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Configuration Guide - DCN and Server Management

CloudEngine 8800, 7800, 6800, and 5800 V200R003C00

This document describes the configurations of Trill, FCoE, DCB, and NLB Server Cluster Association.
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PFC

PFC

Background

SAN traffic is sensitive to packet loss on a converged network.

The Ethernet Pause mechanism ensures the lossless transmission service. When a downstream device detects that its receive capability is lower than the transmit capability of its upstream device, it sends Pause frames to the upstream device, requesting the upstream device to stop sending traffic for a period of time. The Ethernet Pause mechanism stops all traffic on a link, whereas FCoE requires link sharing:
  • Burst traffic of one type cannot affect forwarding of traffic of other types.
  • A large amount of traffic of one type in a queue cannot occupy buffer resources of traffic of other types.

Priority-based Flow Control (PFC) addresses the contradiction between the Ethernet Pause mechanism and link sharing.

Principles

PFC is also called Per Priority Pause or Class Based Flow Control (CBFC). It enhances the Ethernet Pause mechanism. PFC is a kind of flow control mechanism based on priority. As shown in Figure 3-1, eight priority queues on the transmit interface of DeviceA correspond to eight buffers on the receive interface of DeviceB. When a receive buffer on DeviceB is congested, DeviceB sends a backpressure signal to DeviceA, requesting DeviceA to stop sending packets in the corresponding priority queue.

Figure 3-1 PFC working mechanism
Table 3-1 describes the mappings between packet priorities and interface queues.
Table 3-1 Mappings between packet priorities and interface queues

Packet Type

Priority

Queue

Unicast

0

0

1

1

2

2

3

3

4

4

5

5

6

6

7

7

Unknown unicast

NOTE:

On the CE8860EI, CE8850EI, CE7855EI, CE7850EI, CE6860EI, CE6850HI, CE6856HI, CE6855HI, CE6851HI, and CE6850U-HI, one priority corresponds to one queue.

0

0

1, 2, 3

1

4 and 5

2

6 and 7

6

A backpressure signal is an Ethernet frame. Figure 3-2 shows the PFC frame format.
Figure 3-2 PFC frame format
Table 3-2 Fields in a PFC frame

Item

Description

Destination address

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

Source address

Source MAC address.

Ethertype

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 and the backpressure time is Time(n). When E(n) is 0, backpressure is disabled for queue n.

Time(0) to Time(7)

Backpressure timer.

If Time(n) is 0, backpressure is canceled.

Pad(transmit as zero)

Reserved.

The value is 0 during PFC frame transmission.

CRC

Cyclic Redundancy Check (CRC).

When receiving backpressure signals, a device only stops traffic in one or several priority queues, but does not stop traffic on the entire interface. PFC can pause or restart any queue, without interrupting traffic in other queues. This feature enables traffic of various types to share one FCoE link. The system does not apply the backpressure mechanism to the priority queues with PFC disabled and directly discards packets in these queues when congestion occurs.

In an FCoE environment, an administrator can apply PFC to queues of FCoE traffic to ensure lossless transmission of FCoE service.

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Updated: 2019-05-08

Document ID: EDOC1100004349

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