Queuing and Traffic Shaping

Shaping tools support a variety of queuing tools that can be applied to the packets waiting in the shaping queue(s). At the same time, IOS supports queuing tools for the interface output queue(s) associated with the physical interface. Deciding when to use queuing tools on shaping queues, when to use them on the interface, and how the configurations differ in each case, can be a little confusing. This section clears up some of that confusion.

To begin, Table 5-5 lists the traffic-shaping tools, and the queuing tools supported by each for the shaping queues.

Table 5-5 Options for Queuing in Traffic-Shaping Tools

To begin, Table 5-5 lists the traffic-shaping tools, and the queuing tools supported by each for the shaping queues.

Table 5-5 Options for Queuing in Traffic-Shaping Tools

Shaping Tool

Queuing Tools Supported for the Shaping Queue(s)

GTS

WFQ

CB shaping

FIFO, WFQ, CBWFQ, LLQ

DTS

FIFO, WFQ, CBWFQ, LLQ

FRTS

FIFO, WFQ, CBWFQ, LLQ, PQ, CQ

When a shaper uses a queuing tool, instead of having a single shaping queue, multiple shaping queues exist. If FRTS were configured to use Priority Queuing (PQ), for instance, PQ would create four queues for shaping, named High, Medium, Normal, and Low. Figure 5-11 shows the basic idea, with shaping enabled on the physical interface, FIFO Queuing on the physical interface, and PQ configured for shaping the only VC.

Figure 5-11 FRTS, with FIFO Queuing for the Physical Interface, Plus PQ for the Shaping Queue

Shaping Queues for a Single VC

PQ High -Shaping Queue

PQ Medium -Shaping Queue

PQ Normal -

Shaping Queue

>

Shape to

PQ Low -

96 kbps 1

Shaping Queue

Interface Output Queue

TX Ring

AR 128 kbps

The shaping queues exist separately from the interface output queues, as seen in the figure. With PQ applied to the shaper, four shaping queues exist for this VC. When the shaper decides to allow another packet to be sent, it takes the next packet from the PQ shaping queues, according to PQ scheduler logic. Those packets are placed into queues associated with the physical interface and then forwarded out the interface.

In some cases, the shaping queues are bypassed, and in other cases, the interface output queues are bypassed. To understand why, consider Figure 5-12, which demonstrates part of the logic behind the decision for determining when each queue should be used.

Packets are held in a shaping queue or interface output queue only if there is some reason why the packet must wait to take the next step. For instance, you already know that if the TX Ring is not full, packets are immediately placed into the TX Ring, bypassing the interface output queue. Likewise, if shaping decides that a packet does not need to be delayed, it can go directly to the interface output queue, or even to the TX Ring.

Many QoS designs call for shaping per VC, as mentioned in the preceding section. Suppose that a router has two 64-kbps CIR VCs sharing an access link, each configured on a separate point-to-point subinterface. Shaping queues will be created for each VC. A single set of interface output queues will be created, too. Figure 5-13 depicts the overall idea.

Figure 5-12 Decision Logic for Queuing with Shaping Enabled

Packet Routed Out This

Figure 5-12 Decision Logic for Queuing with Shaping Enabled

Packet Routed Out This

PQ High -Shaping Queue

Shaping Queue

PQ Normal -

Shaping Queue

PQ Low -Shaping Queue

Any Packets in Interface Output Queues?

Figure 5-13 Fancy Queuing for the Physical Interface and for Two Sets of Shaping Queues

Shaping Queues for Subinterface 1

Figure 5-13 Fancy Queuing for the Physical Interface and for Two Sets of Shaping Queues

Shaping Queues for Subinterface 1

64 kbps

The shaping tool creates a set of queues for each subinterface or VC, based on the queuing tool configured for use by the shaper. IOS creates only one set of output interface queues for the physical interface, based on the queuing configuration on the physical interface, as covered in Chapter 4, "Congestion Management." In Figure 5-13, two sets of shaping queues have been created, one per VC. Both VCs feed the single set of interface output queues.

Finally, in this particular example, congestion can occur at the physical interface. The total of the two shaping rates listed in Figure 5-13 is 160 kbps, which exceeds the access rate. Because interface output queues can fill, it helps to apply a queuing tool to the interface output queues in this case.

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