Call Manager providing DHCP services to Remote Site

[ErrolDC2]

Hi. We are getting a cisco phone system. The call managers and vm servers will need to support a remote site connected to our main campus via 2 T-1 that will be used to transport packet voice and an 8MB link for other data. The call mgrs will be located on the main campus. The plan is to define two scopes on the call mgr. One for the main campus and the other for the remote site. Using the DHCP helper service on the router at the remote site, we are going to proxy dhcp requests to the call mgrs and obtain an IP for the phones at the remote site from the second scope. Will this work?
I don't think so. Since the links between the sites are layer 3 links, this means that the phones on the remote site and the phones on the main campus have to be on different subnets. If they share the same DHCP server, there won't be way to define a unique scope for use soley by the remote site right?
Can someone share some insights with me?

 

[rtfmdude]

if you use an ip helper-address command on the ethernet intfc of the remote router, the dhcp request will be unicasted from the ethernet interface of the remote, so, yes - this should work fine.

For survivabilities sake, I would rather run dhcp on the local router - if you lose your wan link and go into srst, what use is srst if (for whatever reason) your phones need dhcp?

 

[ErrolDC2]

we are going to use surviable remote. thx for the clarification. I didn't realize that the helper-address was unicast as the source address of the router's interface.

 

[sirroutesalot]

I used to manage a 40 site network a portion of which used Cisco IP phones. I did run the DCHP from a utility server at the Headquarters office. Remote sites had 6 vlans (subnets) each, so we had 200+ scopes on the server. As long as the scope network ID matches the network ID from which the packet request is sourced, and as long as there is an IP helper address on the interface which receives the original broadcast packets, it will work. In the case of SRST, the phones already had the IP address and in testing the SRST functions, they worked OK with a dead WAN. I suppose if we kept the WAN down till the lease expired we would have had a problem. But then if we let the WAN stay down that long, they would have fired me anyway. The reason we did it centralized was for the ease of management of the DHCP service. The thought of having 40 instances of DHCP running gives me the willies.