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3750 Catalyst Switch config issue - VoIP phones hopping vlans

Our network has 2 vlans one for voice which points to a router connected to a T1 for voice and the other for data connected to a router with a T1 for data. This has worked fine with a couple 24 port 2950's.

We just installed a new switch. Now some phones work perfectly while others are getting their IP address from our DHCP server on the data vlan rather than our voip providers dhcp server.

The phones are Cisco 7940's. The only thing that changed on the network was everything was unplugged from the old switch and connected to the new switch.

The switch is 48 ports. In the attached config I removed ports 2-48 as they are all configured identically.

Is there anything obvious here that could be causing this? Any help would be immensely appreciated!


Re: 3750 Catalyst Switch config issue - VoIP phones hopping vlan

Check to make sure your layer 2 vlans are active , "show vlan" .In th dispay you should see vlan 1 and 2 showing active. Also if the phone vlan is not being routed on the 3750 you do not need the layer 3 SVI for vlan 2 , "interface vlan 2" .

Community Member

Re: 3750 Catalyst Switch config issue - VoIP phones hopping vlan

In the config snippet below port 42 is how all the ports were configured. I had to reconfigure port 43 so that it looks like it looks below. Port 43 is where the DHCP server is plugged in.

interface GigabitEthernet1/0/42

switchport trunk encapsulation dot1q

switchport mode trunk

switchport voice vlan 2

switchport priority extend cos 3

spanning-tree portfast


interface GigabitEthernet1/0/43

spanning-tree portfast

Now everything seems to be working fine.

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