Specifications

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5-4
Cisco Nexus 3000 Series NX-OS Unicast Routing Configuration Guide, Release 5.0(3)U2(2)
OL-25782-02
Chapter 5 Configuring Basic BGP
Information About Basic BGP
BGP Router Identifier
To establish BGP sessions between peers, BGP must have a router ID, which is sent to BGP peers in the
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The best-path algorithm runs each time that a path is added or withdrawn for a given network.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:
Step 1 Compares two paths to determine which is better (see the “Step 1—Comparing Pairs of Paths” section
on page 5-5).
Step 2 Iterates over all paths and determines in which order to compare the paths to select the overall best path
(see the “Step 2—Determining the Order of Comparisons” section on page 5-6).
Step 3 Determines whether the old and new best paths differ enough so that the new best path should be used
(see the “Step 3—Determining the Best-Path Change Suppression” section on page 5-6).
Note The order of comparison determined in Part 2 is important. Consider the case where you have three
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 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.