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Understanding Comcast/Edgemarc/Polycom network setup

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bbaker73

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I'm remotely helping a new client with just a few phones. Phones are not registering. I'm waiting on the model# of the Comcast modem so I can look into the configuration, SIP ALG etc. They don't have anyone technical onsite. With the information they've given, I'm trying to understand the network. May need to call someone in, but maybe you guys have some experience with the Comcast/Edgmarc/Polycom network design and can help me understand how it is working so I can deploy 3CX phones alongside.

From the info I've been given:

All endpoint ethernet drops go into an unmanaged Netgear switch.
Out of the Netgear, there is connection to Comcast Business/Cisco modem/router, which I assume does dhcp etc for the PC's.
Also out of the Netgear, there is connection to Edgemarc router, which then goes to an Arris modem.

So the local lan appears to be shared between PC's and the Comcast Polycom phones, but somehow through the same unmanaged switch, the phone traffic goes out the Edgemarc then Arris and the PC traffic goes out the Comcast Business modem.

Is this as simple as the devices are all on the same subnet, Comcast is the DHCP server, but the Polycom phones are provisioned statically with the Edgemarc IP as their gateway?
 
Oops posted to wrong thread, couldn't figure out how to delete this, just edit it.
 
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Got a little more info. Got a phone going. Appears that PC's get DHCP from comcast modem, but the IP phone got the def GW to be the Edgemarc. Statically configured the phone for comcast GW and is working.

What method are they using in this scenario of the pc, polycoms, yealinks same subnet but IP phones gets GW of Edgemarc? I'm not familiar with the Edgemarc. Is it intercepting the DHCP request for IP phones?
 
The how is not really relevant since you can't change it except by eliminating the Comcast voice portion at which point it doesn't matter. But from a technical standpoint there's really only one way it can happen and that's with a DHCP policy that gives out different settings for IP phones vs the rest of the equipment. I'm assuming the Edgemarc is doing it because I don't think the standard Comcast modems are that smart (unless it's just a DHCP relay upstream). If you can get on a PC it should tell you what the DHCP server is and then you'll know what device is doing it. But as I said, they how isn't really relevant.

You basically have two options. Statically assign the IP info or put another router behind the Comcast modem and then DHCP phones behind that.
 
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I haven't worked with Edgemarc in a long time but I seem to remember that networks built on their equipment will be using SIP-ALG.
 
Forgot to mention make sure you either put the Comcast modem in bridge mode or get a static IP so you aren't doing double NAT.
 
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