HP 5400ZL - Manual (Page 264)

Quality of Service: Managing Bandwidth More Effectively
Globally-Configured QoS
IPv4 ToS/IPv6 Traffic Class Byte
IPv4 packet headers contain a Type of Service (ToS) byte; IPv6 packet headers
contain a Traffic Class byte. In an IPv6 packet, the Traffic Class byte is used
in the same way as the ToS byte in an IPv4 packet. A ToS/Traffic Class byte
includes a DSCP codepoint and precedence bits:
■ Differentiated Services Codepoint (DSCP): Consists of the upper six
bits of the ToS/Traffic Class byte. There are 64 possible codepoints.
In the switches covered in this guide, the default QoS configuration
includes some codepoints, such as Assured Forwarding and Expedited
Forwarding, that are preconfigured with an 802.1p priority setting. All
other codepoints are not configured with an 802.1p priority and display
No-override as shown in the default DSCP Policy table (Table 6-11).
Use the qos dscp map command to configure the switch to assign different
802.1p priorities to IP packets with different codepoints. Also, you can
configure the switch to assign a new codepoint with its associated priority
level (0-7) to matching packets as follows:
a. Configure a DSCP codepoint with the desired priority in an edge
switch.
b. Configure the local switch to mark specified inbound traffic with the
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 6-88.
■ Precedence Bits: A subset of the DSCP codepoint, consisting of the
upper three bits of the ToS/Traffic Class byte. When a global IP-Prece­
dence classifier is configured, the switch uses the precedence bit set to
determine the priority for selected packets as shown in Table 6-8. (The
switch does not change the setting of the precedence bits.)
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