Reference Guide

170 | Border Gateway Protocol
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4-Byte AS Numbers
FTOS Version 7.7.1 and later support 4-Byte (32-bit) format when configuring Autonomous System
Numbers (ASNs). The 4-Byte support is advertised as a new BGP capability (4-BYTE-AS) in the OPEN
message. If a 4-Byte BGP speaker has sent and received this capability from another speaker, all the
messages will be 4-octet. The behavior of a 4-Byte BGP speaker will be different with the peer depending
on whether the peer is 4-Byte or 2-Byte BGP speaker.
Where the 2-Byte format is 1-65535, the 4-Byte format is 1-
4294967295. Enter AS Numbers using the
traditional format. If the ASN is greater than 65535, the dot format is shown when using the
show ip bgp
commands. For example, an ASN entered as 3183856184 will appear in the show commands as
48581.51768; an ASN of 65123 is shown as 65123. To calculate the comparable dot format for an ASN
from a traditional format, use ASN/65536. ASN%65536.
When creating Confederations, all the routers in a Confederation must be either 4-Byte or 2-Byte identified
routers. You cannot mix them.
Configure the 4-byte AS numbers with the
four-octet-support command.
AS4 Number Representation
FTOS version 8.2.1.0 supports multiple representations of an 4-byte AS Numbers: asplain, asdot+, and
asdot.
ASPLAIN is the method FTOS has used for all previous FTOS versions.It remains the default method with
FTOS 8.2.1.0 and later. With the ASPLAIN notation, a 32 bit binary AS number is translated into a
decimal value.
• All AS Numbers between 0-65535 are represented as a decimal number when entered in the CLI as
well as when displayed in the show command outputs.
• AS Numbers larger than 65535 are represented using ASPLAIN notation as well.
65546 is
represented as 65546.
Table 8-2. 4-Byte ASN Dot Format Examples
Traditional Format Dot Format
65001 Is 0.65501
65536
The
1.0
100000 Same As 1.34464
4294967295 65535.65535
Note: The ASDOT and ASDOT+ representations are supported only in conjunction with the 4-Byte AS
Numbers feature. If 4-Byte AS Numbers are not implemented, only ASPLAIN representation is supported.