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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 10-13 illustrates the concept of buffer overrun.

Figure 10-13 Buffer Overflow

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.

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Table 10-24 defines a few of the Layer 2 queues available in Cisco Catalyst switches.

Table 10-24 Layer 2 Queues

Figure 10-14 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 10-25 describes the possible supervisor and switching engine combinations on a Catalyst 6500.

Table 10-25 Supervisor and Switching Engine Combinations

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Table 10-27 describes the supported receive queues of the Catalyst 6500.

Table 10-27 Supported Receive Queues of the Catalyst 6500

Table 10-28 describes the supported transmit queues of the Catalyst 6500.

Table 10-28 Supported Transmit Queues of the Catalyst 6500

Table 10-29 lists the supported receive and transmit queues of the Ethernet modules for the Catalyst 6500.

Table 10-29 Supported Receive and Transmit Queues of the Ethernet Modules for the Catalyst 6500

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Table 10-29 Supported Receive and Transmit Queues of the Ethernet Modules for the Catalyst 6500 (Continued)

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Table 10-30 Catalyst 4500/4000 Supervisor Engine Matrix (Continued)

Table 10-31 lists the default queue assignment for the Catalyst 4500/4000 with a Supervisor I or II after QoS has been enabled.

Table 10-31 Catalyst 4500/4000 Supervisor I or II Default Queue Admission

Table 10-32 lists the available QoS features of a Catalyst 4500/4000 with a Supervisor II Engine.

Table 10-32 Catalyst 4500/4000 Supervisor II QoS Features

Table 10-33 lists the default queue assignments for the Catalyst 4500/4000 with a Supervisor III or IV.

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Table 10-33 Catalyst 4500/4000 Supervisor III or IV Default Queue Admission

Table 10-34 lists the available QoS features of a Catalyst 4500/4000 with a Supervisor III of IV Engine.

Table 10-34 Catalyst 4500/4000 Supervisor III or IV QoS Features

Table 10-35 describes the default queue admission criteria.

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Table 10-35 Catalyst 3550 Default Queue Admission

Table 10-36 lists the available QoS features of a Catalyst 3550.

Table 10-36 Catalyst 3550 QoS Features

Table 10-37 describes the default queue admission criteria.

Table 10-37 Catalyst 3524 Default Queue Admission

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Table 10-38 lists the available QoS features of a Catalyst 3524.

Table 10-38 Catalyst 3524 QoS Features

Table 10-39 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 10-39 Default CoS-to-DSCP Mapping

Table 10-40 summarizes the priority queues available on the Catalyst IOS switches.

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Table 10-40 Priority Queues

Table 10-41 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 10-41 Summary of RX and TX Queues

Understand the different roles of the distribution and access layer switch. Table 10-25 lists common network layer positioning for Cisco switches in real-time application environments.

Table 10-42 Network Layer Postitioning

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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.