Route Server Filters at IXPs and 4-byte ASNs

Sebastian Spies s+Mailinglisten.nanog at
Sat Jan 25 15:48:32 UTC 2014

Am 25.01.2014 16:38, schrieb Bryan Socha:
> Re-reading, I was thinking of someone connecting to an IXP, not a new
> IXP needing a 2Byte.    This is an interesting situation and you are
> correct, my comment was off topic.

Sorry for not mentioning the beef: Extended Communities effectively
leave 6 bytes to the user. So, it is not possible, that a 4-byte-ASN IXP
encodes its own 4-byte-ASN and a 4-byte-ASN customer in one extended
community string. This is needed, as the IXP RS usually interpret their
own ASN and a peer ASN as: "Send this prefix out to the peer ASN".

To make things worse: even if the IXPs ASN is 2-byte, I would assume,
that RS implementors chose to interpret extended community strings as
always being in the format 4-byte:2-byte (see RFC5668).

Best regards,

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