02-23-2009 10:48 AM - edited 03-15-2019 04:24 PM
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: 172.16.99.1
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 10.16.96.1 255.255.255.0
ip helper-address 172.16.99.1
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?
02-23-2009 10:52 AM
Hi,
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.
02-23-2009 12:07 PM
i think you are missing the "switchport access vlan X" command
02-23-2009 01:06 PM
That's been entered, but for some reason doesn't show in the config.
02-23-2009 03:23 PM
Because it's the default of 1.
02-24-2009 08:56 AM
Hello,
Try adding the ip helper at the voice vlan interface
interface vlan100
ip helper-address x.x.x.x
Hope this helps
Mark
02-24-2009 11:42 AM
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.
02-24-2009 11:41 AM
(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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