IP Precedence DSCP and CoS relationships

As part of studying the 642-642 QoS exam course is seems quite important to understand the relationship between the various ways of marking traffic. Below are some notes I’ve made so far about these relationships. This is note a post about the inner working of the various PHBs or their use.

DSCP – Differentiated Services Code Point. This is a field that uses 6bits of the TOS field in an IP packet header.

The four assured forwarding classes each include three drop probability classes. In general the higher the first number the better QoS treatment of the packet and the higher the second number the higher the probability the packet will be dropped during a period of congestion. A simple formula to remember the decimal value is AF(8*A)+(2*B). E.g. AF31 = (8*3)+(2*1 )= 26

IP Precedence – This was used prior to the DSCP. It also uses bit from the TOS field found in IP Packet headers although only 3bits rather than the 6bits of DSCP.  These three Precedence bits overlap with the most significant bits of the DSCP.

CoS – Class of service. A 3bit field defined under the 802.1p spec and only found in 802.1q tagged Ethernet frames (cisco ISL also supports CoS). Different from Precedence and DSCP as its a layer 2 marking and some networking equipment can only act on layer 2 CoS markings.

A Table showing the relationship between IP Precedence and DSCP:

Per Hop BehaviourDSCP
DSCP (binary value)DSCP (decimal value)IP PrecedenceIP Precedence (binary)
Default (no qos)BE00000000000

Assured Forwarding
Expedited ForwardingEF101110465101

To enhance the backward compatibility with IP precedence DSCP has seven Class Selector PHBs. These all have zeros in the three least significant bits of the DSCP field. E.g. xxx000. The ideam being along the line that if a IP precedence marking router send a marked packet to a DSCP marking router it will have a measure of understanding.

PHBDSCP (binary value)CoS
Default (BE)0000000
Class Selector 1 (CS1)0010001
Class Selector 2 (CS2)0100002
Class Selector 3 (CS3)0110003
Class Selector 4 (CS4)1000004
Class Selector 5 (CS5)1010005
Class Selector 6 (CS6)1100006
Class Selector 7 (CS7)1110007

On Cisco kit these mapping can of course be customised and changed through the cli.

m00nie :)

  1. Денис

    Спасибо, очень познавательно!
    Thank you, very usefull information!

  2. Thanks. Its good.

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