VNC issues through IP Phones

[iSub]

Hi everybody,

I have inherited a Cisco IP phone system and whilst I have picked up some basics, I am stuck with one particular problem. We cannot VNC into computers that are connected through the Cisco IP Phones. Here is what I know:

• We are running Cisco Unified CM Administration - System version: 11.0.1.20000-2
• We use a combination of Cisco CP-3905 and Cisco CP-7841 phones.
• Phones are on VLAN 140 and PCs on VLAN 110. Both devices get an appropriate IP for their VLAN.
• We have Meraki switches and can VNC into the PCs if we remove the phone and connect directly to the same port.

I presume there is a setting to change on the phones but I have no idea which one. Any thoughts?

[Jonathan Schulenberg]

That’s very strange, especially because phones have never touched PC traffic and those series run on different code bases. The phone is just a three-port switch with the PC running in the untagged/native VLAN.

VLC isn’t trying to use jumbo frames is it?

The only other long-shot guess I have is port synchronization. Are the speed and duplex settings the same between the phone and switch as they are between the phone and PC? If not, there is a setting on the phone configuration page that you can to force this behavior. Essentially train down to the lowest common denominator.

One other test you could try: turn off the Voice VLAN on that port and see if the problem persists when the phone and PC are both in the untagged VLAN.

Either way I suspect you may be in for a network tap and some packet captures to clearly discern whether to point blame at the switch or the phone.

[HARIS_HUSSAIN]

We are able to use VNC through the phones. As suggested by Jonathan Schulenberg check the duplex settings.

Thanks,
Haris