dhcp server for voice vlan

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Feb 14th, 2007

Hi all, when setting up voice, where would you put the dhcp server, and how would you stop other pcs on your network from getting the dhcp scope for the vlan, would you put the server actually in the voice vlan ?

I have this problem too.
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david.lowe Wed, 02/14/2007 - 20:21

I'm assuming that you're referring to Cisco's IP Telephony platform.

It's a good idea to have DHCP services configured on the very first layer 3 hop device (usually a router or layer 3 switch) and then on the Call Manager server(s) as well for redundancy purposes.

As far as the dynamic assignment of IP's in relation to VLAN assignments, the switch port and the IP end point (phone) negotiate the type of VLAN that the phone should be assigned to via Cisco Discovery Protocol. Because of CDP, it enables the IP end point to be dynamically assigned to the voice VLAN on the switch access (or trunk) port. The access layer switch port has a manual configuration within the physical port that designate the VLAN ID of the voice VLAN so that's how it knows. Once the phone knows which VLAN to use, it will tag it's frames with the appropriate 802.1q tags and then request an IP address from a DHCP server located on that specific VLAN.

Hope that helps.

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