[Idr] RFC 4893 - Aggregation and mismatching AS paths

Neil Matthew <neil@abelon.com> Mon, 31 March 2008 13:32 UTC

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Date: Mon, 31 Mar 2008 14:32:36 +0100
From: Neil Matthew <neil@abelon.com>
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Subject: [Idr] RFC 4893 - Aggregation and mismatching AS paths
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Hi Folks

I'm seeking clarification on a couple of points.

1. Aggregation (should an AS_SET contain multiple AS_TRANS entries)?
When performing route aggregation a four-octet peer may create an AS_SET 
with multiple non-mappable four-octet ASes.  When calculating the number 
of ASes in a path an AS_SET counts as one regardless of how many ASes it 
contains.  Am I therefore correct in thinking that it is an 
implementation choice as to whether a four-octet peer advertising to a 
two-octet peer creates an AS_SET with a single AS_TRANS entry 
representing all non-mappable AS numbers or multiple AS_TRANS entries, 
one per non-mappable AS?

2. Non Matching AS_PATH and AS4_PATH Attributes
I wonder whether RFC 4893 defines strictly enough the required 
relationship between AS_PATH and AS4_PATH. The RFC stipulates that the 
number of AS numbers in an AS4_PATH must not exceed that of the 
AS_PATH.  However this does not rule out the AS4_PATH containing more 
segments (and by implication differing segment sizes).  If this were to 
occur it would suggest something bad has happened. What should an 
implementation do in such a situation? Accept the AS4_PATH or raise an 



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