Brocade Network OS Administrator’s Guide v4.1.1 - Manual (Page 472)

LOG indication for the disabled interface. This feature is supported on Brocade VDX 8770 series,
VDX 6740, and VDX 6740-T platforms.
• Data Center Bridging. DCB 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 eight
multicast traffic class queues.
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.
In standalone mode:
• All incoming priority 7 tagged packets are counted in queue 7 (TC7).
• Untagged control frames are counted in queue 7 (TC7).
NOTE
The user priority mapping discussed in this chapter applies to both unicast and multicast traffic.
Congestion control
Queues can begin filling up for a number of reasons, such as over subscription of a link or
backpressure from a downstream device. Sustained, large queue buildups generally indicate
congestion in the network and can affect application performance through increased queuing delays
and frame loss.
Congestion control covers features that define how the system responds when congestion occurs or
active measures taken to prevent the network from entering a congested state.
NOTE
You cannot configure CoS thresholds and multicast tail drop on Brocade VDX 8770-4 and VDX 8770-8
platforms. Random Early Discard (RED) is supported only on Brocade VDX 6740, VDX 8770-4 and
VDX 8770-8 platforms.
Rewriting
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