Priority Queuing

Priority Queuing's most distinctive feature is its scheduler. PQ schedules traffic such that the higher-priority queues always get serviced, with the side effect of starving the lower-priority queues. With a maximum of four queues, called High, Medium, Normal, and Low, the complete logic of the scheduler can be easily represented, as is shown in Figure 4-9.

As seen in Figure 4-9, if the High queue always has a packet waiting, the scheduler will always take the packets in the High queue. If the High queue does not have a packet waiting, but the Medium queue does, one packet is taken from the Medium queue—and then the process always starts over at the High queue. The Low queue only gets serviced if the High, Medium, and Normal queues do not have any packets waiting.

The PQ scheduler has some obvious benefits and drawbacks. Packets in the High queue can claim 100 percent of the link bandwidth, with minimal delay, and minimal jitter. The lower queues suffer, however. In fact, when congested, packets in the lower queues take significantly longer to be serviced than under lighter loads. In fact, when the link is congested, user applications may stop working if their packets are placed into lower-priority queues.

Figure 4-9 PQ Scheduling Logic

Most of the rest of the details about PQ can be easily understood. PQ classifies packets based on the content of the packet headers. It uses a maximum of four queues, as mentioned earlier. The only drop policy is tail drop—in other words, after classifying the packet, if the appropriate queue is full, the packet is dropped. The length of each queue, which of course affects packet loss and delay, can be changed—in fact, PQ can set the queue length to a value of zero, which means the queue length is infinite. (Infinite really means that when the router runs out of memory, the packet cannot be queued; however, you have worse problems than queuing a packet if the router is out of memory!) Figure 4-10 summarizes these key features of PQ.

The figure represents the internals of a router, after the routing decision has identified the output interface for the packet. The following list describes each component of the queuing process, with the numbers in the list matching the numbers in the figure.

Figure 4-10 PQFeatures

1) Classification ill


-Extended ACL -Multiprotocol -Source Interface -Packet Length -Fragments -TCP and UDP Ports

2) Drop Decision

3) Maximum Number of Queues

4) Maximum Queue Length

5) Scheduling Inside Queue

Tail Drop

4 Queues Max

Unlimited Length


6) Scheduler Logic

6) Scheduler Logic

Defaults per Queue:

Drop Policy: Tail Drop (Only Option)

Queue Sizes: 20, 40, 60, 80 (High, Medium, Normal, Low)

1 PQ can classify packets using access-control lists (ACLs) for most Layer 3 protocols, matching anything allowed by any of the types of ACLs. PQ can also directly match, without using an ACL, the incoming interface, packet length, and TCP and UDP port numbers.

2 Tail drop is the only available drop policy.

3 Four queues maximum

4 Maximum queue length can be set to zero, which means the queue has theoretically infinite length. Defaults are 20, 40, 60, and 80 packets for High, Medium, Normal, and Low queues, respectively.

5 Inside a queue, PQ uses FIFO logic.

6 When scheduling among the queues, PQ always services the highest-priority queue.

Independent of the process leading up to placing the packet into the queue, the scheduler continually reacts when the TX Ring empties a packet out the interface, implying that there is now more room in the TX Ring/TX Queue for another packet. When the TX Ring frees space, the PQ scheduler then performs the logic described in Figure 4-9, taking a packet from the highest-priority queue that has a packet waiting. The PQ scheduler moves the packet to the TX Ring, for later transmission on the interface.

As with other queuing tools, none of this work happens if the interface is not congested. When the interface is not congested (in other words, the TX Ring is not full), new packets are placed into the TX Ring directly. When the TX Ring fills, PQ performs queuing. When all the PQ queues drain, as well as the TX Ring drains, congestion has abated. Newly arriving packets would then be placed directly into the TX Ring, until it fills again, which in turn restarts the queuing process with PQ.

PQ works great for QoS policies that need to treat one type of traffic with the absolute best service possible. It has been around since IOS 10.0. However, PQ's service for the lower queues degrades quickly, making PQ impractical for most applications today. For instance, even running one FTP connection, one web browser, one NetMeeting call, and two VoIP calls when creating the output for this section of the book, the TCP connections for the FTP and HTTP traffic frequently timed out.

Table 4-3 summarizes some of the key functions and features of PQ. For those of you pursuing the QoS exam, look at Appendix B for details of how to configure PQ.

Table 4-3 PQ Functions and Features

Table 4-3 summarizes some of the key functions and features of PQ. For those of you pursuing the QoS exam, look at Appendix B for details of how to configure PQ.

Table 4-3 PQ Functions and Features

PQ Feature



Classifies based on matching an ACL for all Layer 3 protocols, incoming interface, packet size, whether the packet is a fragment, and TCP and UDP port numbers.

Drop policy

Tail drop.

Maximum number of queues


Maximum queue length

Infinite; really means that packets will not be tail dropped, but will be queued.

Scheduling inside a single queue


Scheduling among all queues

Always service higher-priority queues first; result is great service for the High queue, with potential for 100% of link bandwidth. Service degrades quickly for lower-priority queues.

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