DSCP Values
Differentiated services provide levels of service based on the value of certain
bits in the IP or ISL header or the 802.1Q tag. Each hop along the way must
be configured to treat the marked traffic the way you want—this is called
per-hop behavior (PHB).
In the Layer 3 IP header, you use the 8-bit ToS field. You can set either IP
Precedence using the top 3 bits or Differentiated Services Code Points
(DSCP) using the top 6 bits of the field. The bottom 2 bits are set aside for
congestion notification. The default DSCP value is zero, which corresponds
to best-effort delivery
The six DSCP bits can be broken down into two sections: The first 3 bits
define the DiffServ Assured Forwarding (AF) class, and the next 2 bits
define the drop probability within that class. The sixth bit is 0 and unused.
AF classes 1–4 are defined, and within each class, 1 is low drop probability,
2 is medium, and 3 is high (meaning that traffic is more likely to get dropped
if there is congestion). These are shown in Table 7-1. Each hop still needs to
be configured for how to treat each AF class.
Table 7-1 DSCP Assured Forwarding Values
Low Drop Medium Drop High Drop
Class 1 AF11 AF12 AF13
Class 2 AF21 AF22 AF23
Class 3 AF31 AF32 AF33
Class 4 AF41 AF42 AF43
Voice bearer traffic uses an Expedited Forwarding value of DSCP 46 to give
it higher priority within the network.