Quality of Service (QoS): Managing Bandwidth More Effectively
Using QoS Classifiers To Configure Quality of Service for Outbound Traffic
Details of QoS IP Type-of-Service
IP packets include a Type of Service (ToS) byte. The ToS byte includes:
■ A Differentiated Services Codepoint (DSCP): This element is com-
prised of the upper six bits of the ToS byte). There are 64 possible
codepoints.
• In the 5300xl switches, the default qos configuration includes some
codepoints with 802.1p priority settings for Assured-Forwarding and
Expedited Forwarding (codepoint 101110), while others are unused
(and listed with No-override for a Priority).
• In the 3400cl/6400cl switches, the default qos configuration includes
the codepoint (7) having the 802.1p priority setting for Expedited
Forwarding, while all others, including the Assured-Forwarding code-
points, are unused (and listed with No-override for a Priority).
Refer to figure 8-14 on page 8-63 for an illustration of the default DSCP
policy table.
Using the qos dscp map command, you can configure the switch to assign
different prioritization policies to IPv4 packets having different code-
points. As an alternative, you can configure the switch to assign a new
codepoint to an IPv4 packet, along with a corresponding 802.1p priority
(0-7). To use this option in the simplest case, you would:
a. Configure a specific DSCP with a specific priority in an edge switch.
b. Configure the switch to mark a specific type of inbound traffic with
that DSCP (and thus create a policy for that traffic type).
c. Configure the internal switches in your LAN to honor the policy.
(For example, you could configure an edge switch to assign a codepoint
of 000001 to all packets received from a specific VLAN, and then handle
all traffic with that codepoint at high priority.)
For a codepoint listing and the commands for displaying and changing the
DSCP Policy table, refer to
“Differentiated Services Codepoint (DSCP)
Mapping” on page 8-62.
Restriction: On the 3400cl/6400cl switches, “mixing” ToS DSCP policies
and 802.1p priorities is not recommended. Refer to the Note on page 8-10.
■ Precedence Bits: This element is a subset of the DSCP and is comprised
of the upper three bits of the ToS byte. When configured to do so, the
switch uses the precedence bits to determine a priority for handling the
associated packet. (The switch does not change the setting of the prece-
dence bits.) Using the ToS Precedence bits to prioritize IPv4 packets relies
on priorities set in upstream devices and applications.
8-46
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