[86164] in North American Network Operators' Group
Re: prepending 2 bytes of zeros....
daemon@ATHENA.MIT.EDU (Geoff Huston)
Mon Oct 24 15:02:57 2005
Date: Tue, 25 Oct 2005 05:02:21 +1000
To: bmanning@vacation.karoshi.com, nanog@merit.edu
From: Geoff Huston <gih@apnic.net>
In-Reply-To: <20051024174645.GA18411@vacation.karoshi.com.>
Errors-To: owner-nanog@merit.edu
At 03:46 AM 25/10/2005, bmanning@vacation.karoshi.com wrote:
>I am greatful to Geoff for his consistant ability to get me interested in
>breaking things... so, for the assembled mutlitude, what would the impact
>on various peers be if I was to change my orign AS (ok, so i'll have to
>change the router code on my end to support this) from
>
> 4554
>
> to
>
> 00004554
>
>Any ideas on how IOS (various flavors) will deal w/ this? (yes, there is
>some lab work to do first, but i don' think there is a comprehensive enough
>lab to cover the full range of possibilities...)
So lets say you are running a BGP version that supports 4-Byte AS code, and
you local AS is 4554
When you open a session with your BGP peer (using in this case 4554 in the
2-byte My AS field of the OPEN packet) you will send a BGP capacility
advertisement to your peer, indicating your willingness to exchange 4-Byte
AS numbers with your peer.
At this stage IOS (various flavours) do not (to my limited knowledge)
support this capability, so you will not get a positive response to your
capability announcement, so you are now on a NEW / OLD transition boundary.
Your BGP will now operate in this NEW / OLD transition mode. The behaviour
now depends on whether you are an isolated 0:4554 island, or whether there
are any 4-Byte AS's that directly peer with your AS.
In the former case you will strip off the leading zero 2-Byte fields from
your BGP Updates when you pass them to your BGP peer
The the latter case the action will vary, depending on whether the 4-Byte
AS's in the path are all zero-strippable or not. In the latter case the
UPDATES will cause a NEW_AS_PATH attribute to be added to the UPDATES you
generate. in the former case its zero stripping.
So does this answer your question?
Geoff
