Hello
I am not quite sure of this question but I am assume its in relation to the differences of the as-path sequencing between a normal bgp route advertisement and summarization?
As you may be aware when a prefix passes through different bgp ASN domains the ASN of each domain is registered in the AS-path of the prefix which is seen as as-path sequence numbers in the bgp route table, The sequence number of the route increases as it goes through the bgp network so as-path of the route lengthens.
Now If this prefix then becomes part of an aggregation route, then when that aggregation route is advertised its possible the original prefix as-path history isn't advertised as such it has no record from what ASN it originated, which could in turn cause a loop as that aggregated route could then be re-advertised back into the same ASN from where originated in addition to that also the overall as-path length has now been shortened due to this removal.
However if you append the as-set option command to the aggregated route it will add an as-set number to the as-path which will have the details of the previous asn's included in it thus assisting in preventing any future loop.
If I remember there is a caveat to this and that is the whole as-set number no mater how may asn;s it has included will only be counted as just one as-path value, meaning if you summarise the same route from two aggregation points (with/without as-set option) then the aggregation route without the as-set option would most probably be the preferred route due to it shorter as-path length but it will have a higher risk of introducing a loop.
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Kind Regards
Paul
