Users Guide

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.

• All 802.1p priorities are enabled 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).

• For PFC to be applied on an Aggregator port, the auto-configured priority traffic must be supported

by a PFC peer (as detected by DCBx).

• A dcb-map for PFC applied to an interface may become invalid if dot1p-queue mapping is

reconfigured. 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 re-synchronized with the peer devices.

• Dell Networking OS does not support MACsec Bypass Capability (MBC).

Configuring Enhanced Transmission Selection

ETS provides a way to optimize bandwidth allocation to outbound 802.1p classes of converged Ethernet

traffic.

Different traffic types have different service needs. Using ETS, you can create groups within an 802.1p

priority class to configure different treatment for traffic with different bandwidth, latency, and best-effort

needs.

For example, storage traffic is sensitive to frame loss; interprocess communication (IPC) traffic is latency-

sensitive. ETS allows different traffic types to coexist without interruption in the same converged link by:

• Allocating a guaranteed share of bandwidth to each priority group.

• Allowing each group to exceed its minimum guaranteed bandwidth if another group is not fully using

its allotted bandwidth.

To configure ETS and apply an ETS output policy to an interface, you must:

1. Create a Quality of Service (QoS) output policy with ETS scheduling and bandwidth allocation

settings.

Data Center Bridging (DCB)