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what is the diffrence between DSCP or IP precedence and what is the Criteria to implement qos ...when we will use IP precedence and when DSCP, and what is the default DSCP or IP precedence for IP packet example :if any ip packet originating from PC and IP voice packet which is originating from IP Phone and why IP precedence to DSCP mapping is required



Both Prec and DSCP are defined in the TOS byte in the IP header.

Prec uses first leftmost 3 bits and DSCP uses 6 leftmost bits.

Prec Defines the classes and the DSCP is code point in the class. Something for example like subnetting.

you consider a subnet of as the main class.

And then you can have multiple subnets within it like etc.

So if you match only the subnet you will ultimately match all the subnets contained like /30 /32 etc in the same subnet.

Similarly Each PREC class has 8 DSCP values within it. So if you match of map a single PREC with a behaviour it affects all the Code points (DSCP values within it with same behaviour), now when you want granular control or behaviour for a certain DSCP you mathc upon the DSCP value. Or map that DSCP value to a PREC value.

Your IP phone and PC question is not clear, but you may need IP prec to DSCP mapping in a case where you have defined a certain behaviour based on Prec and want to map a given DSCP value which doesnt fall under that PREC class, you can create your map by using a Policy map with class and match and set the incoming DSCP value to a certain PREC.

You can read a little on QOS books to give you more idea as its difficult to explain without the question being clear.



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