Figure 27. Illustration of Traffic CongestionThe system supports loading two DCB_Config files:• FCoE converged traffic with priority 3.• iSCSI storage traffic with priority 4.In the Dell Networking OS, PFC is implemented as follows:• PFC is supported on specified 802.1p priority traffic (dot1p 0 to 7) and is configured per interface. However, only two lossless queuesare supported on an interface: one for Fibre Channel over Ethernet (FCoE) converged traffic and one for Internet Small ComputerSystem Interface (iSCSI) storage traffic. Configure the same lossless queues on all ports.• PFC supports buffering to receive data that continues to arrive on an interface while the remote system reacts to the PFC operation.• PFC uses DCB MIB IEEE 802.1azd2.5 and PFC MIB IEEE 802.1bb-d2.2.• A dynamic threshold handles intermittent traffic bursts and varies based on the number of PFC priorities contending for buffers, while astatic threshold places an upper limit on the transmit time of a queue after receiving a message to pause a specified priority. PFC trafficis paused only after surpassing both static and dynamic thresholds for the priority specified for the port.• By default, PFC is enabled when you enable DCB. If you have not loaded FCoE_DCB_Config and iSCSI_DCB_Config, DCB is disabled.When you enable DCB globally, you cannot simultaneously enable link-level flow control.• Buffer space is allocated and de-allocated only when you configure a PFC priority on the port.Enhanced Transmission SelectionEnhanced transmission selection (ETS) supports optimized bandwidth allocation between traffic types in multiprotocol (Ethernet, FCoE,SCSI) links.ETS allows you to divide traffic according to its 802.1p priority into different priority groups (traffic classes) and configure bandwidthallocation and queue scheduling for each group to ensure that each traffic type is correctly prioritized and receives its required bandwidth.For example, you can prioritize low-latency storage or server cluster traffic in a traffic class to receive more bandwidth and restrict best-effort LAN traffic assigned to a different traffic class.NOTE: Use the following command to enable etsacl: cam-acl l2acl 2 ipv4acl 2 ipv6acl 0 ipv4qos 0 l2qos 0l2pt 0 ipmacacl 0 vman-qos 0 fcoeacl 2 etsacl 3. After executing this command, you must save theconfiguration and then reload the system.The following figure shows how ETS allows you to allocate bandwidth when different traffic types are classed according to 802.1p priorityand mapped to priority groups.254 Data Center Bridging (DCB)
