Layer 2

Class of Service - COS

• 3 bit value (8 possible values, 0 - 7)

But 6 and 7 are reserved.

In Layer 2 we have 2 encapsulation technique

• .1q (Dot1Q is an IEEE's open standard)

• ISL (Inter-Switch Link is cisco proprietary)

Layer 3

Type of Service - TOS

We have

• IP Precedence (which is three bits)

• DSCP (Differentiated Servie Field Code Points, which takes 6 bits out of 8 leaving 6 and 7 for reserved)

2^6 = 64

The IETF pre-selected 21 Values from the 64 possible values and gave them names.

The name is PHB - Per Hop Behaviors

AFdd - the first digit is the DSCP equivalent IP Precedence.

Ex:

AF21 - if this marking is intercepted by the Router that supports only IP Precedence then it will consider this as a IP Precedence 2.

This is mainly for the Backward compatibility. So the DSCP values can make sense if that is intercepted by the router that only speaks IP Precedence.

ECN in IPv4

RED

RED and WRED

WRED

Shaping and Policing