Deployment Guide

9 Repeat Steps 1 to 8 on all PFC-enabled peer interfaces to ensure lossless trac service.

Dell Networking OS Behavior: As soon as you apply a DCB policy with PFC enabled on an interface, DCBx starts exchanging information

with PFC-enabled peers. The IEEE802.1Qbb, CEE, and CIN versions of PFC Type, Length, Value (TLV) are supported. DCBx also validates

PFC congurations that are received in TLVs from peer devices.

By applying a DCB input policy with PFC enabled, you enable PFC operation on ingress port trac. To achieve complete lossless handling

of trac, also enable PFC on all DCB egress ports or congure the dot1p priority-queue assignment of PFC priorities to lossless queues.

To remove a DCB input policy, including the PFC conguration it contains, use the no dcb-input policy-name command in

INTERFACE Conguration mode. To disable PFC operation on an interface, use the no pfc mode on command in DCB Input Policy

Conguration mode. PFC is enabled and disabled as the global DCB operation is enabled (dcb enable) or disabled (no dcb enable).

You can enable any number of 802.1p priorities for PFC. Queues to which PFC priority trac is mapped are lossless by default. Trac may

be interrupted due to an interface ap (going down and coming up) when you recongure the lossless queues for no-drop priorities in a

PFC input policy and reapply the policy to an interface.

To apply PFC, a PFC peer must support the congured priority trac (as detected by DCBx).

To honor a PFC pause frame multiplied by the number of PFC-enabled ingress ports, the minimum link delay must be greater than the

round-trip transmission time the peer requres.

If you apply an input policy with PFC disabled (no pfc mode on):

• You can enable link-level ow control on the interface. To delete the input policy, rst disable link-level ow control. PFC is then

automatically enabled on the interface because an interface is by default PFC-enabled.

• PFC still allows you to congure lossless queues on a port to ensure no-drop handling of lossless trac.

NOTE

: You cannot enable PFC and link-level ow control at the same time on an

interface.

When you apply an input policy to an interface, an error message displays if:

• The PFC dot1p priorities result in more than two lossless port queues globally on the switch.

• Link-level ow control is already enabled. You cannot be enable PFC and link-level ow control at the same time on an interface.

• In a switch stack, congure all stacked ports with the same PFC conguration.

A DCB input policy for PFC applied to an interface may become invalid if you recongure dot1p-queue mapping. This situation occurs when

the new dot1p-queue assignment exceeds the maximum number (2) of lossless queues supported globally on the switch. In this case, all

PFC congurations received from PFC-enabled peers are removed and resynchronized with the peer devices.

Trac may be interrupted when you recongure PFC no-drop priorities in an input policy or reapply the policy to an interface.

How Priority-Based Flow Control is Implemented

Priority-based ow control provides a ow control mechanism based on the 802.1p priorities in converged Ethernet trac received on an

interface and is enabled by default. As an enhancement to the existing Ethernet pause mechanism, PFC stops trac transmission for

specied priorities (CoS values) without impacting other priority classes. Dierent trac types are assigned to dierent priority classes.

When trac congestion occurs, PFC sends a pause frame to a peer device with the CoS priority values of the trac that needs to be

stopped. DCBx provides the link-level exchange of PFC parameters between peer devices. PFC creates zero-loss links for SAN trac that

requires no-drop service, while at the same time retaining packet-drop congestion management for LAN trac.

PFC is implemented on an Aggregator as follows:

• If DCB is enabled, as soon as a dcb-map with PFC is applied on an interface, DCBx starts exchanging information with PFC-enabled

peers. The IEEE802.1Qbb, CEE and CIN versions of PFC TLV are supported. DCBx also validates PFC congurations received in TLVs

from peer devices.

• To achieve complete lossless handling of trac, enable PFC operation on ingress port trac and on all DCB egress port trac.

Data Center Bridging (DCB)