Bgp Router Identifier; Bgp Path Selection; Step 1-Comparing Pairs Of Paths - Cisco Nexus 7000 Series Configuration Manual

Information About Basic BGP
S e n d d o c u m e n t c o m m e n t s t o n e x u s 7 k - d o c f e e d b a c k @ c i s c o . c o m .

BGP Router Identifier

To establish BGP sessions between peers, BGP must have a
OPEN message when a BGP session is established. The BGP router ID is a 32-bit value that is often
represented by an IPv4 address. You can configure the router ID. By default, Cisco NX-OS sets the router
ID to the IPv4 address of a loopback interface on the router. If no loopback interface is configured on
the router, then the software chooses the highest IPv4 address configured to a physical interface on the
router to represent the BGP router ID. The BGP router ID must be unique to the BGP peers in a network.
If BGP does not have a router ID, it cannot establish any peering sessions with BGP peers.

BGP Path Selection

Although BGP might receive advertisements for the same route from multiple sources, BGP selects only
one path as the best path. BGP puts the selected path in the IP routing table and propagates the path to
its peers.
The best-path algorithm runs each time that a path is added or withdrawn for a given network. The
best-path algorithm also runs if you change the BGP configuration. BGP selects the best path from the
set of valid paths available for a given network.
Cisco NX-OS implements the BGP best-path algorithm in the following steps:
Compares two paths to determine which is better (see the
Step 1
on page
Step 2 Iterates over all paths and determines in which order to compare the paths to select the overall best path
(see the
Step 3 Determines whether the old and new best paths differ enough so that the new best path should be used
(see the
The order of comparison determined in Part 2 is important. Consider the case where you have three
Note
paths, A, B, and C. When Cisco NX-OS compares A and B, it chooses A. When Cisco NX-OS compares
B and C, it chooses B. But when Cisco NX-OS compares A and C, it might not choose A because some
BGP metrics apply only among paths from the same neighboring autonomous system and not among all
paths.
The path selection uses the the BGP AS-path attribute. The AS-path attribute includes the list of
autonomous system numbers (AS numbers) traversed in the advertised path. If you subdivide your BGP
autonomous system into a collection or confederation of autonomous systems, the AS-path contains
confederation segments that list these locally defined autonomous systems.
Step 1—Comparing Pairs of Paths
This first step in the BGP best-path algorithm compares two paths to determine which path is better. The
following sequence describes the basic steps that Cisco NX-OS uses to compare two paths to determine
the better path:
1.
Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
10-4
10-4).
"Step 2—Determining the Order of Comparisons" section on page
"Step 3—Determining the Best-Path Change Suppression" section on page
Cisco NX-OS chooses a valid path for comparison. (For example, a path that has an unreachable
next-hop is not valid.)
Chapter 10 Configuring Basic BGP
router ID, which is sent to BGP peers in the
"Step 1—Comparing Pairs of Paths" section
10-6).
10-6).
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