6
PE2 runs the enhanced BGP best-path calculation.
7
PE2 installs the best path in the RIB passing the appropriate cost community value.
8
PE2 RIB has two paths for 10.1.1.0/24: one with cost community cost2 added by EIGRP and another with
the cost community cost1 added by BGP. Because both the route paths have cost community, RIB compares
the costs first. The BGP path has the lower cost community, so it is selected and downloaded to the RIB.
9
PE2 RIB redistributes the BGP path into EIGRP with VRF. EIGRP runs a diffusing update algorithm
(DUAL) because there are two paths, and selects the BGP-redistributed path.
10
PE2 EIGRP advertises the path to CE2 making the path the next hop for the prefix to send the traffic over
the MPLS network.
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, on page 38 for information on
the 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
path.
• Part 3—Determines whether the old and new best paths differ enough so that the new best path should
be used.
The order of comparison determined by Part 2 is important because the comparison operation is not
transitive; that is, if three paths, A, B, and C exist, such that when A and B are compared, A is better, and
when B and C are compared, B is better, it is not necessarily the case that when A and C are compared,
A is better. This nontransitivity arises because the multi exit discriminator (MED) is compared only among
paths from the same neighboring autonomous system (AS) and not among all paths.
Note
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
38 OL-30423-03
Implementing BGP
BGP Best Path Algorithm
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