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Troubleshooting Guide

CloudEngine 16800, 12800, 12800E, 8800, 7800, 6800, and 5800 Series Switches

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How the Backpressure Timer Value and the Number of Required PFC Frames Are Calculated?

How the Backpressure Timer Value and the Number of Required PFC Frames Are Calculated?

PFC is a Priority-based Flow Control mechanism. After DCB is enabled on the device and negotiation is successful, the device sends a backpressure signal to a queue where congestion occurs.

The backpressure timer value and number of required PFC frames are calculated as follows:

  • Backpressure timer value = 512 x Time(n)/Interface bandwidth

  • Number of required PFC frames = Interface bandwidth/(512 x Time(n))

Figure 22-133 shows the PFC frame format.
Figure 22-133 PFC frame format
Table 22-96 Fields in a PFC frame



Destination address

Destination MAC address, which has a fixed value of 01-80-c2-00-00-01.

Source address

Source MAC address.


Ethernet frame type. The value is 88-08.

Control opcode

Control code. The value is 01-01.

Priority enable vector

Priority-enable vector. E(n) corresponds to queue n and determines whether backpressure is enabled for queue n.

  • When E(n) is 1, backpressure is enabled for queue n.

  • When E(n) is 0, backpressure is disabled for queue n.


Backpressure timer. The unit is the time at which an interface sends 512 bits. If Time(n) is 0, backpressure is canceled.

Pad(transmit as zero)


The value of this field is 0 during PFC frame transmission.


Cyclic Redundancy Check (CRC).

Updated: 2020-01-07

Document ID: EDOC1000060766

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