Route Server Filters at IXPs and 4-byte ASNs

Rubens Kuhl rubensk at
Sat Jan 25 14:16:08 UTC 2014

> What happens, if the IXP uses a 4-byte ASN? RFC5668 (4-Octet AS Specific
> BGP Extended Community) defines <Global Admin,4bytes>:<Local Admin,
> 2bytes>.
> I have been asking some IXP operators, about their practice and their
> reply was "4-byte ASNs are supported by our RS". What's your experience?
> Did you see IXPs, that do not support them? Do you think, the IXP
> operators are aware of this limitation? Have you seen IXPs with 4-byte
> ASNs or do RIRs reserve 2-byte ASNs for future IXPs?  What other
> operational problems did you experience while using 4-byte ASNs?

Do you see an issue with future IXP or future IXP members ?
You can see at this list of members of one IXP that there are both 2-byte
and 4-byte ASNs living in harmony, with a large number of 4-byte ones:

The IXP itself is using a 2-byte public ASN, so if at some point
communities are used (although there are route-servers capable of granular
policy without communities), they could use standard 2-byte community

And with a bit coordination, 2-byte private ASNs and communities could be
used to overcome limitations in member routers support of 4-byte ASNs.




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==============================================> show ip bgp summary

BGP router identifier, local AS number 20121

RIB entries 32868, using 3081 KiB of memory

Peers 4, using 18 KiB of memory

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down
State/PfxRcd  4 26162 3294906 1619885        0    0    0 05w0d23h    19247  4 26162 3468468 1617924        0    0    0 01w5d06h    19227

Total number of neighbors 2

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