Foundation Summary

Foundation Summary 303

Figure 4-24 Two Output Queues, with Scheduler Always Servicing Queue 1 Rather Than Queue 2 When Packets Are in Queue 1

Assumed Behavior if

No TX Queue

R1 — Serial 0

Output Queue 1 — Preferred Queue

Output Queue 2

Actual Behavior with TX Queue

The following list summarizes the key points about TX Rings and TX Queues in relation to their effect on queuing:

•The TX Queue/TX Ring always performs FIFO scheduling, and cannot be changed.

•The TX Queue/TX Ring uses a single queue, per interface.

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•IOS shortens the interface TX Queue/TX Ring automatically when an output queuing method is configured

•You can configure the TX Ring/TX Queue length to a different value.

To delay the traffic, traffic shaping places the packets into the queue associated with the subinterface or DLCI and drains the traffic from the shaping queue at the shaped rate. Figure 4-25 shows the structure of the queues on a subinterface, interface, and the TX Queue, when shaping is enabled.

Figure 4-25 Shaping Queues, Interface Queues, and TX Ring

Router1

Table 4-19 summarizes some of the key features of PQ.

Table 4-19 PQ Functions and Features

Table 4-20 summarizes some of the key features of CQ.

Table 4-20 CQ Functions and Features

Flow-Based WFQ, or simply WFQ, classifies traffic into flows. Flows are identified by at least five items in an IP packet.

•Source IP address

•Destination IP address

•Transport layer protocol (TCP or UDP) as defined by the IP Protocol header field

•TCP or UDP source port

•TCP or UDP destination port

For perspective on the sequence of events for WFQ, marking the sequence number, and serving the queues, examine Figure 4-26.

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Figure 4-26 WFQ—Assigning Sequence Numbers and Servicing Queues

3)Maximum Number of Queues

4)Maximum Queue Length

WFQ calculates the sequence number (SN) before adding a packet to its associated queue. The formula for calculating the SN for a packet is as follows:

Previous_SN + weight * new_packet_ength

Table 4-21 lists the weight values used by WFQ before and after the release of 12.0(5)T/12.1.

Table 4-21 Weight Values Used by WFQ

WFQ discards some packet when a queue’s congestive discard threshold (CDT) has been reached. To appreciate how the CDT is used, examine Figure 4-27.

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Table 4-24 WFQ Functions and Features

Table 4-25 summarizes some of the key features of CBWFQ.

Table 4-25 CBWFQ Functions and Features

All the commands for CBWFQ are repeated for reference in Tables 4-26 and 4-27.

Table 4-26 Command Reference for CBWFQ

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Table 4-26 Command Reference for CBWFQ (Continued)

To prevent LLQ from having the same problem as PQ, where packets in the highest-priority queue could dominate, LLQ’s scheduler actually works as shown in Figure 4-28.

Figure 4-28 Servicing Queues with LLQ and CBWFQ—The Real Story

The single additional configuration command for LLQ is listed in Table 4-28.

Table 4-28 Command Reference for LLQ

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Table 4-31 Summary of Scheduler, Drop, and Number of Queues (Continued)

*WFQ’s modified tail drop includes a per-queue limit, an aggregate limit for all queues, with the ability to dequeue a previously enqueued packet if the new packet has a better SN.

Also make sure and review Tables 4-16 and 4-17 immediately preceding the Foundation Summary.