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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 dcb-map 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 dcb-map with PFC disabled (no pfc mode on):
● You can enable link-level flow control on the interface. To delete the dcb-map, 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 a dcb-map 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 MAP 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 a dcb-map or reapply the dcb-map to an interface.
NOTE:
All these configurations are available only in PMUX mode and you cannot perform these configurations in
Standalone mode.
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 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:
Data Center Bridging (DCB)
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