Specifications

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5-3
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
Note The administrative distance does not influence the BGP path selection algorithm, but it does influence
whether BGP-learned routes are installed in the IP routing table.
For more information, see the “Administrative Distance” section on page 1-7.
BGP Peers
A BGP speaker does not discover another BGP speaker automatically. You must configure the
relationships between BGP speakers. A BGP peer is a BGP speaker that has an active TCP connection
to another BGP speaker.
BGP Sessions
BGP uses TCP port 179 to create a TCP session with a peer. When a TCP connection is established
between peers, each BGP peer initially exchanges all of its routes—the complete BGP routing
table—with the other peer. After this initial exchange, the BGP peers send only incremental updates
when a topology change occurs in the network or when a routing policy change occurs. In the periods of
inactivity between these updates, peers exchange special messages called keepalives. The hold time is
the maximum time limit that can elapse between receiving consecutive BGP update or keepalive
messages.
Cisco NX-OS supports the following peer configuration options:
• Individual IPv4 or IPv4 address—BGP establishes a session with the BGP speaker that matches the
remote address and AS number.
• IPv4 prefix peers for a single AS number—BGP establishes sessions with BGP speakers that match
the prefix and the AS number.
• Dynamic AS number prefix peers—BGP establishes sessions with BGP speakers that match the
prefix and an AS number from a list of configured AS numbers.
Dynamic AS Numbers for Prefix Peers
Cisco NX-OS accepts a range or list of AS numbers to establish BGP sessions. For example, if you
configure BGP to use IPv4 prefix 192.0.2.0/8 and AS numbers 33, 66, and 99, BGP establishes a session
with 192.0.2.1 with AS number 66 but rejects a session from 192.0.2.2 with AS number 50.)
Cisco NX-OS does not associate prefix peers with dynamic AS numbers as either interior BGP (iBGP)
or external BGP (eBGP) sessions until after the session is established. See Chapter 6, “Configuring
Advanced BGP,” for more information on iBGP and eBGP.
Note The dynamic AS number prefix peer configuration overrides the individual AS number configuration
that is inherited from a BGP template. See Chapter 6, “Configuring Advanced BGP,” for more
information on templates.