best-path algorithm, as specified in Section 9.1 of the Internet
Engineering Task Force (IETF) Network Working Group draft-ietf-idr-bgp4-24.txt document.
The BGP best-path algorithm implementation is in three parts:
• Part 1—Compares two paths to determine which is better.
• Part 2—Iterates over all paths and determines which order to compare the paths to select the overall best
• Part 3—Determines whether the old and new best paths differ enough so that the new best path should
path.
be used.
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
Adding Routes to the Routing Information Base
BGP Best Path Algorithm, on page 25 for information on the
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