18.
Quality of Service
MN700004 Rev 01
212
NOTE
When changing the priority on the port the priority of the dynamic MAC address is also
changes.
Traffic Scheduling
Congestion management features allow you to control congestion by determining the order in
which packets are transmitted based on priorities assigned to those packets. Congestion
management entails the creation of queues, assignment of packets to those queues based on
the packet’s classification, and scheduling of the packets in a queue for transmission. If you
use congestion management features, packets accumulating at an interface are queued until
the interface is free to transmit them; they are then scheduled for transmission according to
their assigned priority and the queuing mechanism configured for the interface. The
determines the order of packet transmission by controlling which packets are placed in which
queue and how queues are serviced with respect to each other.
First-In, First-Out Queuing
In FIFO queuing, also known as first-come, first-served (FCFS) queuing, packets are queued
when the network is congested, and forwarded in order of arrival when the network is no
longer congested.
FIFO applies no prioritization or classification of traffic and. There is only one queue, and all
packets are treated equally. Packets are sent out in the order in which they arrive. Higher
priority packets are not transmitted faster than lower priority packets.
When FIFO is used, ill-behaved sources can consume all the bandwidth, bursty sources can
cause delays in time-sensitive or important traffic, and important traffic can be dropped
because less important traffic fills the queue.
Strict Priority (SP)
With Strict Priority (SP) queue handling, the queues are ranked in order. The highest ranking
queue
txq7
is serviced first until it is empty, then the lower queues
txq6
,
txq5
,
txq4, txq3
,
txq2
,
txq1
and
txq0
are serviced in sequence. SP provides absolute preferential treatment to high
priority traffic, ensuring that mission-critical traffic traversing various WAN links gets
priority treatment. In addition, SP provides a faster response time than do other methods of
queuing.
Use the SP mechanism to guarantee a fixed portion of available bandwidth to one type of
application - for example, interactive multimedia applications - possibly at the expense of less
critical traffic. But when you choose to use SP, consider that lower priority traffic is often
denied bandwidth in favor of higher priority traffic, so use of SP could, in the worst case,
result in lower priority traffic never being transmitted. To avoid inflicting these conditions on
lower priority traffic, you can use rate-limit to control the rate of the higher priority traffic.
Figure 18-2 illustrates the SP process.
