Default PHB
All the bits in the TOS byte are set to “0,” which gives best-effort delivery.
Any unmarked traffic is mapped to this PHB.
Assured Forwarding and Class Selector PHB
Figure 4-1 shows the TOS byte in the IP header.
Figure 4-1 The TOS Byte in the IP Header
CCNP ONT
[ 280 ] CCNP ONT Quick Reference
IP Precedence or AF bits Class Selector bits Congestion Notification bits
TOS Byte
The six DSCP bits can be broken down into two sections: The highest three
bits define the DiffServ Assured Forwarding (AF) class (the area in green),
and the next three bits are called the “Class Selector (CS)” (the area in yellow).
When the three CS bits are all zero, you have a value that is equal to IP
Precedence. The lowest two bits (the area in white) are not used in DiffServ
marking—they allow the sending of congestion notification information.
Each AF class becomes its own queue at the interface. AF uses the first two CS
bits to define the drop probability within that queue. The last bit is always
zero and is not used in calculating drop probability values. 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).
AF guarantees a specified amount of bandwidth to a class. By default, it allows
the traffic to burst above that amount if there is extra bandwidth available,
although this can be policed.
Table 4-3 lists the classes and their associated AF values.
Table 4-3 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
DiffServ Expedited Forwarding PHB
Another predefined DiffServ classification is Expedited Forwarding (EF),
which is DSCP 46. This is equivalent to IP precedence 5. EF traffic becomes
a separate queue at the QoS-enabled router interface. You must configure
each hop in the network for the type of service you want EF traffic to
receive. EF is usually used to put traffic in a low-latency queue, which
results in low delay, guarantees a specified amount of bandwidth, and also
polices the traffic to prevent it from exceeding that bandwidth.