QoS and Switches

[ARC1450]

Well (and pardon me in my inebriated state), the big thing about QoS is that is controls bandwidth. There are several mechanisms to control bandwidth, and those are drop priority, shaping, and simply priority (IMHO). Keep in mind when I say this, I'm plastered off my ass.

With drop priority, generally what you can do is remark traffic so that the less important it is, the more likely it is that it will get dropped. Shaping is generally employed with drop priority, and it includes allocating a certain amount of bandwidth, and once that bandwidth restriction is reached, packets either get marked to a higher drop priority or dropped completely, depending on how important they are. There's also priority queuing, and this is simply setting aside a certain amount of bandwidth for a single application. This is really only good and useful on low-bandwidth (<2M) links.

Keep in mind that there are, for lack of better terms, derivations of all of these (IIRC, which I probably don't). With some switches, you can specify queue length and queue priority, thus dropping anything out of lower priority queues by what is called tail dropping, and that is the last packet in a queue gets dropped if there is no more space in the queue essentially.

And yes, Cisco's QoS explanation is heavier than lead, and at times extremely convuluted.

[edit]I should mention though, that these are generally something resembling border or core QoS techniques. For switches (such as access layer switches, or those closest to the user), generally you'll either trust the QoS you're receiving (bad idea many times), or you'll remark the traffic to the proper priority depending on where it comes from. Most used are DSCP values, at least in sco equipment.