Priority-Based Flow Control Configuration (S-, 7100-Series)

Priority-based Flow Control (PFC), as defined by 802.1Qbb, functions similarly to 802.3 PAUSE, but allows for per-priority pausing instead of per-port pausing. This is achieved by pausing the egress queues to which the priorities are mapped. PFC can be enabled for up to 6 priorities on the 7100‐Series and up to 2 priorities on supported S‐Series platforms.

PFC is supported on all 7100‐Series and the S140/S180 and SSA180/SSA150A platforms.

Traffic congestion is determined on a per-queue basis. When the ingress queue that maps to a particular 802.1p priority reaches a certain (non-configurable) threshold, a priority-based flow control message is sent to the peer to pause transmission of traffic tagged with that priority. Once queue buffer levels return to normal, the ingress queue stops sending priority-based flow control messages, and the peer no longer pauses traffic tagged with that priority.

Priority-based flow control is defined only for a pair of full duplexed MAC devices connected by one point-to-point link. An egress queue is paused by a switch when it receives a message from its peer on the other end of the link that priority-tagged frames mapped to the queue should be paused.

Flows are paused by egress queue, not by priority. If non-PFC priorities are mapped to the same egress queue as PFC priorities, the non-PFC priority data will be paused along with the PFC priority data. For example, if priorities 3 and 6 are set to egress on queue 5 and only priority 3 is enabled for priority-based flow control, when priority 3 is paused, queue 5 will stop transmission and priority 6 will be paused as well.

A buffer allowance is provided for storing incoming traffic between the time a priority-based flow control frame is sent back to the peer and the peer pauses the PFC priority traffic, preventing data loss when initiating priority-based flow control. This buffer amount is called link delay allowance, and is measured in number of bits.

The default priority used primarily for control traffic should not be set for priority-based flow control. Untagged packets bypass egress queues and transmit out with the default priority traffic.

Note

Priority-based flow control and 802.3 PAUSE are mutually exclusive. Enabling one feature automatically disables the other. Enabling PFC both clears PAUSE advertisement and prohibits PAUSE advertisement while PFC is enabled.

Use the set dcb pfc command to enable priority-based flow control on a port, specifying the port and 802.1p priority.

You can configure LLDP to add the LLDP-DCB Priority-Flowctrl TLV to LLDP PDUs transmitted on a port using the set lldp port tx-tlv priority-flowctrl command.

This example shows how to enable priority-based flow control for priority 5 on port ge.1.2 and advertise the setting to its peer:

System(rw)->set dcb pfc ge.1.2 5 enable
System(rw)->set lldp port tx-tlv priority-flowctrl ge.1.2