Voice DHCP and IP helper-address

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Feb 23rd, 2009
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I have a quick question regarding the correct use of ip helper-addresses. I have a typical setup: voice and data on separate vlans, with a Windows DHCP server giving out addresses for both.

Data Vlan: 1

Voice vlan: 100

DHCP Server:

Data DHCP Scope: 172.16.96.XXX

Voice DHCP Scope: 10.16.96.XXX

I have one site whose phones will not get a DHCP lease, unless I disable the data DHCP scope on the server - which suggests to me that the ip helper-address is not configured correctly. As soon as I disable the 172 scope, the phones grab an IP.

On the router, the helper address is currently set on the voice vlan subinterface:

Interface FastEthernet0/0.100

encapsulation dot1q 100

ip dhcp relay information trusted

ip address

ip helper-address

The switchports are configured as such:

Interface FastEthernet0/1

switchport mode access

switchport voice vlan 100

mls qos trust device cisco-phone

mls qos trust cos

auto qos voip cisco-phone

spanning-tree portfast

Is this the correct way to do this? If not, where exactly should the helper address be configured? Is there something else I am missing?

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paolo bevilacqua Mon, 02/23/2009 - 10:52
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I would recommend you to configure dhcp for the phones only, in the router or switch itself. That will eliminate any helper issue, and guarantee phones will get an address no matter what.

balitewiczp Mon, 02/23/2009 - 12:07
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i think you are missing the "switchport access vlan X" command

rcoote5902_2 Mon, 02/23/2009 - 13:06
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That's been entered, but for some reason doesn't show in the config.

paolo bevilacqua Mon, 02/23/2009 - 15:23
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Because it's the default of 1.

markcarat Tue, 02/24/2009 - 08:56
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Try adding the ip helper at the voice vlan interface

interface vlan100

ip helper-address x.x.x.x

Hope this helps


rcoote5902_2 Tue, 02/24/2009 - 11:42
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Well I think I figured it out, getting some help from another thread with a similar problem. Our DHCP server was connected to a switch with a default config, including:

switchport voice vlan 100

I removed this and it the phones are not struggling for an IP anymore it seems.

kwyoung11 Tue, 02/24/2009 - 11:41
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(1) Do not use Vlan 1 as the data vlan

(2) where did you define Option 150 for the TFTP server to know that vlan 100 is the voice vlan ?


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