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Troubleshooting PoE Problems for IP Phones

I was recently involved in a production issue related to Power over Ethernet (PoE).  More specifically, seemingly random IP phones on a given 3750 stack would lose power and sometimes recover if they were reconnected to another port, or if a power brick was used to power them locally instead of through the PoE mechanism.  For the curious the issue turned out to be poor wiring extensions within the building, but they were good enough to work some of the time.  

The main reason for this post is to bring attention to this article, Cisco troubleshooting guide to PoE on 3750 switches.  The quality of the article impressed me and definitely made my bookmark list for future issues - it is excellent. 

So, in the interest of brevity, here are some of the highlights I felt would be most useful for a quick reference.  The article above also goes into some electrical details related to PoE that I felt were a little out of scope for this post - but I definitely recommend reading at least once.

Cisco IP Phone Disconnects or Resets

1.  Make sure the electrical connections between the switch and the powered phone are reliable.  Any unreliable connections will result in random interruptions and make your life very difficult.  Phones may disconnect or reload.

2.  Keep the Ethernet cable length less than 100 meters between the switch and the powered IP phone.  Extending the cable beyond this length is likely to limit the amount of power the PoE switch can provide to the IP Phone.

3.  Use show log on the switch to look for other events on the switch that may correlate to the problem and give you a clue as to what is happening.

4.  Ensure that the IP phone is not losing network connectivity with the call manager just before the phone reloads.  It can be easy to misinterpret the problem description given by end users who have concept of how this works.

5.  Be aware that the switch has a power budget.  Once the power budget has been depleted, you can start to see unexpected results.  IP phones may not power up at all, or some phone models will operate on a low wattage level that results in a dimmer than normal screen.  Commands like show power inline and show interface status help to paint a wider view of how the switch's power usage.

6.  Verify configuration to make sure that the port is not shut, or that power inline never is configured.  If either of these conditions exist on the ports configuration, there will be no power offered to the IP phone.

Useful Troubleshooting Commands
  1. show cdp neighbor detail
  2. show cdp neighbors g5/0/1 detail
  3. show power inline g2/0/14
  4. show interface status
  5. debug ilpower port
  6. debug ilpower powerman
  7. debug ilpower event
  8. show environment all
  9. show log

More Information

Troubleshooting Power over Ethernet
Power over Ethernet:  ILP and 802.af

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