Brocade Network OS Administrator’s Guide v4.1.1 - Manual (Page 344)

Congestion control and queuing
The Brocade VDX hardware supports several congestion control and queuing strategies. As an output
queue approaches congestion, Random Early Detection (RED) is used to selectively and proactively
drop frames to maintain maximum link utilization. Incoming frames are classified into priority queues
based on the Layer 2 CoS setting of the incoming frame, or the possible rewriting of the Layer 2 CoS
field based on the settings of the DCB port or VLAN.
The Brocade VDX hardware supports a combination of two scheduling strategies to queue frames to
the egress ports: Priority queuing, which is also referred to as strict priority, and Deficit Weighted
Round Robin (DWRR) queuing.
The scheduling algorithms work on the eight traffic classes as specified in 802.1Qaz Enhanced
Transmission Selection (ETS).
Queuing features are described as follows:
• RED — RED increases link utilization. When multiple inbound TCP traffic streams are switched to
the same outbound port, and some traffic streams send small frames while other traffic streams
send large frames, link utilization will not be able to reach 100 percent. When RED is enabled, link
utilization approaches 100 percent.
• Classification — Setting user priority.
‐ Inbound frames — Inbound frames are tagged with the user priority set for the inbound
port. The tag is visible when examining the frames on the outbound port. By default, all
frames are tagged to priority zero.
‐ Externally tagged Layer 2 frames — When the port is set to accept externally tagged Layer
2 frames, the user priority is set to the Layer 2 CoS of the inbound frames.
• Queuing
‐ Input queuing — Input queuing optimizes the traffic flow in the following way. A DCB port
has inbound traffic that is tagged with several priority values, and traffic from different
priority settings is switched to different outbound ports. Some outbound ports are already
congested with background traffic while others are uncongested. With input queuing, the
traffic rate of the traffic streams switched to uncongested ports should remain high.
‐ Output queuing — Output queuing optimizes the traffic flow in the following way. Several
ports carry inbound traffic with different priority settings. Traffic from all ports is switched to
the same outbound port. If the inbound ports have different traffic rates, some outbound
priority groups will be congested while others can remain uncongested. With output
queuing, the traffic rate of the traffic streams that are uncongested should remain high.
‐ Multicast rate limit — A typical multicast rate limiting example is where several ports carry
multicast inbound traffic that is tagged with several priority values. Traffic with different
priority settings is switched to different outbound ports. The multicast rate limit is set so
that the total multicast traffic rate on output ports is less than the specified set rate limit.
Multicast rate-limiting in global configuration mode is supported only on the Brocade VDX
6710, VDX 6720, and VDX 6730. Multicast rate-limiting commands are not supported on
the Brocade VDX 6740 or VDX 8770. On the latter platforms, use BUM storm control
instead.
‐ Multicast input queuing — A typical multicast input queuing example is where several ports
carry multicast inbound traffic that is tagged with several priority values. Traffic with
different priority settings is switched to different outbound ports. Some outbound ports are
already congested with background traffic while others are uncongested. The traffic rate of
the traffic streams switched to the uncongested ports should remain high. All outbound
ports should carry some multicast frames from all inbound ports. This enables multicast
traffic distribution relative to the set threshold values.
‐ Multicast output queuing — A typical multicast output queuing example is where several
ports carry multicast inbound traffic. Each port has a different priority setting. Traffic from
all ports is switched to the same outbound port. If the inbound ports have varying traffic
rates, some outbound priority groups will be congested while others remain uncongested.
Congestion control and queuing
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