Users Guide

9 Repeat Steps 1 to 8 on all PFC-enabled peer interfaces to ensure lossless traffic 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 configurations that are received in TLVs from peer devices.

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

of traffic, also enable PFC on all DCB egress ports or configure the dot1p priority-queue assignment of PFC priorities to lossless queues.

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

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

Configuration 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 traffic is mapped are lossless by default. Traffic may

be interrupted due to an interface flap (going down and coming up) when you reconfigure 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 configured priority traffic (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 flow control on the interface. To delete the input policy, first disable link-level flow control. PFC is then

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

• PFC still allows you to configure lossless queues on a port to ensure no-drop handling of lossless traffic.

NOTE

: You cannot enable PFC and link-level flow 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 flow control is already enabled. You cannot be enable PFC and link-level flow control at the same time on an interface.

• In a switch stack, configure all stacked ports with the same PFC configuration.

A DCB input policy for PFC applied to an interface may become invalid if you reconfigure 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 configurations received from PFC-enabled peers are removed and resynchronized with the peer devices.

Traffic may be interrupted when you reconfigure PFC no-drop priorities in an input policy or reapply the policy to an interface.

How Priority-Based Flow Control is Implemented

Priority-based flow control provides a flow control mechanism based on the 802.1p priorities in converged Ethernet traffic received on an

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

specified priorities (CoS values) without impacting other priority classes. Different traffic types are assigned to different priority classes.

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

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

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

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 configurations received in TLVs

from peer devices.

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

Data Center Bridging (DCB)