Configuring Classification Using DSCP

router(config-cmap)#

match ip dscp ip-dscp-value [ip-dscp-value ...]

• Select up to eight DSCP values or names.

• All packets marked with one of the selected DSCP values are matched by this class map.

DSCP

DSCP Class

DSCP

DSCP Class

Value

Name

Value

Name

0 (000000)

default

10 (001010)

af11

8 (001000)

cs1

12 (001100)

af12

16 (010000)

cs2

14 (001110)

af13

24 (011000)

cs3

18 (010010)

af21

32 (100000)

cs4

20 (010100)

af22

40 (101000)

cs5

22 (010110)

af23

48 (110000)

cs6

26 (011010)

af31

56 (111000)

cs7

28 (011100)

af32

46 (101110)

ef

30 (011110)

af33

34 (100010)

af41

36 (100100)

af42

38 (100110)

af43

IP packets can also be classified based on the IP DSCP field. A QoS design can be based on IP precedence marking or DSCP marking. DSCP standards make IP precedence marking obsolete but include backward compatibility with IP precedence by using the Class Selector (CS) values (which are 6-bit equivalents to their IP precedence counterparts and are obtained by setting the three most significant bits of the DSCP to the IP precedence value, while holding the three least significant bits to zero).

Syntax Description

Parameter

Description

[ip]

(Optional) Specifies that the match is for IPv4 packets only. If not used, the match is on both IPv4 and IPv6 packets.

dscp-value

Specifies the exact value from 0 to 63 used to identify an IP DSCP value.

The figure illustrates implementation of a design with these five classes:

■ Voice: Identified by DSCP value EF, which looks like IP Precedence value 5 in non-DSCP-compliant devices.

■ Mission-critical: Identified by DSCP values AF31, AF32, and AF33. The class is also identified by IP Precedence 3.

■ Transactional: Identified by DSCP values AF21, AF22, and AF23. The class is also identified by IP Precedence 2.

■ Bulk: Identified by DSCP values AF11, AF12, and AF13. The class is also identified by IP Precedence 1.

■ Best-effort: Identified by the default DSCP value that is equal to the default IP Precedence value (0).

From a non-DSCP-compliant device, the design looks slightly different:

■ Mission-critical: IP Precedence 3

■ Transactional: IP Precedence 2

■ Best-effort: IP Precedence 0

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