Bgp Best Path Algorithm; Comparing Pairs Of Paths - Cisco NCS 5500 Series Configuration Manual

Information about Implementing BGP
Table 5: Restricted BGP Operations by Attach Point
Command
prepend as-path
most-recent
replace as-path
set med igp-cost
set weight
suppress

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.
Note 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.

Comparing Pairs of Paths

Perform the following steps to compare two paths and determine the better path:
1 If either path is invalid (for example, a path has the maximum possible MED value or it has an unreachable
next hop), then the other path is chosen (provided that the path is valid).
2 If the paths have unequal pre-bestpath cost communities, the path with the lower pre-bestpath cost
community is selected as the best path.
3 If the paths have unequal weights, the path with the highest weight is chosen.
BGP Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.2.x
130
import export
eBGP only eBGP only
eBGP only eBGP only
forbidden eBGP only
n/a forbidden
forbidden forbidden
Implementing BGP
aggregation redistribution
n/a n/a
n/a n/a
forbidden forbidden
n/a n/a
n/a forbidden