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Blog Post

Mar 04

Cisco Enhanced PoE: Cisco 2702 and 3702 APs and the number of AP Antennas


So in buying the latest and greatest AP type (the Cisco 2702/3702 at the time of writing), you’re into the world of the 4x4 antenna AP. This AP can transmit and receive on all its antennas at once – meaning you can have multiple streams to provide that high throughput, as well as improved signal coverage over earlier models.

However, there’s a major "gotcha" we found when first using these APs that’s worth knowing about. The AP will power up with normal PoE, but that last antenna has bumped up the power draw on the AP – now its up at just a little over 16W, more than the 15.4W supplied by normal PoE.

Older switch = Less antennas

If you’ve got an older switch that can only do standard PoE, no great shakes – the AP will disable one TX antenna and continue to operate. The drawback is fewer special streams and the AP won’t provide as perfect and even coverage as it would have done with the extra antenna.

Newer switch = More antennas

The obvious answer is to use a newer switch with 802.3at (PoE+) that can supply up to 25.5W of power per port. There is another option though; some Cisco 802.3af switches support a feature called ePoE that you can turn on in order to enable some extra power (up to 20W per port) – be careful though as the switch only has so much power available. The amount of APs you can support will depend on how large a PSU you bought with the switch.

Check with this command:

  • show inline-power

Enable Cisco enhanced PoE (ePoE)

Firstly you need a switch that can support ePoE, such as the Cisco 3560E or the Cisco 3750E switches. To enable power up to 20W on the switchport, use the interface specific command below:

  • interface xxx
  • power inline port maximum 20000

You might well run into the issue that the command is not appearing. Most likely in this case, you have an older IOS – although its been supported for quite a long time. The feature was added back in 2008 on IOS 12.2(44)SE for Catalyst 3560-E and 3750-E switches, 12.2(33)SXH2 for 6500 line cards and 12.2(44)SG for 4500-E line cards.

Checking ePoE is working

The screen shot below shows how to check the AP is running at full power. Access the WLC GUI and go to the following menu:

Wireless --> Access Points --> Select the AP of interest

Click on the Advanced tab

The item to check is highlighted in the screen shot below – should show ‘full power’ when all antennas are operational.

AP running at full power

AP running at full power

You can also check on the switch, by issuing the show inline-power command to see the power draw on each port:

ePOE Power Draw

Command enabled to configure the full ePoE power

 

Here's a sample of the interface configuration - command used here was do show run int gi0/48:

ePOE Switchport Configuration

ePoE switchport configuration


 
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