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no audio on Cisco 3560 with Polycom CX700


I have a scenario where I cannot get any audio between two Polycom CX700s connected to my Cisco 3560.  This used to work but just stopped.  Unfortunately I just do the networking and have to rely on other people for information on the phone system.

VLAN 10 is data and phones.

All QOS Is fully disabled.

This is Microsoft OCS.

Here’s what I’ve found:

2 x Polycom CX700s on two ports on a single Cisco 3560

Port configs:

                Switchport mode access

                Switchport access vlan 10

Call set-up is fine – no audio

If I plug a hub into the switch and plug the phones into this, then two-way audio is working perfectly.

Removed hub and plugged phones back into 3560.

Decided to try this as a config:

                Switchport trunk encapsulation dot1q

                Switchport mode trunk

                Switchport trunk native vlan 10

The above configuration works but I have no idea why.

This used to work fine until two weeks ago with ‘switchport mode access’ but something weird has happened.

The fact that I’ve got this working by assigning this phone to the native vlan (the same as the data vlan) is a little bizarre.  The native vlan is untagged so in theory this is the same as an access port on this vlan.  Is this correct thinking?

I did set the native vlan to 1 as a test to prove it wasn't using the trunk and nothing happened there.

Am I missing something?

Personally I think something has changed on the phone system for this to happen and me making this change has just masked it, but I cannot prove this.

I look forward to your thoughts.

Cisco Employee


Seems you will have to work with your networks team as there could be a number of things to look at

First I see this phone may need LLDP to find its VLAN information.  Cisco switches would use Cisco Discovery Protocol (CDP) and some can be configured for LLDP is my understanding.

You say the phone registers fine and can call another phone - this seems to indicate signalling is working?

Can you check voicemail do you get an audio path there?

Is it just audio from this particular phone, what about audio from the MOC client when placed in the same VLAN?

Is the phone getting an IP address (i.e. via DHCP)?

Maybe the default gateway is not set up correctly for your VLAN?

Is the VLAN up and running and not shutdown...but how are the phones registering then?

There are some good show and debug commands on the Cisco switches you can run to show how this phone is behaving.

Good luck.