Friday, December 9, 2011

Power Over Ethernet in Switch-port Modules

I recently ran into a rather interesting issue regarding enabling POE for a switch-port module.  Those of you who have worked with routers extensively for years are probably well aware of both the problem and the solution, however those of us who have only a years experience managing them need some nudges in the right direction some times.

Now for the problem. We had done some recent updates on our networks routers adding sticky mac configurations to the switch-port modules inside them.  After the update a few of our users started reporting that their IP phones would no longer power up.  Their computers hanging off the same switch-port modules were all working fine.  But not all switch-port modules were having the issue.

After spending hours going through the configs we sought external help.  I was lucky enough that the first person I talked to had not only heard of the issue but had been able to find a solution.  The solution was so simple it is almost embarrassing to post it.  I learned that POE for routers modules is tied to the default VLAN for that router, in order to allow power to flow through to the switch-port module we needed to trunk the default vlan through on the switch-port modules trunk port, in our case vlan 1.

As soon as we trunked vlan1 through everything powered up and life was good again.  For security reasons we had been trained to shut the default vlan down and never use it.  It never occurred to me to check to see if a vlan, which was always administratively down, had access.  It is still rather odd that power flows even though the interface is down, yet still requires the interface on the trunk port.

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