Administrator Guide

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 Lossless Queues
DCB also supports the manual configuration of lossless queues on an interface when PFC mode is turned off
and priority classes are disabled in a DCB map, apply the map on the interface.
Prerequisite: A DCB input policy with PFC configuration is applied to the interface with the following
conditions:
• PFC mode is off (no pfc mode on).
• No PFC priority classes are configured (no pfc priority priority-range).
Example:
Port A —> Port B
Port C —> Port B
PFC no-drop queues are configured for queues 1, 2 on Port B. PFC capability is enabled on priorities 3, 4 on
PORT A and C.
Port B acting as Egress
During the congestion, [traffic pump on priorities 3 and 4 from PORT A and PORT C is at full line rate], PORT
A and C send out the PFCs to rate the traffic limit. Egress drops are not observed on Port B since traffic flow
on priorities is mapped to loss less queues.
Port B acting as Ingress
If the traffic congestion is on PORT B , Egress DROP is on PORT A or C, as the PFC is not enabled on PORT B.
Data Center Bridging (DCB) 287