IP Helper

Ok I have a CUCM at the core office and have created a new vlan and seperate dhcp scope to cater for the voip phones.

In the core office I have no problems with phones picking up the correct address(Vlan) and all works fine.

In the branch office the phones are picking up a local dhcp address, however the phones do register and calls can be made.
The problem I have is if I set the interface fo the port to the new VLAN, the phones fail to pick-up an ip address and thus cant register.

I believe I need to configure "ip-helper" to point to the core office in order to correct this?

If so I assume the ip helper address would be the core dhcp server and not the CUCM??

Is this correct?

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Comments

dtlokee

The IP helper address will forward the DHCP request to your DHCP server if it's on a different subnet. The alternative solution would be to configure a DHCP server on that vlan (or the switch/router).

laidbackfreak

DT, I have a dhcp server on that vlan (data), based at the remote site. This serves all the local hosts. The dhcp server for the new vlan (voice) is based at the HQ site.

What I'm trying to acheive is to ensure the voip phones conect to the correct Vlan (voice) at the HQ site.

Does that makes sense? with regards to the above... ?

dtlokee

You will need a helper address on the VLAN interface and make sure the scope you created on the DHCP server has option 150 configured to point to your callmanager (you should be able to look at other scopes that work for this information)

laidbackfreak

cheers DT, keep going round in circles as to where to place the helper address lol

I've got the 150option already covered so thats done....

getting there slowly....

malcybood

laidbackfreak wrote:

cheers DT, keep going round in circles as to where to place the helper address lol

I've got the 150option already covered so thats done....

getting there slowly....

Hey man,

Placement of the helper address would be on the router voice LAN sub-interface on the router which your LAN switch connects to.

So if you had VLAN 2 on your switch (subif 0/0.2 on router) as your data subnet / VLAN you would have no helper address on here as your DHCP requests will broadcast on your local LAN.

If your voice VLAN is VLAN 701 (router subif 0/0.701) would require the following configuration on the router voice vlan subif to forward requests to DHCP server 10.20.20.150

router(config-subif)#ip helper-address 10.20.20.150

This will forward bootp/DHCP requests from VLAN 701 to the 10.20.20.150 provided you have the dhcp scope, voice VLAN config on the switch and intervlan routing already in place correctly.

Is your router LAN port setup for dot1q trunking correctly?

dtlokee

There could be a number of things at play here. Are you using a L3 interface on the switch (eg, "interface vlan 301") or a subinterface on a router for routing? That is where you need to put the "ip helper address". Also make sure the remote devices have a route back to the new subent, did you advertise it into your routing protocol?

malcybood

dtlokee wrote:

Also make sure the remote devices have a route back to the new subent, did you advertise it into your routing protocol?

Good point, definitely check this out also. When I've done voice / data vlan separation exercises in the past the most common issues I've had are;

routing as mentioned above, over the WAN
dot1q trunking not setup correctly on the remote router for the new VLAN
LAN switch not cofigured with trunk port on uplink to router
DHCP scope configuration issue for new VLAN

Those are your major elements here in making this work.