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694 Chapter 10: LAN QoS

Example 10-1 DSCP-to-CoS Mapping (Continued)

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interface FastEthernet0/0.1 encapsulation dot1Q 102

service-policy input map-cos-to-dscp service-policy output map-dscp-to-cos

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interface FastEthernet0/0.2 encapsulation dot1Q 2 native

After classification has been established, QoS tools can be used to differentiate the traffic and direct the desired classification to the proper queue.

Layer 2 Queues

Although queues on LAN switches behave similarly as compared with router queues, a slightly different perspective helps with understanding the problems solved by queuing on a LAN switch. Think of a Layer 2 queue as a bucket that buffers packets until they are transported. The larger the bucket, the more packets it holds. In a switch that has one queue, refered to as 1q, for example, all traffic is placed in this queue regardless of traffic classification and traffic is serviced on a first-in, first-out (FIFO) basis. If a packet arrives at the queue during a period of congestion, the packet may be dropped if the queue cannot hold additional packets.

Using multiple queues on a switch interface allocates some of the finite number of switch buffers in each queue. You can protect the voice queue against the possibility of running out of buffers by putting delay-sensitive voice into one queue, and all else into another. A switch that has two queues, refered to as 2q, for example, has the capability to direct traffic matching a specific classification, such as voice traffic with a CoS value of 5, into one queue while directing other traffic that does not meet this critera into the other queue. Because voice packets are smaller and more predictable than data packets, classifying and scheduling high-priority traffic into this second queue decreases the likelihood that the second queue will expreience buffer overrun and discard the high-priority traffic.

Keep in mind that each port has a finite amount of buffer space to support the buckets. One queue will take all of the buffer space, for instance, two queues will divide the buffer space into two parts, three queues divide the buffer space into three parts, and so on. If the buffer space is too small, it will not be effective in momentarily holding the traffic before transport. Because nonpriority queues are serviced in either a round-robin or a Weighted Round-Robin manner, there is no guarantee that the traffic in the buffer is transported next. This limitation can lead to instaneous buffer overrun for the classified traffic of the last example due to the smaller buffer size available.

One remmedy for this situation is the introduction of a single strict-priority queue, refered to as 1p—meaning “one-priority queue.” In a 1p queue, all traffic within the priority queue is prioritized over the traffic in a standard queue by being transported as it is received. For example, a

The Need for QoS on the LAN 695

switch configured with one priority queue and one standard queue, called 1p1q, can be configured to direct traffic matching a specific classification, such as voice traffic with a CoS value of 5, into the priority queue, ensuring immediate transport, while directing other traffic that does not meet this critera into the standard queue.

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

Table 10-2 Layer 2 Queues

Layer 2 Queue

Description

 

 

1q

A single Layer 2 queue. All traffic crossing the interface flows through this queue.

 

 

2q

2 Layer 2 queues. Traffic can be directed to the desired queue based on

 

classification.

 

 

1p1q

1 priority Layer 2 queue and 1 standard Layer 2 queue. Traffic can be directed to

 

the priority queue based on classification. Other traffic can be directed to the

 

standard queue.

 

 

1p2q

1 priority Layer 2 queue and 2 standard Layer 2 queues. Traffic can be directed to

 

the priority queue based on classification. Traffic can be directed to the desired

 

standard queues based on additional classification.

 

 

It is highly recommended that you place real-time applications into the priority queue.

Drop Thresholds

Drop thresholds define the amount of the total Layer 2 buffer use that must be reached before a specified class of traffic is dropped. In other words, this is how much the bucket needs to fill before a decision is made to begin dropping traffic of a specific class. Some switches have one priority queue and one standard queue, for instance, with four drop thesholds on the nonpriority queue, refered to as 1p1q4t. For the traffic placed into the nonpriority queue, the four drop thresholds indicate which class of traffic, based on CoS, should be dropped most agresssively in the event that a percentage of the bucket fills.

Table 10-3 illustrates a possible configuration of the theshold values from the previous example.

Table 10-3 Drop Thesholds

Queue Use Threshold

Traffic to Drop

 

 

50%

CoS 0–1

 

 

60%

CoS 2–3

 

 

80%

CoS 4–5

 

 

100%

CoS 6–7

 

 

696 Chapter 10: LAN QoS

After 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 classified 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 classified with a CoS of 0, 1, 2, 3, 4, or 5 becomes drop candidates to avoid congestion. At 100 percent of capacity, all traffic, regardless of classification, becomes drop candidates.

Figure 10-4 illustrates the drop thresholds.

Figure 10-4 Drop Thresholds

Drop Threshold 4: 100%

Reserved for

CoS 6 and 7

Reserved for CoS 4 and Higher

Reserved for CoS 2 and Higher

Available for Traffic with Any CoS Value

Drop Threshold 3: 80%

Drop Threshold 2: 60%

Drop Threshold 1: 50%

100% Available for CoS 6 and 7

80% Available for CoS 4 and 5

Receive Queue

60% Available for CoS 2 and 3

50% Available for CoS 0 and 1

 

 

 

 

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Traffic Is Dropped

(Default Values Shown)

Thesholds provide an alternative to dividing the buffer space to add more standard queues, eliminating the risk of starving one queue while flooding another. Setting a drop theshold allows the entire buffer space to be used, decreasing the potential for instaneous buffer overrun for higher-priority traffic in the standard queue.

Trust Boundries

As discussed in Chapter 3, trust boundaries represent the point in the network that you begin to trust the packet markings. Establishing a trust boundary becomes increasingly important as PC

The Need for QoS on the LAN 697

network interface cards (NICs) gain the capability to mark traffic and alter the desired QoS design of your network. Figure 10-5 shows a network that uses a trust boundary at the IP Phone. A trust boundary can also be configured on an access switch in the event that an IP Phone is not present.

Figure 10-5 Trust Boundaries

Mark

X

Y

Z

Voice Bearer Traffic Mark CoS 5, IP Precedence 5 and IP DSCP EF

Voice Signaling Traffic Mark CoS 3, IP Precedence 3 and IP DSCP AF31

Re-Mark PC Traffic CoS 0, IP Precedence 0 and IPDSCP Default

Mark

X

Y

Z

If SW1 Is Layer 3 Capable:

Classification and Marking Based on IP Precedence or IP DSCP for Ingress Traffic

If ISL/802.1q to SW1:

Map Incoming CoS Value to IP Precedence or IP DSCP Values

Hannah

IP

SW1

R1

Jessie

 

Mark

 

 

X

 

Trust

Y

 

 

Boundary

 

Mark

 

 

 

 

 

 

 

Z

 

X

 

Layer 3 Switch:

 

 

 

 

Y

 

Classification and Marking

Based on IP

 

Precedence and IP DSCP Values

 

 

Z

 

Layer 2 Switch:

 

 

Classification and Marking Based on CoS

 

 

Hannah’s PC Can Mark

 

Values

 

 

 

 

IP Precedence and IP DSCP,

 

 

 

and CoS if Trunking