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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.

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Published: 2012-11-28