Brocade Communications Systems 53-1001761-01 manual Rewriting, Queueing, User-priority mapping

9 Rewriting

•Scheduling—When multiple queues are active and contending for output on a common physical port the scheduling algorithm selects the order the queues are serviced. Scheduling algorithms include Strict Priority (SP) and Deficit Weighted Round Robin (DWRR) queueing. The scheduler supports a hybrid policy combining SP and DWRR servicing. Under a hybrid scheduler configuration, the highest priority queues are serviced by SP while lower priority queues share the remaining bandwidth using the DWRR service.

•Converged Enhanced Ethernet—CEE describes an enhanced Ethernet that will enable convergence of various applications in data centers (LAN, SAN, and IPC) onto a single interconnect technology.

Rewriting

Rewriting a frame header field is typically performed by an edge device. Rewriting occurs on frames as they enter or exit a network because the neighboring device is untrusted, unable to mark the frame, or is using a different QoS mapping.

The frame rewriting rules set the Ethernet CoS and VLAN ID fields. Egress Ethernet CoS rewriting is based on the user-priority mapping derived for each frame as described later in the queueing section.

Queueing

Queue selection begins by mapping an incoming frame to a configured user priority, then each user-priority mapping is assigned to one of the switch’s eight unicast traffic class queues or one of the four multicast traffic class queues.

NOTE

You need to enter the copy running-configstartup-configcommand to save your configuration changes to NVRAM so that they are not lost if there is a system reload or power outage.

User-priority mapping

There are several ways an incoming frame can be mapped into a user-priority. If the neighboring devices are untrusted or unable to properly set QoS, then the interface is considered untrusted. All traffic must be user-priority mapped using explicit policies for the interface to be trusted; if it is not mapped in this way, the iEEE 802.1Q default-priority mapping is used. If an interface is trusted to have QoS set then the CoS header field can be interpreted.

NOTE

The user priority mapping described in this section applies to both unicast and multicast traffic.

Default user-priority mappings for untrusted interfaces

When Layer 2 QoS trust is set to untrusted then the default is to map all Layer 2 switched traffic to the port default user priority value of 0 (best effort), unless configured to a different value.

92	Converged Enhanced Ethernet Administrator’s Guide
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