Owner manual

Chapter 13: Quality of Service
148 Section II: Advanced Operations
Packet Processing
You can use the switch’s QoS tools to perform any combination of the
following functions on a packet flow:
 Limiting bandwidth
 Prioritizing packets to determine the level of precedence the switch will
give to the packet for processing
 Replacing the VLAN tag User Priority to enable the next switch in the
network to process the packet correctly
 Replacing the TOS precedence or DSCP value to enable the next
switch in the network to process the packet correctly.
Bandwidth Allocation
Bandwidth limiting is configured at the level of traffic classes, and
encompasses the flow groups contained in the traffic class. Traffic classes
can be assigned maximum bandwidths, specified in kbps, Mbps, or Gbps.
Packet Prioritization
The switch has eight Class of Service (CoS) egress queues, numbered
from 0 to 7. Queue 7 has the highest priority. When the switch becomes
congested, it gives high priority queues precedence over lower-priority
queues. When the switch has information about a packet’s priority, it
sends the packet to the appropriate queue. You can specify the queue
where the switch sends traffic, how much precedence each queue has,
and whether priority remapping is written into the packet’s header for the
next hop to use.
Prioritizing packets cannot improve your network’s performance when
bandwidth is over-subscribed to the point that egress queues are always
full. If one type of traffic is causing the congestion, you can limit its
bandwidth. Other solutions are to increase bandwidth or decrease traffic.
You can set a packet’s priority by configuring a priority in the flow group or
traffic class to which the packet belongs. The packet is put in the
appropriate CoS queue for that priority. If the flow group and traffic class
do not include a priority, the switch can determine the priority from the
VLAN tag User Priority field of incoming tagged packets. The packet is put
in the appropriate CoS queue for its VLAN tag User Priority field. If neither
the traffic class / flow group priority nor the VLAN tag User Priority is set,
the packet is sent to the default queue, queue 1.