Foundation Summary

The "Foundation Summary" is a collection of tables and figures that provide a convenient review of many key concepts in this chapter. For those of you already comfortable with the topics in this chapter, this summary could 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.

Why is QoS important on a LAN when bandwidth is abundant? As you have learned, bandwidth is not the only important factor to consider when determining how traffic will flow across your LAN infrastructure. Figure D-13 illustrates the concept of buffer overrun.

You must understand the behavior of the real-time applications present on your network and devise a strategy to support these real-time applications.

The first step in devising your strategy is achieving an understanding of the real-time applications that will reside on your LAN. Following are questions you should consider:

• Will voice traffic reside on your LAN?

• Will video traffic reside on your LAN?

• Will you need to prioritize specific data applications on your LAN?

• What are the bandwidth, delay, and jitter requirements for the expected applications?

After you have an understanding of the real-time applications that will be supported on your LAN, you can begin to design the network to meet your needs. For QoS support in a LAN environment, multiple queues are required on all interfaces to guarantee that loss, delay, and jitter do not affect voice, video, and mission-critical data.

Table D-24 defines a few of the Layer 2 queues available in Cisco Catalyst switches.

Table D-24. Supervisor and Switching Engine Combinations

Supervisor	Switching Engine
Supervisor II (WS-X6K-SUP2-2GE)	Layer 3 Switching Engine II (WS-F6K-PFC2PFC2)
Supervisor Engine I (WS-X6K-SUP1A-2GE) or (WS-X6K-SUP1-2GE)	Layer 3 Switching Engine (WS-F6K-PFC)
Supervisor Engine I (WS-X6K-SUP1A-2GE) or (WS-X6K-SUP1-2GE)	Layer 2 Switching Engine II (WS-F6020A)
Supervisor Engine I (WS-X6K-SUP1A-2GE) or (WS-X6K-SUP1-2GE)	Layer 2 Switching Engine I (WS-F6020)

Figure D-14 illustrates the proccess that drop thresholds follow. When the queue has reached 50 percent of capacity, any traffic classified with CoS of 0 or 1 becomes drop candidates to avoid congestion. If the queue continues to fill in spite of the drops, at 60 percent of capacity any traffic clssified with a CoS of 0, 1, 2, or 3 becomes drop candidates to avoid congestion. If the queue still continues to fill in spite of the drops, at 80 percent of capacity any traffic clssified with a CoS of 0, 1, 2, 3, 4, or 5 becomes drop candidates to avoid congestion. At 100 percent of capacity, all traffic, requardless of classification, becomes drop candidates.

Table D-25 describes the possible supervisor and switching engine combinations on a Catalyst 6500.

Table D-25. QoS Processess of the Catalyst 6500

QoS Process	Catalyst 6500 Component That Performs This Function
Input scheduling	Performed by port application-specific integrated circuits (ASICs) Layer 2 only with or without PFC
Classification	Performed by supervisor or PFC L2 only is done by supervisor L2/3 is done by PFC
Policing	Performed by PFC via L3 forwarding engine
Packet rewrite	Performed by port ASICs L2/L3 based on classification
Output scheduling	Performed by port ASICs L2/L3 based on classification

Table D-26 summarizes the QoS processes of the Catalyst 6500 and lists the component responsible for the function.

Table D-26. Supported Receive Queues of the Catalyst 6500

Catalyst 6500 Receive Queues	Description
1q4t	1 standard queue with 4 tail-drop thresholds
1p1q4t	1 strict-priority queue, 1 standard queue with 4 tail-drop thresholds
1p1q0t	1 strict-priority queue, 1 standard queue with one nonconfigurable (100%) tail-drop threshold
1p1q8t	1 strict-priority queue, 1 standard queue with 8 configurable WRED-drop thresholds and 1 nonconfigurable (100%) tail-drop threshold

Table D-27 describes the supported receive queues of the Catalyst 6500.

Table D-27. Supported Transmit Queues of the Catalyst 6500

Catalyst 6500 Transmit Queues	Description
2q2t	2 standard queues with 2 tail-drop thresholds per queue
1p2q2t	1 strict-priority queue, 2 standard queues with 2 WRED-drop thresholds per queue
1p3q1t	1 strict-priority queue, 3 standard queues with 1 WRED-drop threshold and 1 nonconfigurable tail-drop threshold per queue
1p2q1t	1 strict-priority queue, 2 standard queues with 1 WRED-drop threshold and 1 nonconfigurable (100%) tail-drop threshold per queue

Table D-28 describes the supported transmit queues of the Catalyst 6500.

Table D-28. Supported Receive and Transmit Queues of the Ethernet Modules for the Catalyst 6500

Ethernet Modules	Module Description	RX Queue Type	TX Queue Type	RX Queue Size	TX Queue Size
WS-X6K-S2U-MSFC2 WS-X6K-S2-MSFC2 WS-X6K-S2-PFC2	All Supervisor Engine II uplink ports	1p1q4t	1p2q2t	PQ: 9 KB Q1: 64 KB	Q2: 64 KB Q1: 311 KB
WS-X6K-S1A-MSFC2 WS-X6K-SUP1A-MSFC WS-X6K-SUP1A-PFC WS-X6K-SUP1A-2GE	All Supervisor Engine IA uplink ports	1p1q4t	1p2q2t	PQ: 9 KB Q1: 64 KB	PQ: 64 KB Q2: 64 KB Q1: 311 KB
WS-X6K-SUP1-2GE	Supervisor Engine II uplink ports	1q4t	2q2t	Q1: 80 KB	Q2: 80 KB Q1: 352 KB
WS-X6524-100FX-MM	24-port 100BASE-FX fabric-enabled with MT-RJ connectors	1p1q0t	1p3q1t	PQ: 6 KB Q1: 22 KB	PQ: 272 KB Q3: 272 KB Q2: 272 KB Q1: 272 KB
WS-X6548-RJ-21	48-port 10/100BASE-TX fabric-enabled with RJ-21 connectors	1p1q0t	1p3q1t	PQ: 6 KB Q1: 22 KB	PQ: 272 KB Q3: 272 KB Q2: 272 KB Q1: 272 KB
WS-X6548-RJ-45	48-port 10/100BASE-TX fabric-enabled with RJ-45 connectors	1p1q0t	1p3q1t	PQ: 6 KB Q1: 22 KB	PQ: 272 KB Q3: 272 KB Q2: 272 KB Q1: 272 KB
WS-X6324-100FX-MM	24-port 100BASE-FX with MT-RJ connectors	1q4t	2q2t	Q1: 16 KB	Q2: 22 KB Q1: 90 KB
WS-X6324-100FX-SM	24-port 100BASE-FX with MT-RJ connectors	1q4t	2q2t	Q1: 16 KB	Q2: 22 KB Q1: 90 KB
WS-X6348-RJ-45	48-port 10/100BASE-TX with RJ-45 connectors	1q4t	2q2t	Q1: 16 KB	Q2: 22 KB Q1: 90 KB
WS-X6348-RJ21V	48-port 10/100BASE-TX with RJ-21 connectors and inline power	1q4t	2q2t	Q1: 16 KB	Q2: 22 KB Q1: 90 KB
WS-X6348-RJ-45V	48-port 10/100BASE-TX with RJ-45 connectors and inline power	1q4t	2q2t	Q1: 16 KB	Q2: 22 KB Q1: 90 KB
WS-X6224-100FX-MT	24-port 100BASE-FX with MT-RJ connectors	1q4t	2q2t	Q1: 8 KB	Q2: 16 KB Q1: 40 KB
WS-X6248-RJ-45	48-port 10/100BASE-TX with RJ-45 connectors	1q4t	2q2t	Q1: 8 KB	Q2: 16 KB Q1: 40 KB
WS-X6248-TEL	48-port 10/100BASE-TX with RJ-21 connectors	1q4t	2q2t	Q1: 8 KB	Q2: 16 KB Q1: 40 KB
WS-X6248A-TEL	48-port 10/100BASE-TX with RJ-21 connectors	1q4t	2q2t	Q1: 16 KB	Q2: 22 KB Q1: 90 KB
WS-X6148-RJ-45V	48-port 10/100BASE-TX with RJ-45 connectors and inline power	1q4t	2q2t	Q1: 16 KB	Q2: 22 KB Q1: 90 KB
WS-X6148-RJ21V	48-port 10/100BASE-TX with RJ-21 connectors and inline power	1q4t	2q2t	Q1: 16 KB	Q2: 22 KB Q1: 90 KB
WS-X6024-10FL-MT	24-port 10BASE-FL with MT-RJ connectors	1q4t	2q2t	Q1: 8 KB	Q2: 16 KB Q1: 40 KB
WS-X6816-GBIC	16-port 1000BASE-X dual-fabric with GBIC connectors and onboard DFC	1p1q4t	1p2q2t	PQ: 9 KB Q1: 64 KB	PQ: 64 KB Q2: 64 KB Q1: 311 KB
WS-X6516-GBIC	16-port 1000BASE-X with GBIC connectors	1p1q4t	1p2q2t	PQ: 9 KB Q1: 64 KB	PQ: 64 KB Q2: 64 KB Q1: 311 KB
WS-X6516-GE-TX	16-port 10/100/100BASE-T with RJ-45 connectors	1p1q4t	1p2q2t	PQ: 9 KB Q1: 64 KB	PQ: 64 KB Q2: 64 KB Q1: 311 KB
WS-X6408-GBIC	8-port 1000BASE-X with GBIC connectors	1q4t	2q2t	Q1: 80 KB	Q2: 80 KB Q1: 352 KB
WS-X6408A-GBIC	8-port 1000BASE-X with GBIC connectors	1p1q4t	1p2q2t	PQ: 9 KB Q1: 64 KB	PQ: 64 KB Q2: 64 KB Q1: 311 KB
WS-X6416-GBIC	16-port 1000BASE-X with GBIC connectors	1p1q4t	1p2q2t	PQ: 9 KB Q1: 64 KB	PQ: 64 KB Q2: 64 KB Q1: 311 KB
WS-X6416-GE-MT	16-port 1000BASE-SX with MT-RJ connectors	1p1q4t	1p2q2t	PQ: 9 KB Q1: 64 KB	PQ: 64 KB Q2: 64 KB Q1: 311 KB
WS-X6316-GE-TX	16-port 1000BASE-T with RJ-45 connectors	1p1q4t	1p2q2t	PQ: 9 KB Q1: 64 KB	PQ: 64 KB Q2: 64 KB Q1: 311 KB
WS-X6502-10GE	1-port 10 Gigabit Ethernet with OIM connectors	1p1q8t	1p2q1t	51 KB 205 KB	PQ: 15.3 MB Q2: 17.9 MB Q1: 30.7 MB
WS-X6501-10GEX4	1-port 10 Gigabit Ethernet with SC connectors	1p1q8t	1p2q1t	51 KB 205 KB	PQ: 15.3 MB Q2: 17.9 MB Q1: 30.7 MB
OSM	All optical services modules (Layer 2 Gigabit Ethernet ports only)	1p1q4t	1p2q2t	PQ: 9 KB Q1: 64 KB	PQ: 64 KB Q2: 64 KB Q1: 311 KB

Table D-29 lists the supported receive and transmit queues of the Ethernet modules for the Catalyst 6500.

Table D-29. Catalyst 4500/4000 Supervisor Engine Matrix

Catalyst Switches That Support the Supervisor Engine I	Catalyst Switches That Support the Supervisor Engine II	Catalyst Switches That Support the Supervisor Engine III	Catalyst Switches That Support the Supervisor Engine IV
Catalyst 4003	Catalyst 4506	Catalyst 4506	Catalyst 4507
	Catalyst 4503	Catalyst 4503	Catalyst 4506
	Catalyst 4006	Catalyst 4006	Catalyst 4503
			Catalyst 4006

Table D-30 lists the 4500/4000 Catalyst switches that support each supervisor engine model.

Table D-30. Catalyst 4500/4000 Supervisor I or II Default Queue Admission

Classification	Queue
CoS values 0 through 7	1
Multicast and broadcast traffic	2

Table D-31 lists the default queue assignment for the Catalyst 4500/4000 with a Supervisor I or II after QoS has been enabled.

Table D-31. Catalyst 4500/4000 Supervisor II QoS Features

QoS Feature	Setting on 4000/4500 with Sup II
QoS Feature	Setting on 4000/4500 with Sup II
TX queue	2q1t
Scheduling	Round-robin
Trust	Switch wide
Extend trust	Not available
Classification	CoS
Operating system	Catalyst OS

Table D-32 lists the available QoS features of a Catalyst 4500/4000 with a Supervisor II Engine.

Table D-32. Catalyst 4500/4000 Supervisor III or IV Default Queue Admission

Classification	Queue
DSCP 015 CoS 01	1
DSCP 1631 CoS 23	2
DSCP 3247 CoS 45	3
DSCP 4863 CoS 67	4

Table D-33 lists the default queue assignments for the Catalyst 4500/4000 with a Supervisor III or IV.

Table D-33. Catalyst 4500/4000 Supervisor III or IV QoS Features

QoS Feature	Setting on 4000/4500 with Sup III or IV
TX queue	1p3q1t
Scheduling	1 priority queue 3 WWR Queues
Trust	Per port
Extend trust	Per port to IP Phone
Classification	CoS IP precendence IP DSCP ACLs
Operating system	IOS

Table D-34 lists the available QoS features of a Catalyst 4500/4000 with a Supervisor III of IV Engine.

Table D-34. Catalyst 3550 Default Queue Admission

Classification	Queue
DSCP 015 CoS 01	1
DSCP 1631 CoS 23	2
DSCP 3247 CoS 45	3
DSCP 4863 CoS 67	4

Table D-35 describes the default queue admission criteria.

Table D-35. Catalyst 3550 QoS Features

QoS Feature	Setting on 3550
TX queue	1p3q1t
Scheduling	1 priority queue 3 WWR queues
Trust	Per port
Extend trust	Per port to IP Phone
Classification	CoS IP precendence IP DSCP ACLs
Operating system	IOS

Table D-36 lists the available QoS features of a Catalyst 3550.

Table D-36. Catalyst 3524 Default Queue Admission

CoS Value	Queue
03	1
47	2

Table D-37 describes the default queue admission criteria.

Table D-37. Catalyst 3524 QoS Features

QoS Feature	Setting on 3524
TX queue	1p1q
Scheduling	1 priority queue 3 WWR queues
Trust	Switch wide
Extend trust	Per port to IP Phone
Classification	CoS
Operating system	IOS

Table D-38 lists the available QoS features of a Catalyst 3524.

Table D-38. Default CoS-to-DSCP Mapping

CoS Value	DSCP Value
0	0
1	8
2	16
3	24
4	32
5	40
6	48
7	56

Table D-39 lists the default CoS-to-DSCP mapping in Catalyst switches. To match the recommended settings of DSCP = AF31 (or decimal 26) for VoIP call control and DSCP = EF (or decimal 46) for VoIP bearer traffic, these DSCP values must be remapped to CoS values of 3 for VoIP call control and 5 for VoIP bearer traffic.

Table D-39. Priority Queues

Platform	Priority Queue	Configuration Command
6500	1p	On by default
4500	3	tx-queue 3 priority high
3550	4	priority-queue out
3524	2	On by default

Table D-40 summarizes the priority queues available on the Catalyst IOS switches.

Table D-40. Summary of RX and TX Queues

Switch or Module	Receive Queue	Transmit Queue
Catalyst 6500 Supervisor Engine II or Ia	1p1q4t	1p2q2t
Catalyst 6500 Supervisor Engine II or Ia	1q4t	2q2t
Catalyst 6500 65xx series line cards	1p1q0t	1p3q1t
Catalyst 6500 63xx series line cards	1q4t	2q2t
Catalyst 4500/4000 with Supervisor Engine III or IV	1q	1p3q1t
Catalyst 4500/4000 with Supervisor Engine II	1q	2q1t
Catalyst 3550 series	1q	1p3q1t
Catalyst 3524	1q	2q1t

Table D-41 summarizes the receive queues and transmit queues present on the switches that have been discussed in this chapter. For the purposes of QoS, a priority queue is always preferred over a standard queue. Use a switch or module that supports priority queues in your designs whenever possible.

Table D-41. Network Layer Postitioning

Access Layer Switch	Distribution Layer Switch
Catalyst 6500 with Layer 2 or Layer 3 (PFC) switching engine	Catalyst 6500 with PFC
Catalyst 4500/4000 with Supervisor Engine II, III or IV	Catalyst 4500/4000 with Supervisor Engine III or IV
Catalyst 3550 / 3524 series

Understand the different roles of the distribution and access layer switch. Table D-25 lists common network layer positioning for Cisco switches in real-time application environments.

The distribution layer switch provides aggregation for traffic from the access layer switches. CoS or DSCP values received from the access layer switches is trusted by the distribution layer.

It is the responsibility of the access layer switch to mark the traffic flow with the desired CoS or DSCP value. Trust the CoS values received on the access layers switch from attached IP Phones; however, be sure to rewrite the CoS value received from the attached PC to 0. Set the DSCP and CoS classification marking on the access layer switch for those devices that cannot set these values on their own, such as video servers.

After you have designed your network around your real-time application, you can trace the path of the real-time traffic flow and examine the effects of your queuing strategy. Consider the answers to these questions:

• Are your real-time applications correctly classified?

• Do your real-time applications use the priority queue?

• Are there any link mismatches or oversubscribed links that may cause instantaneous buffer overrun?

Remember that QoS in the LAN is not a bandwidth management issue as much as it is a buffer management issue. TX queue congestion can cause packet loss, which can adversely affect performance of applications that are sensitive to loss, delay, and jitter.