Hi,
Now that I have LLDP working on my board, I can request more power from a cisco port
with the DTE Power via MDI TLV.
This TLV has two 16bits fields PDrequest and PSEallocated.
It would have been great if those fields, when in the LLDP packet from a consumer (eg. PD),
were used for
power request (in PDrequest) and max power request (in PSEallocated),
so the PSE can set aside a max budget for the port.and grant the request.
The two fields, when in the LLDP packet from the source (eg. PSE),
were used for
power budget (in PDrequest) and power allocated (in PSallocated).
The power budget is a dynamic value and depending on policy, priority and attached
poe devices to the switch, the power buget for the port could be decreased or
increased (increase to max request only).
That way the PD always knows the amoount of power that can be requested.
An excerpt from
http://www.cisco.com/en/US/docs/switches/lan/catalyst3750x_3560x/software/release/12.2_55_se/configuration/guide/swlldp.html
When LLDP is enabled and power is applied to a port, the power TLV determines the actual power requirement of the endpoint device so that the system power budget can be adjusted accordingly. The switch processes the requests and either grants or denies power based on the current power budget. If the request is granted, the switch updates the power budget. If the request is denied, the switch turns off power to the port, generates a syslog message, and updates the power budget. If LLDP-MED is disabled or if the endpoint does not support the LLDP-MED power TLV, the initial allocation value is used throughout the duration of the connection.
The bold line worries me.
What if I were to request 51W (UPOE) after powerup (board gets 13W at powerup), then when connected to a Poe+ port which only can deliver 30W,
the port shuts down, whereas I would like the port to just allocate the power that is available and inform the board about that via
the granted power value in the LLDP packet. The board can then disable parts of the board to limit power consumption.
Can I find out the amount of power the board can use, in which case I would never request more power than can be granted, so the port
never shuts down?
regards peter
