Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

trust device lost on a port with IP phone


On a 2950 switch with identical port config one phone do not send periodically cdp mesages (not checked on monitoring session, only with sh cdp nei command on the 2950 switch), which results trust device lost event.

it seems the phone tries to get IP address in the access vlan not in the voice vlan. (discards the cdp packet sent by the switch?)

This phone should autoregister to the ccm 4.1(3)sr5. Autoregistration works on other switches.

If I configure this phone on the ccm, it starts working, it sends cdp more often - in every 120 sec the the cdp hold time restarts.

What could be the source of this problem?




Re: trust device lost on a port with IP phone

When the voice VLAN feature is enabled, it allows voice traffic from the attached IP phone and data traffic from a daisy-chained PC to be transmitted on different VLANs. This capability provides flexibility and simplicity in IP address allocation and prioritizing voice over data.

Untagged traffic is sent according to the default Class of Service (CoS) priority of the Catalyst switch port. The CoS value is trusted for 802.1p or 802.1q tagged traffic.

The voice VLAN feature enables access ports to carry IP voice traffic from an IP phone. The switch can connect to a Cisco 7960 IP phone and carry IP voice traffic. Because the sound quality of an IP phone call can deteriorate if the data is unevenly transmitted, the switch supports QoS based on IEEE 802.1p CoS.

QoS uses classification and scheduling to send network traffic from the switch in a predictable manner. The Cisco 7960 IP phone is a configurable device and you can configure it to forward traffic with an 802.1p priority. You can configure the switch to trust or override the traffic priority assigned by a Cisco 7960 IP phone.

The Cisco 7960 IP phone contains an integrated three-port 10/100 switch. The ports are dedicated connections to these devices:

Port 1 connects to the switch or other VoIP device.

Port 2 is an internal 10/100 interface that carries the IP phone traffic.

Port 3 connects to a PC or other device.

New Member

Re: trust device lost on a port with IP phone


Thanks for your reply.

Did I ask a sort summary about any CP phone?

Please try to focus on the problem, and give me suggestions why CP7912 does not understand CDPv2.