Quality of Service: Managing Bandwidth More Effectively
Globally-Configured QoS
P r e r e q u i s i t e
5-40
Global IP Type-of-Service Classifier
Global QoS Classifier Precedence: 3
The global IP Type-of-Service classifier enables you to classify and mark IP
packets according to the following modes:
■
IP-Precedence Mode: All IP packets generated by upstream devices and
applications include a precedence bit set in the ToS/Traffic Class byte. In
IP-precedence mode, the switch uses the precedence bits to compute and
assign the corresponding 802.1p priority.
IP Differentiated Services (Diffserv) Mode: The Diffserv mode uses
■
the codepoints set in IP packets by upstream devices and applications to
assign an 802.1p priority to packets. You can use Diffserv mode to mark
packets in the following ways:
•
Assign a new DSCP policy: A "policy" includes both a codepoint
and a corresponding 802.1p priority. This option selects an incoming
IP packet on the basis of its codepoint and assigns a new codepoint
and corresponding 802.1p priority. (Use the qos dscp-map command
to specify a priority for any codepoint—page 5-90.)
•
Assign an 802.1p priority: This option reads the DSCP of an
incoming IP packet and, without changing this codepoint, assigns the
802.1p priority to the packet, as configured in the DSCP Policy Table
(page 5-90). This means that a priority value of 0 - 7 must be configured
for a DSCP before the switch can perform a QoS match on the packet's
DSCP bits.
A DSCP codepoint must have a preconfigured 802.1p priority (0 - 7) before
you can use the codepoint to mark matching packets. If a codepoint you want
to use shows No-override in the Priority column of the DSCP Policy table (show
qos dscp-map command), you must first configure a priority for the codepoint
before proceeding (qos dscp-map priority command). See "Differentiated Ser-
vices Codepoint (DSCP) Mapping" on page 5-90 for more information. Note
that some 802.1p priorities are assigned by default to well-known DSCP
codepoints, such as the "Assured Forwarding" and "Expedited Forwarding"
codepoints (see Table 5-11).
Unless IP-Precedence mode and Diffserv mode are both disabled (the default
setting), enabling one automatically disables the other. For more information
on Type-of-Service operation, refer to "IPv4 ToS/IPv6 Traffic Class Byte" on
page 5-41.