IP Phone also in Vlan 10 - why?

Unanswered Question
Aug 22nd, 2006
User Badges:


We have 2 vlans: Vlan 10 for Data and Vlan 20 for Voice. Why does the switch show the IP phone to be part of Vlan 10 also when the show mac-address-table cmd. is issued?

Can any one help me understand the "real technical" reason as to why? Thank you.


Output of #show mac-address-table|inc Fa0/10 cmd?

10 0006.5bcb.e24c DYNAMIC Fa0/10

10 0013.c4aa.be4e DYNAMIC Fa0/10

20 0013.c4aa.be4e DYNAMIC Fa0/10

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
gpulos Wed, 08/23/2006 - 03:55
User Badges:
  • Blue, 1500 points or more

what type switch and version software?

i've seen this before with certain versions and upgraded to allieviate.

rajju Wed, 08/23/2006 - 10:27
User Badges:


It's a Cisco 3550 running Version 12.1(22)EA1a.

You might disregard: I was told this is normal behavior. I see the same for other ip phones too.


gpulos Thu, 08/24/2006 - 03:35
User Badges:
  • Blue, 1500 points or more

i wouldnt call this normal behavior only because it really isn't.

you cannot have a cisco ip phone in two vlans simultaneously.

as i've stated, i've seen this before in certain IOS and had to upgrade to allieviate.

the fact that you see two subnets via that port tells me it is in trunking mode, so you should be ok there.

rajju Thu, 08/24/2006 - 07:57
User Badges:

Here is the explanation I received. Please let me know if there is a better way to understand this.

"The IP Phone can tag the packets with 802.1Q frames which means it can tag the packets with different VLAN. It learns its vlan through CDP. So when the switchport is defined with access vlan 10 and voice vlan 20, the phone sends voice packets with vlan 20 and data packets coming from the attached pc with vlan 10. In essence, IP phone acts as another switch to the catalyst switch and hence the trunk behavior on the port."


gpulos Tue, 08/29/2006 - 11:48
User Badges:
  • Blue, 1500 points or more

this is correct, the phone must operate in a trunk mode to support the data vlan and the voice vlan.

this does not answer why you see the phone MAC in both vlans. this is not normal and is not correctly representing the configuration as shown with your 'show mac...' command.

have you tried to upgrade the IOS? i'm sure if you do, this symptom will disapear. (as it did with me when i upgrade to allieviate the same MAC in two vlans for my phones)

tsherman Fri, 09/08/2006 - 21:34
User Badges:
  • Cisco Employee,

The reason you see both MAC addresses is because on that hardware for that switch, the MAC address of the CDP messages are counted. CDP traffic is not 802.1q tagged and goes to the native vlan. On othe hardware you will not see this because CDP traffic will not be shown on the port unless there is an problem with the IOS, as you have stated.

tbramwe Thu, 08/24/2006 - 03:17
User Badges:
  • Silver, 250 points or more

Please check the DHCP scope (if not using CCM for DHCP). Also, if the voice VLAN is not trunked the IP phone may get an address from the PC, or other, VLAN.


This Discussion