Before I continue to the configuration of QoS, first some short guidelines. Markings determine traffic classes, as explained in part I. It is up to the network device to decide what to do with a certain class of traffic. For consistency, some general guidelines. Note that there is nothing stopping you from using other values than the ones recommended, but using these values will make troubleshooting easier.

Layer 2 – CoS
The CoS field is three bits, so that means there are eight CoS values possible: 0 to 7. By default the CoS marking is zero for most frames. Wikipedia has a page with commonly used traffic classes. The most important ones, depending on the environment, are CoS 5 for voice payload and CoS 3 for FCoE.

Layer 3 – IP Precedence
The IP Precedence field is the first three bits of the DSCP field described below. These days it’s not used anymore by itself but as part of the DSCP field. Nevertheless, it still has a one-to-one mapping to the CoS values. So the guideline is the same as for CoS: IPP 5 is commonly used for Voice. FCoE doesn’t have an IP header, so no IP precedence or DSCP value. By default the marking in this field is also zero, with one notable exception: router generated control packets such as OSPF and HSRP hellos are marked with IPP 6 by default.

Layer 3 – DSCP
Differentiated Services Code Point has six bits, so 64 combinations. It is more complex than IPP and CoS, but the first three bits still mean the same: they indicate a priority for the traffic class. The last three bits are commonly used to mark drop probability. Priority is used for latency mostly: the higher priority a packet is treated, the less time it spends in a queue, and the lower the latency. Drop probability used to differentiate between traffic classes in case a queue is full and packets must be dropped. A full explanation can be found on Wikipedia again, but here are some general guidelines to help understanding the logic:

  • As far as drop probability goes (the last three bits), often only the first two are used in practice.
  • The commonly used DSCP values have a name. Most of these are called Assured Forwarding, with two numbers. The first number indicates the priority, the second drop probability. Because the last bit isn’t used in this naming, AF11 stands for ‘001 01 0’ or DSCP value 10. It is meant to indicate a low priority, low drop probability.
  • Priority in assured forwarding is counted 1 to 4, drop probability 1 to 3. AF43, ‘101 11 0’, means high priority, high drop probability. AF12, ‘001 10 0’ means lower priority, medium drop probability, and so on.
  • Voice traffic is usually given the value Expedited Forwarding (EF), DSCP value 46: ‘101 11 0’. Why this value? It makes sense if you think about it: the first three bits are ‘110’ or 5, which means IPP 5 and maps to CoS 5, the value used for voice on layer 2. The last three bits are ‘110’, or high drop probability. Since voice traffic needs to be as real-time as possible, queueing it is of little use. So in case of a filled queue, which would mean a lot of latency, it is better to drop it in favor of packets that can tolerate more delay.

So what are the AF classes used for? Answer: any application that fits the description. FTP, for example, is very tolerant of latency and can tolerate some packet loss, so AF12 could be a good DSCP value. An interactive application such as Remote Desktop Protocol will perform better in a low latency environment, and preferably less packet loss, so AF41 could be used. Of course, this all depends on the needs.

Also, the QoS mechanisms work by differentiating between traffic flows: if you mark every application in the network with DSCP EF, it is the same as not marking it at all, because everything will be treated the same again.

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