ZyXEL Communications Plumbing Product metrogigabit switch User Manual

MGS3700-12C User’s Guide

This chapter introduces the queuing methods supported.

Queuing is used to help solve performance degradation when there is network 
congestion. Use the Queuing Method screen to configure queuing algorithms for 
outgoing traffic. See also Priority Queue Assignment in Switch Setup and 
802.1p Priority in Port Setup for related information.

Queuing algorithms allow switches to maintain separate queues for packets from 
each individual source or flow and prevent a source from monopolizing the 
bandwidth. 

Strictly Priority Queuing (SPQ) services queues based on priority only. As traffic 
comes into the Switch, traffic on the highest priority queue, Q7 is transmitted first. 
When that queue empties, traffic on the next highest-priority queue, Q6 is 
transmitted until Q6 empties, and then traffic is transmitted on Q5 and so on. If 
higher priority queues never empty, then traffic on lower priority queues never 
gets sent. SP does not automatically adapt to changing network requirements. 

Weighted Fair Queuing is used to guarantee each queue's minimum bandwidth 
based on its bandwidth weight (portion) (the number you configure in the Weight 
field) when there is traffic congestion. WFQ is activated only when a port has more 
traffic than it can handle. Queues with larger weights get more guaranteed 
bandwidth than queues with smaller weights. This queuing mechanism is highly 
efficient in that it divides any available bandwidth across the different traffic 
queues. By default, the weight for Q0 is 1, for Q1 is 2, for Q2 is 3, and so on. 
Guaranteed quantum is calculated as Queue Weight x 2048 bytes.