Difference between DSCP, AF, and CS

[John Blakley]

What is the difference between these when coloring traffic on an ingress port?

I'm under the assumption that AF sets individual queues up, and each one of those queues can have something done to it by a match statement.

I'm not clear on what the CS values are for. Maybe L2 CoS?

The DSCP value: When you use "set dscp ?", you'll get <0-some number>, all af values, ef, and all cs values.

I've noticed that if I mark with an AFxx, I get a chart when doing a show policy-map <name>, but if I just do a "set dscp 2", then that particular queue looks a little different under the show command.

Thanks,

John

[John Blakley]

So, from the routers prospective it doesn't really matter, but a switch's perspective it does. When the switch receives a marked packet, it has internal coding that would treat packets a certain way?

John

[Edison Ortiz]

Yes, for instance, see table 35-2 and other tables on this document:

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_46_se/configuration/guide/swqos.html#wp1199585

HTH,

__

Edison.

[Jon Marshall]

John

No not really. I think, altho Edison will confirm, that what Edison was saying is that Cisco routers and switches will recognize certain values whether CoS or DSCP and treat them accordingly - think AutoQos as a good example.

But again there is nothing to stop you changing this behaviour. That is what the CoS-to-DSCP, DSCP-to-CoS maps etc.. are for on a switch.

And Cisco mark certain network traffic with CS6/CS7 and Cisco VOIP kit will mark packets but the key point i'm making is that these values in isolation are meaningless. It is up to you or Cisco as to how they treat them ie. if you simply marked a packet as AF41 and left it at that with no queueing and only tail drop used then the AF41 packets would get no better treatment than anything else.

Jon

[Mohamed Sobair]

Hi,

The "CS" is backward compatibilty with IP precedence values.

The IP-Precedence represent 3 bit in the IP header from the TOS field.

On the other hand, The DSCP is 8 bits long of the "TOS" field in the IP Header. The actually most left 6 bits are used to mark packets, while the most right 2 bits are "ECN" field and has different usage.

Assured Forwarding "AF" and "EF" are all PHP codes for the corresponds DSCP binary value.

DSCP has 4 classes with 3 drop probabilities.

Each class represented by "X" and each priority or drop probability is represented by "Y".

The Correspond Binary Value for the AF marking is defined by the equation bellow"

8x+2y (Binary)

where x = class , y = priority

The sum represent the Binary value.

On the other hand, the L2 COS are 3 bits in the layer-2 header , marking the layer-2 frames with 802.1p.

There is a recommended marking for each type of traffic based on RFC.

The Coloring has no affect on the queue, the Coloring "Marking" is part of the laer-2 and laer-3 packet headers , So it doesn't sets queues up.

HTH

Mohamed

[Joseph W. Doherty]

Although I believe some of the (many other) posts touch on this, just want to emphasis there's often a big gap between how things are defined and what's provided on a particular vendor's network device.

The ToS (type of service) byte goes pretty far back as being part of the IP header. Over the years, RFCs have defined different interpretations to this byte's contents. DSCP is one of the later interpretations for the high 6 order bits (0..63, decimal value). Later RFCs provided suggestions on how DSCP might be used, e.g. assured forwarding (AFxy), expedited forwarding (EF), scavenger, etc. Then there's also a RFC suggesting QoS classes and how AFs and EF might be used.

Although RFC can be dry and/or difficult to understand, they are what defines the standards, so you might benefit from reading the RFCs on these issues.

What's often missing from RFCs, is how and why you might want to do things. For this, often books on a subject will make it clear(er). Cisco has some good ones under its label; their whitepapers and technotes are also very helpful (especially with regard to using features specific to Cisco).

Once you covered the standard meanings, and have a feel for the theory, then you can jump into what you might do on actual network equipment. Although you can learn from the equipment's features, you're often left with many gaps in understanding.

As with many other things, to obtain full benefit of some technology, you often need to know much beyond the subject at hand and/or an understanding of its history to reach "critical mass" (in understanding).

These forums are great, but they might not be the best place to easily obtain any subject's "critical mass", directly. However, if you ask for suggestions on books, articles, classes, you might acquire "critical mass" sooner. These forums can be an excellent source for information tidbits and/or some informational point clarification, but from your original questions, and the lack of information they imply (NOT intended as a negative!!!), I'm concerned some answers might confuse as much as explain.