Queuing Strategies and Tools

As I'd mentioned at the beginning of my notes, queuing impacts for major characterisics of QoS. Those are bandwidth, delay, jitter, and packet loss. The default FIFO queue that IOS creates in memory to handle packets affects drop, delay, and jitter. This is because of its size: once full, tail drop of packets will happen. You can prevent this by lengthening the queue, but that has impact on delay and usually jitter. And of course, if traffic rates exceed the interface bandwidth, your chances of a drop are higher regardless of queue length.

The Cisco QoS Exam guide offers better insight into the queuing process than does the CCIE book. It displays a sample with two queues to better explain the process. Here's a diagram I created which is similar to the one in the book.

This diagram shows the major questions a chosen queuing method will answer. Note that while question 1 appears to be classification/marking, this is actually part of the queuing method's decision process. In other words, it is a queuing choice based on the markings which have already been applied by marking/classification. Don't get confused!

Each queue will normally be FIFO, unless you set class-default to use WFQ. This was mentioned before as an option. The interface subcommand is fair-queue.

After queuing occurs, queue scheduling will be done. This is the pattern, or algorithm, used to service queues. This can take several forms, such as always servicing a particular queue first, or a bandwidth cap may be observed, etc.


Weighted Random Early Detection (WRED)
This queuing mechanism monitors the usage of a queue and progressively drops traffic as it is more heavily utilized. This is done in hopes that TCP is the main protocol in use; if TCP is present, it will retransmit lost traffic and throughput will be better managed. If not, you will have a problem.

There are settings WRED uses you'll want to be familiar with. Among them are:

Average queue depth- this is compared to thresholds you set and then actions are taken (drop/remark traffic)
Mark probability denominator- used to specify how much traffic is being dropped; 1/X, where X is is the mark probability denominator. This calculation sets the max threshold drop percentage.