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New Member

Inconsistent behaviour if voice VLAN == access VLAN

My customer has been using a working switchport config that included the same VLAN no. for voice & access. This is supported and documented, e.g. here:

Now we are in the process of a platform exchange due to lifecycle reasons, with a similar config as much as possible. Surprisingly, the end devices (PC and phone, cascaded) connected to the new switch can't communicate anymore. The LED on the affected ports stay orange, the links are up/up and spanning tree wouldn't start changing state at all (the ports are missing in the 'sh spanning-tree vlan xxx' output). It's important to note that the new switch has the necessary VLANs configured.

As soon as I change the voice VLAN so that it differs from the access VLAN, or, as I remove the 'switchport voice vlan' command, the LEDs turn green and communication starts to flow. So it seems to me that there's some kind of bug or behaviour change on the new switch or its S/W.

Any idea what's happening here?

This is the configuration of the switches:

Old switch (WS-C3750-24PS-S, 12.2(44)SE2):

interface FastEthernet1/0/11
 switchport access vlan 173
 switchport mode access
 switchport voice vlan 173
 srr-queue bandwidth share 10 10 60 20
 priority-queue out 
 mls qos trust device cisco-phone
 mls qos trust cos
 auto qos voip cisco-phone 
 spanning-tree portfast
 service-policy input AutoQoS-Police-CiscoPhone

New switch (WS-C2960X-48LPS-L, 15.0(2)EX5):

interface GigabitEthernet1/0/35
 switchport access vlan 173
 switchport mode access
 switchport voice vlan 173
 spanning-tree portfast

[Yes, we've decided to try running VoIP without QoS to see how it'll turn out.]

Thanks for any hint


make all ports with access

make all ports with access vlan 173 and without voice vlan and it will work.

interface GigabitEthernet1/0/35
 switchport access vlan 173
 switchport mode access
 spanning-tree portfast



so phones and pc will be in the same vlan 173

New Member

Thanks Tagir. I've already

Thanks Tagir. I've already figured out that it works when removing the 'switchport voice vlan' command. The questions is, why do I have to remove it and is it a bug? If not, where is this documented?