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Key Concepts 627
The boundary nodes (shown in the Topology example) are complex
classification devices and can perform the appropriate marking of
packets. This means that any interior nodes that are simple classification
devices are still able to determine the correct traffic prioritization to
perform.
Queuing The first two aims of traffic prioritization (forwarding important traffic
through the device faster than other traffic and reducing the risk of
dropping important traffic) are provided by the queuing mechanism as
follows:
1 The classified traffic is queued for output on a particular port in multiple
queues rather than in a single queue (as performed on devices that are
not capable of traffic prioritization) and those queues are serviced in a
particular way that favors the more important traffic over other traffic.
2 Each of the queues is shared between one or more different types of
traffic that have been defined as having a similar level of importance.
3 If a particular queue fills up with packets then any further packets for that
queue will be dropped until the queue is serviced. This will clear some of
the packets from the queue.
4 Packets are taken off the queues and transmitted out of the port
according to the particular queue servicing mechanism that is in place. A
device may support one or more of the various queue servicing
mechanisms that are available, which include weighted round robin and
strict priority queueing. Each of these queuing mechanisms has its own
characteristics yet they are all aimed at ensuring higher priority traffic is
processed and forwarded by the device quicker than other traffic.
An example of the results of a queuing servicing mechanism is illustrated
in
Figure 344 . It shows how important traffic flows through the network
quicker than the less important traffic and reduces the likelihood of the
more important traffic being dropped. The device in the figure defines
traffic as being gold, silver or bronze (gold being the most important)
with a queue for each type of traffic.