Cisco VoIP phones pulling wrong VLAN

[cbenenhaley]

I have an interesting situation that I haven't seen before.  We are running 6509s for our access switches, and, for the most part, Cisco 7975 VoIP phones.  We have a voice vlan (we'll say vlan 20), and a data vlan (say, vlan 10).  I've seen on a couple of occasions where the phones will drop vlan 20 (voice) and pull vlan 10 (data).  And if my memory serves me correctly, it was after a power outage.  Any ideas why this could be happening?  Thanks in advance!

[Mark Malone]

Hi

are you using recommended setup for access voice ports and there not just set as trunks ?  ....switchport voice vlan x

[cbenenhaley]

Hi Mark... The ports in question are all set as access ports.  If you issue a "sh run" on any of those ports, it does show that the port is configured for both the voice vlan and the data vlan.  Why it will jump from the voice vlan to the data vlan is what I can't figure out.

[paul driver]

Hello

do you have native vlans defined? Is your dhcp server reside on a native vlan?

res

paul

[cbenenhaley]

Paul, we do have native vlans defined.  I'm not sure about the dhcp server being on a native vlan, but we statically configure the devices in question anyway..

[paul driver]

Hello

 an you elaborate on "they pull from another. Lan"

i was on the assumption your voip phones were "pulling" addressing from dhcp for the data vlan

what could happen if you have different trunk interconnects with different native vlans then one vlan can leak over to another 

res

paul

[cbenenhaley]

Paul, to clarify, let me give you an example:  We have an office with a bunch of VoIP phones hooked up.  The ports they connect to are configured for a data vlan and a voice vlan.  I got a call that some of them were down.  Not knowing exactly which ports the phones were connected to, I issued the "sho mac address-table | inc xxxx" command, where the xxxx is the last 4 of the phone's MAC.  The output is what told me that is was on the wrong vlan, which of course is why they weren't working.  Saying that they were pulling from the wrong vlan was just poor verbiage on my part. 

To your last point, could they leak over even if neither one of them are configured on a native vlan?

[lpassmore]

Just to throw my 2 cents in... When a Cisco phone starts it will perform a DHCP request (or CDP request) on the Data VLAN to get the address of the configuration server or the Voice VLAN.  This will cause the MAC address to appear on the data VLAN on the switch interface.  The keeping of the MAC address in the Data VLAN can be device and IOS specific.

Once it has the configuration information, it will continue to use just the Voice VLAN for traffic.  Eventually the MAC address on the Data VLAN might timeout and disappear.

From your description it appears as if the phone is booting on the Data VLAN correctly but never getting the configuration to apply the Voice VLAN.  Look for either a disconnect with CDP (make sure it is enabled) or with the DHCP server configuration (Check for Option 150 I think it is).  Also check for any port security issues.  

Perhaps check that the phone is actually getting an IP address by matching the entry in the router's ARP table.

And make sure you have spanning-tree portfast applied to the phone interfaces

LP

[cbenenhaley]

lpassmore,

What you're saying makes a lot of sense.  As I mentioned before, the phones have a static IP, so DHCP shouldn't come into play. Otherwise, you've given me some things to look into. Thanks for the help!

[paul driver]

Hello

Then ,No as far as I am aware it should happen.

I thin Ipassmore has a point -

The voip phone mac I guess will be seen on both vlans. Its physical port is actually attached to the switchport which has a data/voice vlan assigned.

So given this, The phone physical mac would be seen on both vlans and pc attach to the phone will be seen on just the data vlan.

res
Paul

[cbenenhaley]

I definitely think I have some things to go on now. Thank you for all your help and input!

[ramesh_kumar]

You need to enable CDP on the interface