Classification and Marking on Catalyst 2950 Switches

Port can be configured to trust CoS, DSCP, or Cisco IP Phone (default = untrusted)

Has default CoS-to-DSCP and DSCP-to-CoS maps

Can set the default CoS by port

Can use class-based marking to set DSCP

No VLAN-based classification Limited ACLs—no port range

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IP precedence is encoded into the three high-order bits of the type of service (ToS) field in the IP header. It supports eight classes. two of which (IP precedence 6 and 7) are reserved for control-plane traffic and should not be used for user-defined classes. IP Precedence 0 is the default value and is usually used for the best-effort class.

To set the precedence value in the IP header, use the set ip precedence QoS policy-map configuration command. To leave the precedence value at the current setting, use the no form of this command.

set ip precedence ip-precedence-value

Syntax Description




A number from 0 to 7 that sets the precedence bit in the IP header

This topic describes the Cisco IOS commands that are required to mark IP packets using DSCP with class-based marking.

Differentiated Services (DiffServ) is a new standards-based Layer 3 marking model that supersedes—and is backward compatible with—IP precedence. DiffServ uses six prioritization bits that permit classification of up to 64 values (0 to 63). A DiffServ value is called a DSCP. The set ip dscp command is used to mark packets of a class with a DSCP value.

To mark a packet by setting the IP DSCP in the ToS byte, use the set ip dscp QoS policy-map configuration command. To remove a previously set IP DSCP, use the no form of this command.

set [ip] dscp ip-dscp-value

Syntax Description




A number from 0 to 63 that sets the IP DSCP value. Reserved keywords EF (Expedited Forwarding), AF11 (Assured Forwarding Class 1 Drop Preference 1), AF12 (Assured Forwarding Class 1 Drop Preference 2), and so on can be specified instead of numeric values.

The set dscp command works for both IPv6 and IPv4. The set ip dscp command only works for IPv4 packets.

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