Preferential Queue Treatment For Packet Forwarding - Enterasys Matrix 7G4270-09 Configuration Manual

Quality of Service Overview
Figure 1 Assigning and Marking Traffic with a Priority
The ICMP protocol, used for error messaging, has a low bandwidth requirement, with a high
tolerance for delay and jitter, and is appropriate for a low priority setting. HTTP and FTP
protocols, used respectively for browser‐generated and file transfer traffic, have a medium to high
bandwidth requirement, with a medium to high tolerance for delay and jitter, and are appropriate
for a medium priority level. Voice (VoIP), used for voice calls, has a low bandwidth requirement,
but is very sensitive to delay and jitter and is appropriate for a high priority level.
See RFC 1349 for further details on ToS. See RFCs 2474 and 2475 for further details on DSCP.

Preferential Queue Treatment for Packet Forwarding

There are three types of preferential queue treatments for packet forwarding: strict, weighted fair,
and hybrid.
Strict Queuing
With Strict Priority Queuing, a higher priority queue must be empty before a lower priority queue
can transmit any packets. Strict queuing is depicted in Figure
upper left and proceed to the appropriate queue, based upon the TxQ configuration in the CoS.
Outbound packets exit the queues on the lower right. At this time only queue 3 packets are
forwarded. This will be true until queue 3 is completely empty. Queue 2 packets will then be
forwarded. Queue 1 packets will only forward if both queue 2 and queue 3 are empty. Queue 0
packets will only forward if all other queues are empty. Strict queuing assures that the highest
priority queue with any packets in it will get 100 percent of the bandwidth available. This is
particularly useful for one or more priority levels with low bandwidth and low tolerance for delay.
The problem with strict queuing is that should the higher level queues never fully empty, lower
level queues can be starved of bandwidth.
February 22, 2008
2. Inbound packets enter on the
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