94 Dell Converged Enhanced Ethernet Administrator’s Guide53-1002116-01Rewriting10• Scheduling—When multiple queues are active and contending for output on a commonphysical port the scheduling algorithm selects the order the queues are serviced. Schedulingalgorithms include Strict Priority (SP) and Deficit Weighted Round Robin (DWRR) queueing. Thescheduler supports a hybrid policy combining SP and DWRR servicing. Under a hybridscheduler configuration, the highest priority queues are serviced by SP while lower priorityqueues share the remaining bandwidth using the DWRR service.• Converged Enhanced Ethernet—CEE describes an enhanced Ethernet that will enableconvergence of various applications in data centers (LAN, SAN, and IPC) onto a singleinterconnect technology.RewritingRewriting a frame header field is typically performed by an edge device. Rewriting occurs on framesas they enter or exit a network because the neighboring device is untrusted, unable to mark theframe, or is using a different QoS mapping.The frame rewriting rules set the Ethernet CoS and VLAN ID fields. Egress Ethernet CoS rewriting isbased on the user-priority mapping derived for each frame as described later in the queueingsection.QueueingQueue selection begins by mapping an incoming frame to a configured user priority, then eachuser-priority mapping is assigned to one of the switch’s eight unicast traffic class queues or one ofthe four multicast traffic class queues.NOTEYou need to enter the copy running-config startup-config command to save your configurationchanges to NVRAM so that they are not lost if there is a system reload or power outage.User-priority mappingThere are several ways an incoming frame can be mapped into a user-priority. If the neighboringdevices are untrusted or unable to properly set QoS, then the interface is considered untrusted. Alltraffic must be user-priority mapped using explicit policies for the interface to be trusted; if it is notmapped in this way, the iEEE 802.1Q default-priority mapping is used. If an interface is trusted tohave QoS set then the CoS header field can be interpreted.NOTEThe user priority mapping described in this section applies to both unicast and multicast traffic.Default user-priority mappings for untrusted interfacesWhen Layer 2 QoS trust is set to untrusted then the default is to map all Layer 2 switched traffic tothe port default user priority value of 0 (best effort), unless configured to a different value.
