Some questions about QoS

Unanswered Question
Jul 1st, 2009

Could somebody please help me understand QoS a little better...

1/ Hardware Queue / TX Ring

My understanding is that if the hardware queue/tx ring on an interface is not full, then outgoing packets are put into this queue ready for transmission out of the interface.

Whatever QoS you've configured will not get looked at because no scheduling needs to be done when the hardware queue is not congested.

This is based on what I got from reading this QoS cheat sheet.

http://packetlife.net/static/cheatsheets/qos.pdf

2/ If the WAN link has been rate limited between A and B to be 100M but the interfaces connecting these two end points are GigE, do I have to use hierarchical QoS policy with shaping on the egress to enforce my QoS policy because the hardware queue on a GigE card will never be exhausted.

Eg:

A (ISP1 GigE) ---> (ISP2 Rate Limit 100M) ---> B (ISP1 GigE)

3/ What happens to packets that exceed the policing rate which I've elected to have remarked. Do these packets get remark and then buffered?

That's it for now...

Thanks.

Andy

I have this problem too.
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Collin Clark Thu, 07/02/2009 - 13:22

Andy-

I'm no expert on QoS, I only get to play with on occasion and I took the class a few years ago. I do believe I can answer your questions though.

1. That is correct, if the Tx ring is not full the router will use the default queueing mechanism to transfer packets.

2. I realize this is an example, but here it would be best to set the links speed to 100MB. Hardly realistic I know. You would need to police the traffic and possibly shape it.

3. It's up to you. You can drop them, create a burst and forward them, remark them, there are a bunch of options you can do.

An excellent resource for QoS is the SRND. Actually the QoS class offered by GK uses the SRND for the training manual. You can find it here-

http://www.cisco.com/en/US/docs/solutions/Enterprise/WAN_and_MAN/QoS_SRND/QoS-SRND-Book.html

Hope that helps.

Joseph W. Doherty Sat, 07/11/2009 - 05:38

NB: Very similar to Collin's response. Guess Collin must be correct. ;)

#1 Correct. (NB: can often be adjust using interface tx-ring-limit. Reducing might be necessary when working with something like VoIP.)

#2 "Have to", no, but shaping, I think, for such would be better. (The downstream 100 Mbps policer should [not precisely] emulate a 100 Mbps link.) Also for shaping, you don't need hiearchical QoS, GTS might be used or shaping within non-hiearchical CBWFQ. (NB: Even better, if possible [e.g. copper], would be to run your gig connections at 100 Mbps. Shapers and actual port interfaces, at the same nomimal bandwidth, don't meter traffic exactly the same way. Also, there are sometimes limitations when using hiearchical QoS.)

#3 Can't say. All your question note is packets have been remarked, what happens because of that is - "it depends".

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