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2 Messages

Fri, Jul 22, 2016 4:00 PM

Is the Cisco DPC3941B gateway configurable for use with a 3rd party VoIP?

I am a new member here in this forum. I have looked at quite a few posts to try and answer my question, but the picture is still pretty fuzzy to me. I hope you can help.



We want to setup RingCentral's Office VoIP service on our network. Can we do that?



  • We have Comcast Business package installed and configured by Comcast techs as follows:
    Internet Deluxe 25 (25 Mbps/10 Mbps) with the Cisco DPC3941B gateway
    Business Phone (2 lines) with the Arris TM502G/CT Telephony Modem
    TV Select PRIVATE (never used it, never connected the TV box)


Can I configure the DPC3941B to accomodate RingCentral's VoIP port needs?


Do I need to put the gateway into "Bridge Mode" (or "True Bridge Mode") and add a RingCentral approved router such as a NetGear AC1750?


Any other problems I hould be aware of?


Thanks for your help!




Trusted Forum Contributor


1.4K Messages

5 years ago

Hello FCIHC_DanielG and welcome,


Yes, there are many Comcast Business customers that use their dedicated VOIP Router to serve their business requirements. Configuring your Comcast Gateway DPC3941B (CGDPC) in true bridge mode will disable all your CGDPC routing functionailty including DHCP LAN, WIFI, Natting, et al. The best vehicle for this is to obtain a Comcast Static IP Address (CSIP) for your CGDPC, then program your NetGear AC1750  (NGAC) WAN interface with the CSIP. You must also make sure that all CGDPC CSIP UDP ports are opened like 5060 - 5065 and any other SIP ports embedded in your Ethernet subpackets.


Hope this helps you out. 



2 Messages

5 years ago

Thanks so much for your reply, VBSSP-RICH.


I have a few follow-up questions. Sorry for the length of the post, however I think these questions really ought to be fully and clearly addressed on this forum. Perhaps a Comcast tech could address this (Comcast_John?).


First, is it true that a static IP address is required in general for "reliable" VoIP service, or does it depend on the VoIP service provider and/or the internet service provider and/or the particular reliability needs of a given customer? I have seen a number of opinions:


  • required ("Dynamic IP addressing should not be used for VOIP, VPN, playing online games or game hosting because Dynamic IP addressing is less reliable then Static IP addressing and could cause the service to disconnect while you are on a VOIP")
    • Really?! You can't reliably play online games without a static IP?! Funny - my  teenage son has never complained...

  • required (same exact quote, word for word, as above... huh....)

  • not required ("the phone adapter you receive is designed to work well with DHCP networks")

  • Google Fiber: not required ("Most Fiber customers do not need any static IP addresses. The vast majority of Internet features work without static IPs, including web browsing, email sending and receiving, video streaming, and voice over IP (VoIP).")


This is a non-trivial issue. Obtaining a static IP address from Comcast costs an additional $20/month, and in addition there is a significant increase in administrative costs to install, configure, and maintain this more complicated networking configuration.


Second, there are a number of posts on the forum from Comcast_John and others that state Comcast does not allow the use of a static IP address on a modem/gateway set to "True Bridge Mode". Therefore, your solution (as I understand it) is not possible. Is that correct, or did I misunderstand you?


What I want to know is whether a small office (one IP deskphone and headset, one computer with a softphone app, 7 smartphones with softphone apps), can I use the Comcast provided Cisco DPC3941B gateway in one or more of the following scenarios with a dynamic WAN-side IP address provided by Comcast:


  1. Default configuration with some firewall ports opened/forwarded/triggered to support the needs of a given VoIP service provider. In this scenario, we would continue to use the firewall, DHCP, NAT, routing, etc. of the gateway; no separate router is installed. To increase reliability, perhaps one should assign static LAN-side IP addresses to all VoIP devices... Not sure...

  2. Default configuration with "Bridge Mode" enabled by the customer (me) using the homepage of the admin interface on the DPC3941B gateway. If I understand correctly, enabling "Bridge Mode" in the customer's admin interface is not the same as "True Bridge Mode" which must be enabled by a Comcast technician, and is some sort of poorly defined "Pass Through Mode". This scenario requires that the customer installs a separate router, and this router's firewall is tweaked to open/forward/trigger the necessary ports for the given VoIP service provider. In this scenario, one wonders will the QoS functionality of the customer's router pass cleanly through the gateway unadulterated? Also, is it recommended to assign static LAN-side IP addresses to all VoIP devices in this scenario?


If either scenario 1 and 2 will actually work, and it just comes down to whether these scenarios are "reliable", I think the best way forward is to implement one of these scenarios and test it out to see if it is "reliable enough".








4 Messages

5 years ago

This is exactly what I am having issues with.  I have 5 static IP's however I have to call Comcast to get put into "true bridge mode" to get it to work with my Peplink?