Specifications

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5-2
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 selects a single path, by default, as the best path to a destination host or network. Each path carries
well-known mandatory, well-known discretionary, and optional transitive attributes that are used in BGP
best-path analysis. You can influence BGP path selection by altering some of these attributes by
configuring BGP policies. See the “Route Policies and Resetting BGP Sessions” section on page 6-3 for
more information.
BGP also supports load balancing or equal-cost multipath (ECMP). See the “Load Sharing and
Multipath” section on page 6-6 for more information.
This section includes the following topics:
• BGP Autonomous Systems, page 5-2
• Administrative Distance, page 5-2
• BGP Peers, page 5-3
• BGP Router Identifier, page 5-4
• BGP Path Selection, page 5-4
• BGP and the Unicast RIB, page 5-7
• BGP Virtualization, page 5-7
BGP Autonomous Systems
An autonomous system (AS) is a network controlled by a single administration entity. An autonomous
system forms a routing domain with one or more interior gateway protocols (IGPs) and a consistent set
of routing policies. BGP supports 16-bit and 32-bit autonomous system numbers. For more information,
see the “Autonomous Systems” section on page 1-5.
Separate BGP autonomous systems dynamically exchange routing information through external BGP
(eBGP) peering sessions. BGP speakers within the same autonomous system can exchange routing
information through internal BGP (iBGP) peering sessions.
4-Byte AS Number Support
BGP supports 2-byte or 4-byte AS numbers. Cisco NX-OS displays 4-byte AS numbers in plain-text
notation (that is, as 32-bit integers). You can configure 4-byte AS numbers as either plain-text notation
(for example, 1 to 4294967295), or AS.dot notation (for example, 1.0). For more information, see the
“Autonomous Systems” section on page 1-5.
Administrative Distance
An administrative distance is a rating of the trustworthiness of a routing information source. By default,
BGP uses the administrative distances shown in Table 5-1.
Table 5-1 BGP Default Administrative Distances
Distance Default Value Function
External 20 Applied to routes learned from eBGP.
Internal 200 Applied to routes learned from iBGP.
Local 200 Applied to routes originated by the router.