I do not believe it is a GRE tunnel.

I have seen (Charter) and heard quite a few run RIP or some other routing protocol on the CPE. 

Though I have not seen anything specific about Comcast specifically.


Brandon Jackson

On Thu, May 7, 2020, 16:54 Brandon Martin <lists.nanog@monmotha.net> wrote:
On 5/7/20 4:49 PM, Javier Gutierrez Guerra wrote:
> Just wanted to reach out and get an idea how is people managing customers with static Ips, more specifically on Docsis networks where the customer could be moved between cmts's when a node is split

Around here, Comcast seems to provision a GRE tunnel from the CPE back
to some router within their network and run it over whatever IP address
the CMTS hands out to the modem.  Not very efficient, and it mandates
that you use their CPE (they won't give you the necessary info to set it
up yourself).

AFAIK, such service is only available on their "business class" DOCSIS
product and is upcharged even then.
--
Brandon Martin