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New Member

7905 phones - half-duplex collisions

I have some 7905 phones connected to Catalyst 2950T switches. I also have some 7960s and 7912s at the same sites.

Gateways are 2801s and 2811s.

I know 7905 phones work at 10 Mbps Half-duplex only.

My problem is: either if I force the switch port connected to a 7905 phone to 10Mbps half-duplex, or if I let it autonegotiate, after some days working I´m finding some 7905s being blocked by the switch (port in err-disabled state,

logs in switch:


cted, collision threshold exceeded on FastEthernet0/3.

%PM-4-ERR_DISABLE: loopback error detected on Fa0/3, putting Fa0/3 in err

-disable state

The gateway ethernet interface in the voice network is set to autonegotiate and using 100 Mbps full-duplex, but I assume this should be no problem.

Any ideas?

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Re: 7905 phones - half-duplex collisions

This sounds like your issue:

In the case of Cisco IP Phones with 10Mb ports, such as the 7902G, 7905G, and 7910G most network traffic sent to an unpowered phone returns as loop back messages.

Please rate this post if it is helpful.

New Member

Re: 7905 phones - half-duplex collisions

Thanks a lot, it really seems to be my problem.

I´ve applied the port protection commands and

errdisable detection and recovery commands and I

hope this will solve it.

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