Technical Documentation

Default Forwarding Classes

Table 1 shows the default forwarding class and packet loss priority (PLP) for the well-known DSCPs. It is important to note that although several DSCPs map to the expedited-forwarding and assured-forwarding classes, by default no resources are assigned to these forwarding classes. All of these settings can be changed through configuration.

Table 1: Default Behavior Aggregate Classification

DSCP and DSCP IPv6

Forwarding Class

PLP

ef

expedited-forwarding

low

af11

assured-forwarding

low

af12

assured forwarding

high

af13

assured forwarding

high

af21

best-effort

low

af22

best-effort

low

af23

best-effort

low

af31

best-effort

low

af32

best-effort

low

af33

best-effort

low

af41

best-effort

low

af42

best-effort

low

af43

best-effort

low

be

best-effort

low

cs1

best-effort

low

cs2

best-effort

low

cs3

best-effort

low

cs4

best-effort

low

cs5

best-effort

low

nc1/cs6

network-control

low

nc2/cs7

network control

low

other

best-effort

low

Table 2 shows the router forwarding classes associated with each well-known DSCP code point and the resources assigned to their output queues.

Table 2: Classification Forwarding Classes and Queues

DCSP Alias

DSCP Bits

Forwarding Class

PLP

Queue

ef

101110

expedited-forwarding

low

1

af11

001010

assured-forwarding

low

2

af12

001100

assured-forwarding

high

2

af13

001110

assured-forwarding

high

2

af21

010010

best-effort

low

0

af22

010100

best-effort

low

0

af23

010110

best-effort

low

0

af31

011010

best-effort

low

0

af32

011100

best-effort

low

0

af33

011110

best-effort

low

0

af41

100010

best-effort

low

0

af42

100100

best-effort

low

0

af43

100110

best-effort

low

0

be

000000

best-effort

low

0

cs1

001000

best-effort

low

0

cs2

010000

best-effort

low

0

cs3

011000

best-effort

low

0

cs4

100000

best-effort

low

0

cs5

101000

best-effort

low

0

nc1/cs6

110000

network-control

low

3

nc2/cs7

111000

network-control

low

3

other

best-effort

low

0

Table 3 shows the resources assigned to the four forwarding classes in this example.

Table 3: Resources Assigned to Queues

Queue

Forwarding Class

Transmit Rate

Buffer Size

Priority

0

be (data)

40%

40%

Low

1

ef (financial)

10%

10%

High

2

af (audiovisual)

45%

45%

High (with RED)

3

nc (network control)

5%

5%

Low

The table shows how the 95 percent of output link transmission rate and buffer size (queue) resources assigned by default to Q0 (best-effort) are distributed to Q1 (expedited forwarding) and Q2 (assured forwarding). The audiovisual traffic consumes more bandwidth than other applications, but the financial information, although critical, is carried in fewer packets. In keeping with DiffServ specifications, a RED drop profile is applied to the assured forwarding class. The financial data has a strict set of traffic parameters that must be respected.

The three DiffServ assured forwarding classes supported (af11, af12, and af13, with low, medium, and high packet drop probability, respectively) are distinguished by using a low PLP and RED drop profile for af11 and a high PLP and RED for af12 and af13. All of these parameters should be closely monitored initially for performance and adjusted as necessary.


Published: 2010-04-15