Nortel BES50FE-12T PWR, User Manual

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148 BES50 advanced features fundamentals
packets in a high-priority port queue are transmitted before those in the
lower priority queues. You can set the default priority for each interface and
configure the mapping of frame priority tags to the switch priority queues.
Default priority for interfaces
You can specify the default port priority for each interface on the switch.
All untagged packets entering the switch are tagged with the specified
default port priority and then sorted into the appropriate priority queue at
the output port.
The switch provides four priority queues for each port. It uses Weighted
Round Robin to prevent head-of-queue blockage.
The default priority applies for an untagged frame received on a port set to
accept all frame types (the port receives both untagged and tagged frames).
This priority does not apply to IEEE 802.1Q VLAN tagged frames. If the
incoming frame is an IEEE 802.1Q VLAN tagged frame, the IEEE 802.1p
User Priority bits are used.
If the output port is an untagged member of the associated VLAN, these
frames are stripped of all VLAN tags prior to transmission.
CoS values and egress queues
The switch processes Class of Service (CoS) priority tagged traffic by
using four priority queues for each port, with service schedules based on
Weighted Round Robin (WRR). Up to eight separate traffic priorities are
defined in IEEE 802.1p. The default priority levels are assigned according to
recommendations in the IEEE 802.1p standard as shown in the
"Mapping
CoS values to egress queues table" (page 78) .
The priority levels recommended in the IEEE 802.1p standard for various
network applications are shown in the
"CoS priority levels table" (page 78) .
However, you can map the priority levels to the switch output queues in any
way that benefits application traffic for your own network.
Weighted Round-Robin (WRR) queuing
You can set the switch to service the queues based on a strict rule that
requires all traffic in a higher priority queue to be processed before lower
priority queues are serviced, or you can use WRR queuing that specifies
a relative weight of each queue. WRR uses a predefined relative weight
for each queue that determines the percentage of service time the switch
services each queue before moving on to the next queue. This prevents the
head-of-line blocking that can occur with strict priority queuing.
SMB
Using the Nortel Business Ethernet Switch 50 Series
NN47924-301 01.01 Standard
1.00 October 2006
Copyright © 2006, Nortel Networks Nortel Networks Confidential
.