Cisco Phone 7940 Intermittent Issue on Connecting to Voice Vlan over SG300 Switch

[Aramis Salomon]

I have a local network with 7 SG300-28PP 28-port Gigabit PoE+ Managed Switches and 1 SG500 24 port Gigabit PoE+ switch (main switch). One of the SG300 is having issues connecting Cisco IP Phones 7940 to the voice vlan. I have defined Vlan 2 as voice vlan and vlan 1 is the administrative one.

On SG300 switches, ports 1 – 24 are defined as general ports with access to Vlan 1 (Untagged, PVID) and Vlan 2 (Tagged). Ports 25 – 28 are defined as Trunks with access to Vlan 1 (UP) and Vlan 2 (T).

The IP PBX (an asterisk based system) is connected in one of the SG300 switches and its port only has access to Vlan 2 (TP).

All these switches replaced 2900 switches where everything was working with no issue at all.

I have an issue in one of the SG300 switches, where the 7940 phones, sometimes successfully connects to VLan2 (voice) and sometimes connects to Vlan1 and receive an IP address from that Vlan, so they can’t connects to the IP PBX. Computers connected to these 7940 successfully connect to VLan1 and get their IP addresses correctly. All 7912 Cisco phones connect with no issue, including computers connected to them.

I have some others 7940 phones connected to the rest of the SG300 switches with no issue at all.

Do you have an idea about this issue?

Is this a configuration issue?

Or it may be a switch issue (this is a brand new switch)?

Thank you very much for your help on this one.

Regards,

[Aseem Anand]

Hi Aramis,

7940 rely on CDP to find their voice vlan numbers. Can you double check if CDP is running on the 7940/60 ports?

Also, another way to overcome this problem is by hardcoding the vlan on the phone itself to VLAN 2.

Settings >>>> Network configuration >>> Admin VLAN ID  >>> Set it to 2

Aseem

[Mohammed al Baqari]

Seems that your CDP is fluctuating. Unfortunately 7940 doesn't support LLDP. So the only way to do it to walk across all phones and set admin vlan as mentioned below which overrides learned VLAN.