Bgp Autonomous Systems; 4-Byte As Number Support; Administrative Distance; Chapter 5 Configuring Basic Bgp - Cisco Nexus 3000 Series Configuration Manual
Nx-os unicast routing configuration guide,
nx-os release 5.0(3)u1(1)
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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 3 k - d o c f e e d b a c k @ c i s c o . c o m
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
more information.
BGP also supports load balancing or equal-cost multipath (ECMP). See the
Multipath" section on page 6-6
This section includes the following topics:
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•
•
•
•
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BGP Autonomous Systems
An
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
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
Administrative Distance
An
administrative distance
BGP uses the administrative distances shown in
Table 5-1
Distance
External
Internal
Local
Cisco Nexus 3000 Series NX-OS Unicast Routing Configuration Guide, NX-OS Release 5.0(3)U1(1)
5-2
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
(AS) is a network controlled by a single administration entity. An autonomous
autonomous system
"Autonomous Systems" section on page
is a rating of the trustworthiness of a routing information source. By default,
BGP Default Administrative Distances
Default Value
20
200
200
"Route Policies and Resetting BGP Sessions" section on page 6-3
for more information.
1-5.
1-5.
Table
5-1.
Function
Applied to routes learned from eBGP.
Applied to routes learned from iBGP.
Applied to routes originated by the router.
Chapter 5
Configuring Basic BGP
"Load Sharing and
for
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