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Quality of Service (QoS): Managing Bandwidth More Effectively
Introduction
Overview
QoS settings operate on two levels:
Controlling the priority of outbound packets moving through the
switch: Each switch port has four outbound traffic queues; “low”, “nor-
mal”, “medium”, and “high” priority. Packets leave the switch port on the
basis of their queue assignment and whether any higher queues are empty:
Table 15-1.Port Queue Exit Priorities
Port Queue Priority for Exiting From the Port
High (6 - 7) First
Medium (4 - 5) Second
Normal (0, 3) Third
Low (1 - 2) Fourth
A QoS configuration enables you to set the outbound priority queue to
which a packet is sent. (In an 802.1Q tagged VLAN environment, if QoS is
not configured on the switch, but is configured on an upstream device,
the priorities carried in the packets determine the forwarding queues in
the switch.)
Configuring a priority for outbound packets and a service (prior-
ity) policy for use by downstream devices:
• DSCP Policy: This feature enables you to set a priority policy in
outbound IP packets. (You can configure downstream devices to read
and use this policy.) This method is not dependent on tagged VLANs
to carry priority policy to downstream devices, and can:
– Change the codepoint (the upper six bits) in the TOS byte.
– Set a new 802.1p priority for the packet.
(Setting DSCP policies requires IPv4 inbound packets. Refer to the
“IPv4” entry under
“Terminology” on page 15-5.)
• 802.1p Priority: If an outbound packet is in an 802.1Q tagged VLAN
environment (that is, if the packet is assigned to a tagged VLAN on
the outbound port), then the packet carries an 802.1p priority setting
that was configured in the switch. This priority setting ranges from 0
to 7, and can be used by downstream devices having up to eight
outbound port queues. Thus, while packets within the switch move
at the four priority levels shown in table
15-1, above, they still can
carry an 802.1p priority that can be used by downstream devices
having more or less than the four priority levels in the Series 5300XL
switches. Also, if the packet enters the switch with an 802.1p priority
setting, QoS can override this setting if configured to do so.
15-6