Foundation Summary

The "Foundation Summary" is a collection of tables and figures that provides a convenient review of many key concepts in this chapter. For those of you already comfortable with the topics in this chapter, this summary can help you recall a few details. For those of you who just read this chapter, this review should help solidify some key facts. For any of you doing your final preparation before the exam, these tables and figures are a convenient way to review the day before the exam.

Table 9-10 lists the key differences in QoS capabilities between the Enhanced and Standard OS versions available for the Catalyst 2950.

Table 9-10. Enhanced Versus Standard IOS Images

QoS Feature	Enhanced Image Support	Standard Image Support
Classification capability?	Layers 2 to 4	Layer 2 only
Marking capability?	Layer 2 and 3	Layer 2 only
Priority queue support?	Yes	Yes
Weighted Round Robin capability?	Yes	Yes
Policing capability?	Yes	No
AutoQoS capability?	Yes	No

Figure 9-12 shows the general location of the CoS field inside the 802.1Q headers.

Figure 9-13 outlines the two fields and their positions inside an IP header.

Table 9-11 lists the DSCP classes associated with the DSCP decimal values.

Table 9-11. DSCP Values

DSCP Decimal Value	DSCP Class
0	default dscp (000000)
8	CS1(precedence 1) dscp (001000)
10	AF11 dscp (001010)
16	CS2(precedence 2) dscp (010000)
18	AF21 dscp (010010)
24	CS3(precedence 3) dscp (011000)
26	AF31 dscp (011010)
32	CS4(precedence 4) dscp (100000)
34	AF41 dscp (100010)
40	CS5(precedence 5) dscp (101000)
46	EF dscp (101110)
48	CS6(precedence 6) dscp (110000)
56	CS7(precedence 7) dscp (111000)

Table 9-12 lists the options for configuring the trust state.

Table 9-12. Trust Boundaries

Command	Function
mls qos trust [cos {pass-through} | device cisco-phone | dscp]	mls qos trust cos {pass-through} This command configures the interface to trust the CoS value for all Ethernet frames received. The pass-through option prevents the switch from overwriting the original DSCP value in the received packet with the values indicated in the CoS to DSPC map.
	mls qos trust device cisco-phone This command configures the interface to trust CoS values received only if a Cisco IP phone is attached.
	mls qos trust dscp This command configures the interface to trust the CoS value for all Ethernet packets received.
switchport priority extend [cos value | trust]	switchport priority extend cos value Used in conjunction with mls qos trust device cisco-phone, this command overwrites the original CoS value of all Ethernet frames received from a PC attached to an IP phone with the value specified. switchport priority extend cos 0 is the default.
	switchport priority extend trust Used in conjunction with mls qos trust device cisco-phone, this command enables the switch to trust the CoS value of all Ethernet frames received from a PC attached to an IP phone.

Table 9-13 lists the options for configuring the default CoS value.

Table 9-13. Default CoS Value

Command	Function
mls qos cos [value| override]	mls qos cos value This command configures the interface to attach the specified CoS value to all untagged frames that are received.
	mls qos cos override This command configures the interface to overwrite the original CoS value received with the specified value.

Figure 9-14 illustrates the treatment received by packets as they enter the switch.

Figure 9-15 illustrates the packet flow used by strict priority scheduling.

Figure 9-16 illustrates the packet flow used by WRR scheduling.

Figure 9-16. Weighted Round Robin Scheduling Flow

[View full size image]

Figure 9-17 illustrates the packet flow used by strict priority queuing.

Figure 9-17. Strict Priority Queuing Flow

[View full size image]

Table 9-14 lists the commands used to configure the available scheduling options.

Table 9-14. Scheduling commands

Command	Function
wrr-queue cos-map quid value	This command assigns CoS values from 0 to 7 to one of the four egress queues. For example, wrr-queue cos-map 4 5 places all frames with a CoS value of 5 in egress queue 4.
wrr-queue bandwidth [1-255] [1-255] [1-255] [0-255]	This command allocates the amount of bandwidth the queues will use during WRR scheduling. For example, wrr-queue bandwidth 20 1 80 0 allocates 20 packets to queue 1, 1 packet to queue 2, 80 packets to queue 3, and strict priority queuing to queue 4.

Table 9-15 lists the options available to a policer on a Catalyst 2950.

Table 9-15. Policer Options

Command	Arguments	Function
police rate-bps burst-byte [exceed-action {drop | dscp dscp-value}]	rate-bps	The average receive rate at which the policer will accept conforming traffic Acceptable values for 10/100 1000000 (1 Mbps) to 100000000 (100 Mbps) in multiples of 1000000 (1 Mbps) Acceptable values for 1 Gigabit 8000000 (8 Mbps) to 1000000000 (1 Gbps) in multiples of 8000000 (8 Mbps)
	burst-byte	Allowable burst size that the policer will accept before marking traffic as nonconforming Acceptable values for 10/100 4096 (4 kbps), 8192 (8 kbps), 16384 (16 kbps), 32768 (32 kbps), or 65536 (64 kbps) Acceptable values for 1 Gigabit 4096 (4 kbps), 8192 (8 kbps), 16384 (16 kbps), 32768 (32 kbps), 65536 (64 kbps), 131072 (128 kbps), 262144 (256 kbps), or 524288 (512 kbps)
	exceed-action drop	Specifies that all nonconforming traffic will be dropped
	exceed-action dscp [dscp value]	Specifies that all nonconforming traffic will be remarked with the DSCP value specified and transmitted

AutoQoS on the Catalyst 2950 consists of the two commands listed and defined in Table 9-16.

Table 9-16. AutoQoS Commands

Command	Function
auto qos voip trust	This command enables the trust of CoS values on the interface it is applied to by adding the mls qos trust cos command on that interface. Additionally, this command automatically modifies the CoS-to-DSCP map and configures strict priority queuing for the Catalyst 2950.
auto qos voip cisco-phone	This command enables the trust of CoS values received from a Cisco IP Phone on the interface it is applied to by placing both the mls qos trust device cisco-phone command and the mls qos trust cos command on that interface. Additionally, this command automatically modifies the CoS-to-DSCP map and configures strict priority queuing for the Catalyst 2950.

Figure 9-18 illustrates where the commands might be placed.

Table 9-17 lists the classification and scheduling options configured by AutoQoS.

Table 9-17. AutoQoS Classification and Scheduling

Egress Queue	CoS Value	WRR Bandwidth Allocated
1	0, 1, 2, 4	20 percent WRR
2	Not used	Not used
3	3, 6, 7	80 percent WRR
4	5	Strict Priority Queue

For Further Reading

This book attempts to cover the breadth and depth of QoS as covered on the QoS exam (642-641). However, you might want to read more about topics in this chapter, or other classification and marking topics.

For more on the topics in this chapter:

• Cisco 2950 QoS Configuration Guide (http://www.cisco.com/univercd/cc/td/doc/product/lan/cat2950/12119ea1/2950scg/swqos.htm)

• "Cisco AutoQoS White Paper" (http://cisco.com/en/US/tech/tk543/tk759/technologies_white_paper09186a00801348bc.shtml)

For design-related guidance:

• "Cisco AVVID Network Infrastructure Enterprise Quality of Service Design" (http://cisco.com/application/pdf/en/us/guest/netsol/ns17/c649/ccmigration_09186a00800d67ed.pdf)