[j-nsp] Setting CoS bits on ingress frames

[Victor Sudakov]

Dave Bell wrote:
> >> When frames enter the switch, the switch will need to classify them
> >> and place them into a queue. By default everything goes into queue 0,
> >> regardless of what other switches have done.
> >
> > I am afraid you are mistaken. If a frame enters a switch via a trunk
> > port and the frame already has a non-zero COS value, this COS value is
> > forwarded *intact* out the egress interface.
> >
> > I have experimental proof of this. In my lab, Sw1 (Cisco) classifies
> > frames and their COS value set by Sw1 is visible at HostC.
> 
> When a frame enters port it is classified and placed into a forwarding
> class. There are defaults for some traffic types, but for .1p on the
> EX, everything goes into the forwarding class that is associated with
> queue 0.

Are you speaking now about access or trunk ports? If tagged frames enter a
trunk port and already have non-zero COS values, will they all go into
queue 0 ?

Personally I have no problem with them all going into one queue provided
they leave the switch with the COS fiels unmolested.

> 
> On egress, if there is a rewrite rule on the forwarding class on the
> egress interface, then all traffic gets set regardless if there is
> already markings on the packet.

And this is not desirable for me. I want frames entering trunk ports
to leave unmolested while frames entering access ports to have their
COS set.

> 
> If there is no rewrite rule on the forwarding class, then the marking
> remains unmolested.

Well then, how do I handle the situation when I need to have some frames
rewritten on egress and some not?

> 
> The solution to this is to make sure that all your traffic enters the
> correct forwarding class on ingress. This means when it egresses the
> switch, it will have the intended markings.

This sounds good. Have you looked at the chart at 
ftp://ftp.sibptus.ru/pub/vas/chart2.pdf 
This means that I need some frames to be rewritten and some not,
right?

> 
> Have you tried the configuration I have suggested? 

No, I have not yet tried to apply a classifier to the ingress trunk. I
don't like the idea of reclassifying transit frames on ingress. I'd
prefer some kind of passthru mode the way Cisco works. You just
configure "mls qos trust cos" on trunk ports and you are done.


> Have you looked at
> port counters for your interfaces to see what is happening to the
> traffic?

I am mostly looking at tcpdump and WireShark on HostC. What command do
you suggest to look at the counters? 

-- 
Victor Sudakov,  VAS4-RIPE, VAS47-RIPN
sip:sudakov at sibptus.tomsk.ru