PoE в числах

По команде show power inline на коммутаторах Cisco отображается текущее потребление и вычисляется доступный остаток. Если телефон не поддерживает cdp,lldp или коммутатор не настроен, то на коммутаторе показывается потребление согласно классам энергопотребления, например, 7, 15.4 для второго и третьего класса соответственно.

Телефоны	без LLDP, без CDP	только LLDP	только CDP
Nortel 2001 Class 2	7	4.8	7
Nortel 2004 Class 2 + PC-порт	7	5.4	7
Nortel 1110E Class 2 + PC-порт	7	6	7
Nortel 1020E Class 3 + PC-порт	15.4	9.6	15.4
Cisco 7911G Class 2 + PC-порт	7	5	5
Cisco 7970G Class 2 + PC-порт	15.4	не тестировал	10.3
Cisco 7975G Class 2 + PC-порт	15.4	не тестировал	12

Какая мощность зарезервирована под PoE на коммутаторах

Модель	Доступная мощность, Вт
Cisco 3750 24PS	370
Cisco 3750 48PS	370
Cisco 3750G 48PS	370
Cisco 3750E 48PD-S	420
Cisco 3750E 48PD-SF	800
Juniper EX2200-48P-4G	405
Juniper EX3200-48P-4G	?? (PoE+ 930Вт БП)
Juniper EX4200-48P-4G	?? (PoE+ 930Вт БП)

Попутно выяснилось, что телефон Cisco при включении потребляет 6.3Вт.

Cisco Discovery Protocol and PoE

CDP (Cisco Discovery Protocol) is a Cisco-proprietary Layer 2 protocol that runs on most Cisco equipment and shares information about directly connected Cisco equipment.

Cisco PoE switches using CDP can recognize Cisco powered devices such as IP phones and wireless access points. The actual power requirement can be advertised by the powered device, and the unused class power is returned to the switch power budget.

To use CDP for powered device discovery, enable it on the switch and on the PoE ports. (CDP is typically enabled by default.)

When a Cisco powered device (phone or WAP) powers on, CDP sends a message that tells the switch how much power the powered device actually needs. The switch CPU adjusts the power allocation for the port and adjusts the power budget.

Without CDP, if a powered device is connected but the class is 0 (default) or cannot be identified, the switch must reserve the maximum per-port PoE for the powered device. This can quickly deplete the power budget and, in a worst case, could result in a total artificial power budget depletion. Then the switch might not allocate power to a detected powered device even though power is still available.

For example, if a non-Cisco IEEE 802.3af Class 3 powered device is detected which requires only 9 W, but the actual requirement is not known by the PoE switch, the switch must initially budget the full per-port class power of 15.4 W even though the device will use only 9 W. This wastes 6.4 W, and this quickly adds as multiple similar devices are connected to the switch. An improper classification signature in the powered device can cause the same problem. This might prevent all intended devices from receiving power. Catalyst 3750, 3560, and newer switches monitor the actual per-port power used after powered device detection is complete and PoE is applied.

A Cisco IEEE+CDP powered device such as the 7970G IP phone initially powers up in low-power mode (6.3 W), then sends a CDP message with the actual power requirements of the device. If the required power is less than the default 15.4 W for a Class-3 device, the switch updates its power budget. If the requested power exceeds the power budget available for the switch, power is either be denied, or the port remains in low-power mode (typically 7 W).

Другими словами с cdp, lldp имеем…

Switch-1#sh power inline | ex n/a

Module   Available     Used     Remaining
          (Watts)     (Watts)    (Watts)
------   ---------   --------   ---------
1           370.0       70.4       299.6
Interface Admin  Oper       Power   Device              Class Max
                            (Watts)
--------- ------ ---------- ------- ------------------- ----- ----
Fa1/0/2   auto   on         5.0     IP Phone 7911       2     15.4
Fa1/0/3   auto   on         5.0     IP Phone 7911       2     15.4
Fa1/0/6   auto   on         5.4     Ieee PD             2     15.4
Fa1/0/7   auto   on         9.6     Ieee PD             3     15.4
Fa1/0/9   auto   on         5.0     Ieee PD             2     15.4
Fa1/0/13  auto   on         4.8     Ieee PD             2     15.4
Fa1/0/15  auto   on         5.4     Ieee PD             2     15.4
Fa1/0/16  auto   on         5.4     Ieee PD             2     15.4
Fa1/0/17  auto   on         4.8     Ieee PD             2     15.4
Fa1/0/20  auto   on         5.0     IP Phone 7911       2     15.4
Fa1/0/21  auto   on         5.0     IP Phone 7911       2     15.4
Fa1/0/22  auto   on         5.0     IP Phone 7911       2     15.4
Fa1/0/23  auto   on         5.0     IP Phone 7911       2     15.4

Без lldp

Switch-2#sh power inline | ex n/a

Module   Available     Used     Remaining
          (Watts)     (Watts)    (Watts)
------   ---------   --------   ---------
2           370.0       91.0       279.0
Interface Admin  Oper       Power   Device              Class Max
                            (Watts)
--------- ------ ---------- ------- ------------------- ----- ----
Fa2/0/1   auto   on         7.0     Ieee PD             2     15.4
Fa2/0/4   auto   on         7.0     Ieee PD             2     15.4
Fa2/0/5   auto   on         7.0     Ieee PD             2     15.4
Fa2/0/8   auto   on         7.0     Ieee PD             2     15.4
Fa2/0/9   auto   on         7.0     Ieee PD             2     15.4
Fa2/0/11  auto   on         7.0     Ieee PD             2     15.4
Fa2/0/12  auto   on         7.0     Ieee PD             2     15.4
Fa2/0/13  auto   on         7.0     Ieee PD             2     15.4
Fa2/0/14  auto   on         7.0     Ieee PD             2     15.4
Fa2/0/15  auto   on         7.0     Ieee PD             2     15.4
Fa2/0/16  auto   on         7.0     Ieee PD             2     15.4
Fa2/0/18  auto   on         7.0     Ieee PD             2     15.4
Fa2/0/24  auto   on         7.0     Ieee PD             2     15.4