Re: Third Party VoIP Over Xfinity

[Robert DeVita <radevita () mejeticks com>]

Just push these end users to use softphones. There are very few use cases where a hard phone is necessary.

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From: NANOG <nanog-bounces+radevita=mejeticks.com () nanog org> on behalf of Al Whaley <awnanog () sunnyside com>
Sent: Wednesday, September 11, 2024 6:50 PM
To: nanog () nanog org <nanog () nanog org>
Subject: Re: Third Party VoIP Over Xfinity

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My recent experience with Comcast Business is that it does not permit fixed-IPs with either bridged mode or customer 
modems.

Also they have a 'feature' called SecurityEdge which is now on by default (didn't used to be).  It blocks all sorts of 
things but currently it is possible to go to the account website and turn it back off.  It's pretty evil for anyone 
that wants to, e.g., run a DNS server (evil = not possible due to a number of things including caching things it 
shouldn't etc.).  I don't know about SIP as I have not attempted to run SIP over that particular connection, but I 
wonder if it might interfere.

On 9/11/2024 16:06, Tim Burke wrote:

They do not do CGNAT, as far as I know. (Fixed wired broadband should never be behind CGNAT, but that’s a topic for 
another thread :-) )

Should only apply if you are using Comcast’s CPE for NAT. I had Comcast service some time ago, using their rental CPE 
in bridged mode, and had no issues with third party SIP.

Sent from my iPhone



On Sep 11, 2024, at 17:44, Brandon Ambrose <bpambrose97 () gmail com><mailto:bpambrose97 () gmail com> wrote:


Does this apply only to customers using their proprietary gateway or customers on a CGNAT service with them?

I have been a Comcast customer for years, with my own equipment, and have never had issues using Polycom SIP phones on 
RingCentral.

This is getting a little concerning for me as we are planning to roll out SIP phones direct to users at home soon.

Brandon Ambrose
Network Administrator


On Sep 11, 2024 at 6:15 PM -0400, Matt Hoppes <mattlists () rivervalleyinternet net><mailto:mattlists () 
rivervalleyinternet net>, wrote:
I have not, but we've run these phones with SIP ALG devices before
without issue. I'll have them check.



On 9/10/24 9:17 PM, Tim Burke wrote:
Have you tried placing the CPE in “bridged" mode? It’s been a while since I’ve done anything with Comcast CPE, but I 
remember their CPE doing SIP ALG when acting as a router.



On Sep 10, 2024, at 2:17 PM, Matt Hoppes <mattlists () rivervalleyinternet net><mailto:mattlists () rivervalleyinternet 
net> wrote:

I have an employee who has recently switched to Xfinity cable service. Ever since they switched their internet service 
their work phones will not stay registered for more than about 3 minutes.

These same phones have been used on many ISPs without issues. The same config has been used behind multiple levels of 
NAT without issues.

She was fine, until she switched to XFinity.

Of course, XFinity support is absolutely worthless.

Anyone from XFinity Tier 3 or such that might be able to offer assistance?

I suspect it's something stupid with either NAT overload in the modem or the modem not keeping the SIP channels open.

I've tried playing around with registration times without any success. And again, we've never had issues with these 
phones or this setup with any other ISP.