BGP 4 Byte AS

Standard AS are 16 bits (2 bytes long). The new ASs are 4 bytes. They are represented in either these two formats:

ASDOT – .
                 eg: 1.10 = AS65546 or 0.1221 = AS1221

ASPLAIN –
                    eg AS70000

BGP Peers negotiate with each other (using capability codes) if they support 4 byte AS. If a peer does not support the feature then it will see the new AS numbers as AS23456.

For example:

84.205.64.0/24 – 23456  18508 19151 1103 1125 23456  12654  (2 byte view)
                           393222 18508 19151 1103 1125 196613 12654 (4 byte view)

This means that the AS path length will always be the same.  

A side effect from the compatibility between OLD and NEW BGP speakers is that the usage of “23456” as the local BGP AS number is not possible. If an OLD BGP speaker would be configured to belong to AS23456, then this OLD BGP speaker will see other instances of AS23456 in the AS path and due to AS-Path loop detection drop the BGP routes. This would result in an incomplete view from the OLD BGP speaker perspective.

An additional side effect could be if the NEW BGP speaker belongs AS100.100 and the OLD BGP speaker to AS200. If the OLD BGP Speaker was wrongly configured as belonging to AS23456, then instead of an eBGP peering, an iBGP peering would be established. This would result in the wrong BGP behavior.

Netflow v9 supports 4 byte AS number reporting. IF you are using an older version of netflow then expect to see lots of AS23456.

 4 Byte AS is enabled by default.

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