Problem with VLAN and DHCP

Unanswered Question
Feb 12th, 2007

We have a voice vlan set up to handle our Cisco IP phones. Our edge router at a given site handles the dhcp for the phones (on vlan 3). What is happening is that our phone is sending its dhcp request through it's native vlan (vlan 2) instead of vlan 3 like all of our other sites do so what happens is that dhcp in vlan 2 gets forwarded to a dhcp server on our NOC network. I just want it to get it's ip from the local edge router. Any ideas why this switch is doing this? The edge router is a 3550 12G and the powered switch is a 3550 Powered switch. Both are running 12.2T. All of our other same model switches run the same IOS. So I know that the IOS shouldn't be an issue. The interface config for the phone is as follows:

interface FastEthernet0/2

description Admin Workstion / CISCO PHONE

switchport access vlan 2

switchport trunk encapsulation dot1q

switchport trunk native vlan 2

switchport mode trunk

switchport voice vlan 3

no logging event link-status

no snmp trap link-status

spanning-tree portfast

spanning-tree guard root

I have also tried the new way with no trunking. Neither way works. I have attached the edge router config to help out. THe vlans show in "show vlan" and everything seems to check out. I am at a loss here and am looking for a fresh mind :). Thanks in advance!

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smothuku Mon, 02/12/2007 - 20:30

Hi ,

You configured ip helper-address for vlan 2 and vlan 4 on edge router not for vlan 3.

Thanks,

satish

schadmin Tue, 02/13/2007 - 06:08

But I wouldn't need it since vlan3 is getting it's dhcp from that edge router. It sees the broadcast. the ip helper is there because the server that handles dhcp for those vlan's is in another network. My problem is the phone traffic isnot getting tagged as vlan3... it's just going on the natvie vlan... vlan2... that is my problem...

mifitzgerald Tue, 02/13/2007 - 12:50

Is CDP running? The phone and switch talk CDP to determine which vlan the phone should talk in. No CDP, no voice vlan. For a 3550, the port should be set to access mode, with no trunking, encap, etc. (the "new" way).

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