60 | Data Center Bridging (DCB)w w w . d e l l . c o m | s u p p o r t . d e l l . c o mHow Priority-Based Flow Control is ImplementedPriority-based flow control provides a flow control mechanism based on the 802.1p priorities in convergedEthernet traffic received on an interface and is enabled by default. As an enhancement to the existingEthernet pause mechanism, PFC stops traffic transmission for specified priorities (CoS values) withoutimpacting 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 ofthe traffic that needs to be stopped. DCBX provides the link-level exchange of PFC parameters betweenpeer devices. PFC creates zero-loss links for SAN traffic that requires no-drop service, while at the sametime 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 policy with PFC is applied on an interface, DCBX startsexchanging information with PFC-enabled peers. The IEEE802.1Qbb, CEE and CIN versions of PFCTLV are supported. DCBX also validates PFC configurations received in TLVs from peer devices.• To achieve complete lossless handling of traffic, enable PFC operation is enabled on ingress porttraffic and enabled on all DCB egress port traffic.• All 802.1p priorities are enabled for PFC. Queues to which PFC priority traffic is mapped are losslessby 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 bya PFC peer (as detected by DCBX).• A DCB input policy for PFC applied to an interface may become invalid if dot1p-queue mapping isreconfigured (refer to Create Input Policy Maps). This situation occurs when the new dot1p-queueassignment exceeds the maximum number (2) of lossless queues supported globally on the switch. Inthis case, all PFC configurations received from PFC-enabled peers are removed and re-synchronizedwith the peer devices.• FTOS does not support MACsec Bypass Capability (MBC).
