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Re: [Idr] 4byte ASN support in RFC4273

"Thomas E. Spanjaard" <tgen@netphreax.net> Tue, 22 August 2006 10:23 UTC

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Date: Tue, 22 Aug 2006 10:23:19 +0000
From: "Thomas E. Spanjaard" <tgen@netphreax.net>
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To: Henk Uijterwaal <henk@ripe.net>
Subject: Re: [Idr] 4byte ASN support in RFC4273
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Henk Uijterwaal wrote:
> RFC4273 says:
>    EAch AS is represented as a pair of octets according to the following
>    algorithm:
>      first-byte-of-pair = ASNumber / 256
>      second-byte-of-pair = ASNumber & 255;
> And similar for bgp4PathAttrASPathSegment.
> Question: what will happen when 4byte ASN are deployed?  Can I just
> replace the Octet string by 4 octets (ASN/(265^3), ASN/(256^2), ASN/256,
> ASN%255) (and assume that things will continue to work)?   Or does this
> need an update of the RFC?

I think draft-ietf-idr-as4bytes-12.txt states that the ASPath should 
only be interpreted as 4-octet AS numbers when your peer advertises the 
4byte AS capability and the other peer does so as well. Otherwise, it 
should interpret the AS_PATH as 2-octet AS numbers.

> The same RFC says:
>   bgpPeerRemoteAs OBJECT-TYPE
>     SYNTAX  Integer32 (0...65535)

The 4-octet AS number of the NEW BGP peer is 'advertised' as the 
Capability Value of the Capability Optional Parameter of the Four-octet 
AS Number Capability, sized 4 octets (or if you will, an unsigned 32bits 
integer). The 2-octet AS to use by 4-octet AS peers is AS_TRANS, which 
is defined by IANA.

Cheers,
-- 
         Thomas E. Spanjaard
         tgen@netphreax.net

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