VOIP traffic taking the wrong route

[Vidley]

Hello all,

I am fairly new to Cisco switches and have an issue I can’t seem to remedy.

We have a switch stack with 3 nodes in it (OLD). This connects to the CORE and is being replaced with a another switch stack containing 6 nodes (NEW)

During my testing phase I have delegated a port on OLD as an uplink for NEW so I can check the config on NEW.

NEW has a PC connected to port 2 and a Cisco softphone connected to port 3 while port 1 acts as the uplink to OLD

When I make the uplink between OLD and NEW live the following happens: 

• I can ping the PC connected to NEW
• I can ping the phone connected to NEW
• Within 15 seconds of the uplink going live half the phones connected to OLD fall back to registration mode
• All the PCs connected to OLD still work correctly.

As soon as I disconnect the OLD to NEW uplink all the phones connected to OLD come back to life instantly.

I’m guessing some sort of preferred route is possibly being added to OLD as soon NEW comes up but don’t know what I should be looking for.

Any help would be greatly appreciated

[balaji.bandi]

Looks for me - some kind of STP or VTP issue, how is your configuration looks like on both the switches.

what is the logs show on both the switches.

[Vidley]

The logs show me the following event when i shutdown the connection between OLD and NEW...

entry number 2720703 : SPANTREE-2-UNBLOCK_CONSIST_PORT
Unblocking GigabitEthernet3/0/48 on VLAN0004. Port consistency restored.
timestamp: 1106152593

[balaji.bandi]

Check the cabling any loops, if there is no loop in the cable, setup STP Priority on OLD switch as STP root bridge.

also worth post the config of the switch how they connected.

[Vidley · Accepted solution]

Thanks for the advice, I needed to set the following on the OLD stack port that was acting as the uplink for the NEW stack:

spanning-tree bpdufilter enable

spanning-tree port-priority 240

And now it all works correctly.

[balaji.bandi]

Glad suggestion fixed, appriciated your inputs.