Reference Guide

Ignore Router-ID for Some Best-Path Calculations
FTOS version 8.3.1.0 and later allows you to avoid unnecessary BGP best-path transitions between external paths under
certain conditions. The bgp bestpath router-id ignore command reduces network disruption caused by
routing and forwarding plane changes and allows for faster convergence.
Four-Byte AS Numbers
FTOS version 7.7.1 and later supports 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 is different with the peer depending on whether the peer is a 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 appears 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.
Traditional Format DOT Format
65001 0.65501
65536 1.0
100000 1.34464
4294967295 65535.65535
When creating Confederations, all the routers in a Confederation must be either 4-Byte or 2-Byte identified routers. You
cannot mix them.
Configure 4-byte AS numbers with the four-octet-support command.
AS4 Number Representation
FTOS version 8.2.1.0 supports multiple representations of 4-byte AS numbers: asplain, asdot+, and asdot.
NOTE: The ASDOT and ASDOT+ representations are supported only with the 4-Byte AS numbers feature. If 4-Byte
AS numbers are not implemented, only ASPLAIN representation is supported.
ASPLAIN is the method FTOS has used for all previous FTOS versions. ASPLAIN remains the default method with FTOS
version 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 and 65535 are represented as a decimal number when entered in the CLI and when
displayed in the show commands output.
• AS numbers larger than 65535 are represented using ASPLAIN notation. When entered in the CLI and when
displayed in the show commands output, 65546 is represented as 65546.
ASDOT+ representation splits the full binary 4-byte AS number into two words of 16 bits separated by a decimal point (.):
<high-order 16 bit value>.<low-order 16 bit value>. Some examples are shown in the following table.
• All AS numbers between 0 and 65535 are represented as a decimal number, when entered in the CLI and when
displayed in the show commands outputs.
• AS Numbers larger than 65535 is represented using ASDOT notation as <higher 2 bytes in decimal>.<lower 2
bytes in decimal>. For example: AS 65546 is represented as 1.10.
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