Autonomous System Number Formats In Bgp; 2-Byte Autonomous System Number Format; As-Format Command; Bgp Multi-Instance And Multi-As - Cisco NCS 5500 Series Configuration Manual

Autonomous System Number Formats in BGP

Autonomous System Number Formats in BGP
Autonomous system numbers (ASNs) are globally unique identifiers used to identify autonomous systems
(ASs) and enable ASs to exchange exterior routing information between neighboring ASs. A unique ASN is
allocated to each AS for use in BGP routing. ASNs are encoded as 2-byte numbers and 4-byte numbers in
BGP.

2-byte Autonomous System Number Format

The 2-byte ASNs are represented in asplain notation. The 2-byte range is 1 to 65535.
4-byte Autonomous System Number Format
To prepare for the eventual exhaustion of 2-byte Autonomous System Numbers (ASNs), BGP has the capability
to support 4-byte ASNs. The 4-byte ASNs are represented both in asplain and asdot notations.
The byte range for 4-byte ASNs in asplain notation is 1-4294967295. The AS is represented as a 4-byte
decimal number. The 4-byte ASN asplain representation is defined in draft-ietf-idr-as-representation-01.txt.
For 4-byte ASNs in asdot format, the 4-byte range is 1.0 to 65535.65535 and the format is:
high-order-16-bit-value-in-decimal . low-order-16-bit-value-in-decimal
The BGP 4-byte ASN capability is used to propagate 4-byte-based AS path information across BGP speakers
that do not support 4-byte AS numbers. See
size of an ASN from 2 bytes to 4 bytes. AS is represented as a 4-byte decimal number

as-format Command

The as-format command configures the ASN notation to asdot. The default value, if the as-format command
is not configured, is asplain.

BGP Multi-Instance and Multi-AS

Multi-AS BGP enables configuring each instance of a multi-instance BGP with a different AS number.
Multi-Instance and Multi-AS BGP provides these capabilities:
• Mechanism to consolidate the services provided by multiple routers using a common routing infrastructure
• Mechanism to achieve AF isolation by configuring the different AFs in different BGP instances.
• Means to achieve higher session scale by distributing the overall peering sessions between multiple
• Mechanism to achieve higher prefix scale (especially on a RR) by having different instances carrying
• Improved BGP convergence under certain scenarios.
• All BGP functionalities including NSR are supported for all the instances.
BGP Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.2.x
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into a single IOS-XR router.
instances.
different BGP tables.
draft-ietf-idr-as4bytes-12.txt
Implementing BGP
for information on increasing the