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Defining DSCP in use for Diffserv code points

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Article ID: KB19730 KB Last Updated: 22 May 2011Version: 1.0
Summary:
This article describes the defaults of DSCP code points and their standard aliases.
Symptoms:
Describing the default code-points in use for DSCP.
Solution:
A code-point alias is a name you assign to a set of DiffServ code-point (DSCP) bits. When you configure classes and define classifiers, you can refer to the code points by these alias names. You can configure user-defined classifiers in terms of alias names. If the value of an alias changes, it alters the behavior of any classifier that references that alias.

The table below shows the default mappings between the bit values and standard aliases. For example, it is widely accepted that the alias for DSCP 101110 is ef (expedited forwarding).

Diffserv DesignatorMapping
DSCP Code points 
ef101110
af11001010
af12001100
af13001110
af21010010
af22010100
af23010110
af31011010
af32011100
af33011110
af41100010
af42100100
af43100110
be000000
cs1001000
cs2010000
cs3011000
cs4100000
cs5101000
nc1/cs6110000
cs2/cs7111000
MPLS EXP code points 
be000
be1001
ef010
ef1011
af11100
af12101
nc1/cs6110
nc/cs7111
IEEE 802.1 code points 
be000
be1001
ef010
ef1011
af11100
af12101
nc1/cs6110
nc2/cs7111
Legacy IP Precedence CP 
be000
be1001
ef010
ef1011
af11100
af12101
nc1/cs6110
nc2/cs7111

You use code-point aliases to do the following:
  • Define an alias for bits that currently have no alias
  • Define multiple aliases for the same bits
  • Redefine an alias name to mean a different set of bits than the default

To define a code-point alias, include the code-point-aliases statement at the [edit class-of-service] hierarchy level:
[edit class-of-service]
code-point-aliases {
    (dscp | exp | ieee-802.1 | inet-precedence) {
        alias-name bits;
    }
}


For example, you might set up the following configuration:
[edit class-of-service]
code-point-aliases {
    dscp {
        my1 110001;
                   my2 101110;
                   be 000001;
                   cs7 110000;
    }
}

The sample configuration produces this mapping:
user@host>show class-of-service code-point-aliases dscp
Code point type: dscp


Alias            Bit pattern
ef/my2           101110
af11             001010
af12             001100
af13             001110
af21             010010
af22             010100
af23             010110
af31             011010
af32             011100
af33             011110
af41             100010
af42             100100
af43             100110
be               000001
cs1              001000
cs2              010000
cs3              011000
cs4              100000
cs5              101000
nc1/cs6/cs7      110000
nc2              111000
my1              110001

The following notes explain certain results in the mapping:
  • my1 110001:

    • 110001 was not mapped to anything before, and my1 is a new alias.
    • Nothing in the default mapping table is changed by this statement.

  • my2 101110:

    • 101110 is now mapped to my2 as well as ef.

  • be 000001:

    • be is now mapped to 000001.
    • The old value of be, 000000, is not associated with any alias. Packets with this DSCP value are now classified to the default forwarding class.

  • cs7 110000:

    • cs7 is now mapped to 110000, as well as nc1 and cs6.
    • The old value of cs7, 111000, is still mapped to nc2.

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