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7960G without Power which is plugged into Netgear Switch kills the Network!

Hi together,

yesterday we had an Problem with an 7960G which was plugged into an Netgear FS108 Switch. When we installed the Phone we forgot to Plug in the External-Power-Cube to the Phone. This caused a big Trouble in the whole Network with Ping-Timeouts and so on. No error occours when we plug the phone without power to a HUB or into an Nortel-Bay-Switch.

Also there is no Error when we plugged an older 7960 (without G) into the Netgear Switch.

Does anybody know this Problem ?

Is there an Bug-ID available or an BAD-Switch-List from Cisco ?

Thanks for your Answers


New Member

Re: 7960G without Power which is plugged into Netgear Switch kil

We had a similar experience with our 3Com switches and the 7940Gs.

It may be something to do with the way a Cisco switch detects the presence of an IP phone and switches on the power over the CAT5 cable.

Someone explained it to me once like this. Without any power connected, a Cisco phone by default crosses the TX and RX pairs on the Ethernet port. If the Cisco switch sees the same CDP packets returned then it switches on the power and the phone comes online. If it does not see the CDP packet bounced back (when you connect other equipment) then it does not switch on the power.

I don't know if that is actually correct, but it seems to hold true for us.

So, when you connect the unpowered phone to a non PWR Cisco switch any packets sent through the port come back creating a network loop.

We reduced the risk of the problem happening if a power blocked was removed by ensuring STP was enabled on all the switches which will block the switchport the dead phone is connected to.

Hope this helps.



Re: 7960G without Power which is plugged into Netgear Switch kil

Here's a document about the IP phone power-up process:

The phone does connect the TX and RX pairs. Catalyst switches detect this when link negotiation pulses for Fast Ethernet autonegotiation (FLP, Fast Link Pulses) get looped back to itself. The inline power patch panels use a totally different process using the other four "unused" pairs. CDP comes afterward, when the phone is powered up and Ethernet link is up, and the phone needs to start making decisions like what VLAN to use for its own traffic.

New Member

Re: 7960G without Power which is plugged into Netgear Switch kil

Hi Carsten,

There is a field-notice put out by TAC concerning exactly this issue. The field notice specifies operability problems with Cisco In-line powered switches and Aironet 350 and 1200 Access Points. But most likely carries over to your hardware.


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