Kludging Power over Ethernet Power Allocation

PoE capable switches keep track of the amount of power they've allocated to devices to prevent oversubscription of the power supply. PoE devices are categorised into classes of power consumption:

Class	Power allocation
0	15.4W (default)
1	4.0W
2	7.0W
3	15.4W

A PoE capable device can inform the switch via CDP or LLDP of its power class & the switch will deduct the relevant amount from its power allocation budget. You can view the switch's current power allocation via show power inline:

Available:370.0(w)  Used:115.3(w)  Remaining:254.7(w)


Interface Admin  Oper       Power   Device              Class Max
                            (Watts)                          
--------- ------ ---------- ------- ------------------- ----- ----
Gi0/1     auto   on         12.0    IP Phone 7945       3     15.4
Gi0/2     auto   off        0.0     n/a                 n/a   15.4
Gi0/3     auto   off        0.0     n/a                 n/a   15.4
Gi0/4     auto   off        0.0     n/a                 n/a   15.4
Gi0/5     auto   off        0.0     n/a                 n/a   15.4
Gi0/6     auto   off        0.0     n/a                 n/a   15.4
Gi0/7     auto   on         9.0     IP Phone SPA525G2   0     15.4
Gi0/8     auto   off        0.0     n/a                 n/a   15.4
Gi0/9     auto   off        0.0     n/a                 n/a   15.4
Gi0/10    auto   on         15.4    AIR-AP1042N-E-K9    3     15.4
Gi0/11    auto   off        0.0     n/a                 n/a   15.4
Gi0/12    auto   on         12.0    IP Phone 7945       3     15.4
Gi0/13    auto   off        0.0     n/a                 n/a   15.4
Gi0/14    auto   off        0.0     n/a                 n/a   15.4
Gi0/15    auto   on         12.0    IP Phone 7945       3     15.4
Gi0/16    auto   on         6.3     IP Phone 7941       2     15.4
Gi0/17    auto   on         6.3     IP Phone CP-524S    2     15.4
Gi0/18    auto   off        0.0     n/a                 n/a   15.4
Gi0/19    auto   on         12.0    IP Phone 7945       3     15.4
Gi0/20    auto   on         6.3     IP Phone 7961       2     15.4
Gi0/21    auto   on         12.0    IP Phone 7945       3     15.4
Gi0/22    auto   on         12.0    IP Phone 7945       3     15.4
Gi0/23    auto   off        0.0     n/a                 n/a   15.4
Gi0/24    auto   off        0.0     n/a                 n/a   15.4

Now there's two problems, a lot of switches aren't actually able to provide the whole 15.4W to every port & a lot of devices don't actually consume as much power as their class implies they do. This means that you may encounter situations where you need to power an extra PoE device but the switch has reached its power allocation limit. Fortunately Cisco let you override the automatic power allocation value on a per interface basis, allowing you to squeeze a couple more PoE devices out of the power allocation. The command is power inline consumption:

interface GigabitEthernet0/1
 power inline consumption 4000

Now show power inline now lists interface GigabitEthernet 0/1 as using 4W. Note that the phone is still powered & using however much power it draws, changing the power allocation affects only the switch's record keeping...

Available:370.0(w)  Used:107.3(w)  Remaining:262.7(w)

Interface Admin  Oper       Power   Device              Class Max
                            (Watts)                           
--------- ------ ---------- ------- ------------------- ----- ----
Gi0/1     auto   on         4.0     Ieee PD             3     15.4
Gi0/2     auto   off        0.0     n/a                 n/a   15.4
Gi0/3     auto   off        0.0     n/a                 n/a   15.4
Gi0/4     auto   off        0.0     n/a                 n/a   15.4
Gi0/5     auto   off        0.0     n/a                 n/a   15.4
Gi0/6     auto   off        0.0     n/a                 n/a   15.4
Gi0/7     auto   on         9.0     IP Phone SPA525G2   0     15.4
Gi0/8     auto   off        0.0     n/a                 n/a   15.4
Gi0/9     auto   off        0.0     n/a                 n/a   15.4
Gi0/10    auto   on         15.4    AIR-AP1042N-E-K9    3     15.4
Gi0/11    auto   off        0.0     n/a                 n/a   15.4
Gi0/12    auto   on         12.0    IP Phone 7945       3     15.4
Gi0/13    auto   off        0.0     n/a                 n/a   15.4
Gi0/14    auto   off        0.0     n/a                 n/a   15.4
Gi0/15    auto   on         12.0    IP Phone 7945       3     15.4
Gi0/16    auto   on         6.3     IP Phone 7941       2     15.4
Gi0/17    auto   on         6.3     IP Phone CP-524S    2     15.4
Gi0/18    auto   off        0.0     n/a                 n/a   15.4
Gi0/19    auto   on         12.0    IP Phone 7945       3     15.4
Gi0/20    auto   on         6.3     IP Phone 7961       2     15.4
Gi0/21    auto   on         12.0    IP Phone 7945       3     15.4
Gi0/22    auto   on         12.0    IP Phone 7945       3     15.4
Gi0/23    auto   off        0.0     n/a                 n/a   15.4
Gi0/24    auto   off        0.0     n/a                 n/a   15.4

You can disable PoE completey via power inline never or restore automatic classification via power inline auto.

WARNING! You can cause the switch's power supply to become oversubscribed by forcing it power more devices than it can handle. This will result in brown outs where devices randomly don't receive sufficient power to function properly.