Comcast route server not reflecting their reality

Wed Sep 11 16:44:46 UTC 2019

On Wed, Sep 11, 2019 at 12:29:58PM -0400, Ross Tajvar wrote:
> 
> I'm curious how this works and why it's done this way. I have a friend who
> has transit from AS7922 (the remote AS in his BGP sessions are all 7922),

Are you sure your friend has transit from as7922?  It sounds like your friend is
buying from Comcast Enterprise (Ethernet Dedicated Internet), which places him on
the regional CRAN network, not the 7922 national backbone (ibone).

> but CenturyLink's looking glass shows an AS path of "33657 <his ASN>" and
> includes the no-export community. Why is the AS path rewritten? What is the
> design here?

IIRC, Comcast uses "local-as 7922" path rewriting for all enterprise EDI customers 
running BGP off of regional CRANs, unless I'm mistaken.  So if your regional CRAN
is as7015 (for New England), even if your physical circuit and BGP session land
on a local SUR in your area, you have to use remote-as 7922 to setup the session
with Comcast ENS, regardless of the fact that actual peer's router is using as7015.

What Level 3/CTL looking glass is showing ("33657 <his ASN>") is most likely the
true path reflecting of actual network topology behind the scenes.  Your friend
is connected to 33567/CRAN, not 7922/ibone.

James