New Comcast Router IPv6 DHCP Screwing up SBS2011 and Exchange


I had to have a modem/router from comcast that was giving us issues so they came by on Friday and replaced it with a new Cisco DPC3939B gateway/router.

The problem is that this gateway has IPv6 DHCP built in that it automatically assigns to all my local clients and there is no way to disable it. Comcast told me this is not their problem, even though this is their equipment and we are forced to lease it at $13/month (something that I can't avoid as we have a phone system from them) and even if we buy our own equipment we will still be forced to pay the fee. So I'm trying to get this fixed before I spend money on equipment I don't really need.

The router has a IPv4 DHCP server which I can disable and a IPv6 DHCP server that it will not let me disable. It does however give me 3 options, "Stateless (auto-config)" which I can't turn off, "Stateful (Use DHCP Server) which I can turn off and have turned off, and the ability to assigned DNS manually.

I am hoping that if I can assign the DNS manually I can get this to work.

First off, how do I find out what the IPv6 address is of my server/domain controller hosting exchange 2010? It is SBS2011. I also have a Windows 2003 backup domain controller that I hope to soon eliminate but for now am stuck with. Do I need to have IPv6 on there?

Second, I noticed that in Server 2008 the IPv6 address is obtained automatically and that it also has it's own DHCP server for IPv6. Does it get the IPv6 address from it's own DHCP server or from the comcast router? Should I reset it to be a static IP? If so how do I determine what IP I should use?

I am still new to IPv6 so sorry if these are dumb questions.

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JohnBusiness Consultant (Owner)Commented:
Get a good router, put the Comcast modem/router into Bridged mode, put the new router in front of the Comcast modem (normal hookup) and disable IPv6 on your new router. I have IPv6 disabled on my router and internal equipment works fine.

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Kent FichtnerInformation Technology Systems SupervisorCommented:
if it is a business class Comcast, you can get on your router with Http://  From there do exactly what John says above and it should work great.  We have comcast going in to our Firebox then in to our switches (same basic setup as above).
Pawel_KowalskiAuthor Commented:
The only thing that sucks about this is we are paying them $13 /month for this crap, we really don't need a router that has any different functionality than they already do. All we need is to turn off the IPv6.

But if there is no way to do that we'll go and spend the money on a router, it just bugs me that we have to pay twice, a monthly fee for equipment we don't need since we have to buy our own router anyway all because Comcast won't let me turn off one f-ing setting.
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JohnBusiness Consultant (Owner)Commented:
A decent router is a one time expense (not monthly) and not a modem. So while is costs a small amount, a router will solve your issue neatly.
Pawel_KowalskiAuthor Commented:
Yeah, what I meant is I have to spend $100-$200 on equipment I really wouldn't need since in theory Comcast's router which is Cisco branded should have everything I need and I am forced to pay $130 a year for it. It's just another example of what a crappy company they are in my opinion.
Don JohnstonInstructorCommented:
I have had nothing but trouble with those boxes.  If you have them put it in bridge mode, all you have is a Cable Modem.  The router/voice adapter/wireless functions will not work in bridge mode.

You can request (actually, demand is a better term for what you have to do) a separate cable modem and router/voice voice adapter.  But in some locations, it's almost impossible to get that.

So what I like to do is purchase a cable modem (you will typically recover the cost within two years).  Then tell Comcast you're providing your own cable modem.  They will then install a voice adapter.  

Just make sure to get an approved cable modem.

Approved Cable Modem List
Pawel_KowalskiAuthor Commented:
Thanks for all the help.

I was hoping I didn't need to purchase another router do to the fact we pay $130 a year for this equipment and there is no way around it (since we have a phone system with them they won't let us get around the equipment fee, even with our own modem).

I ended up getting a SOHO router and putting it in DMZ for now. When I have a chance I will call Comcast and have them put the modem into bridge mode (don't want to do it when people are working as I'm scared they will screw something up).
JohnBusiness Consultant (Owner)Commented:
@Pawel_Kowalski  - Thank you for the update and I was happy to help.
I believe this issue surfaced after a recent MS update for SBS2011. I have two similar setups and both were constantly stopping the DHCP service with an error 1053 showing an IPV6 address that I could ping with nothing attached to the server except the Comcast modem. This was never a problem and happened to both within one day. One had an older SMC modem that can shut down the IPV6 DHCP, so that one is OK. The other is a newer Netgear that works like the one in the post. Comcast is bringing us a new modem on Monday, but they claim that they have never heard of this problem before. Many years in this business has taught me that it is never "just you" when something goes suddenly wrong. I have been waiting for this issue to show up on Experts.
Both of the SBS2011 servers had these two patches applied the day the problem showed up:
KB3011780 & KB2992611

Comcast has a request to change the modem from the Netgear CG3000DCR to the SMCD3G tomorrow. I will buy a used one on EBay if Comcast can't supply one.

I see posts on line that say if you go to true bridge mode on the Netgear, Comcast will not put the static IP address on the modem. Comcast Support does not know if this is the case. I would have to wait 24-48 hours to ask a Tier 2 technician.
Pawel_KowalskiAuthor Commented:
I haven't yet attempted to do the bridge mode on the modem, I have heard similar things as you, that when you have a static IP Comcast can't put the modem in to bridge mode, so you are stuck with their crappy router and you need to add another one in front of it (the one that will be exposed to your internal network) as that crappy router doesn't work properly.

I can tell you that this has nothing with SBS2011. It has to do with a stateless IPv6 configuration built in to the modem. SBS 2011 doesn't do IPv6 (that I' aware) so your clients are being configured by the comcast router instead of SBS 2011. This screws everything up.
Comcast changed the modem from the Netgear CG3000DCR to the SMCD3G. I unchecked IPv6 DHCP on the SMCD3G and DHCP Service stays running.
Pawel_KowalskiAuthor Commented:
They no longer have the SMCD3G or at least that's what they told me. The SMC modems I had from them before worked fine. The modem they gave me that had huge DHCP issues with IPv6 was the DPC3939B, which is clearly a router intended for home use, not business use (even though we are business customers).
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