Re: Qos Clarification

So we are talking MLS QoS which has taken on many different behaviors for
different platforms/generations. One of the greatest benefits to MLS QoS is
that it can be performed in hardware at the ASIC level. My personal
drawback to it was the learning curve involved for understanding and
configuring on older 6500s (pre 2T) and 1 u devices like XX60's. Newer
4500's & 6500's still run MLS QoS in hardware but operate more in MQC
fashion like a router. Aligning with above question (and R&S blueprint),
lets stick to how the 3560 operates... still very limited in what can be
done with MQC and no QoS performed out of box by default.

Without enabling QoS globally, frames are forwarded FIFO unaltered. When
MLS QoS is first enabled on 3560, without enabling trust on an interface
CoS/DSCP values are remarked to 0. When trusting CoS/DSCP, the following
chart:
http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/15.0_2_se/configuration/guide/swqos.html#wp1023954outlines
what values are kept or re written based on what is trusted
compared to what comes in both the frame/IP headers. As outlined on the
right side of this chart, The CoS-DSCP mutation is used when CoS is trusted
to assign mapped DSCP value. If DSCP is instead trusted on an interface, it
may never require a mutation. Which to trust depends on end point and best
practices. Also remember that if using trust on a VLAN then it has to be
specified it is VLAN based on the physical interface. This should cover
details of question 1 better. Learn this chart, it is important for
understanding QoS on 3560!

Question 2 is a bit trickier based on the said example. QoS is based
on bandwidth as opposed to packets. packets can vary is size but
more definitive could be the MTU on a frame which is typically set value
around 1500 bytes. At a higher level to answer *"Secondly is cos->dscp
marking is only how switches work or switches and router*
*are same"* QoS is important in 2 situations:

(#1) Going from higher bandwidth to lower bandwidth (traditional LAN-WAN
bottleneck). This is usually when you see a router doing QoS.
(#2) Many streams transmitting on same ring.

MLS QoS is best suited for helping situation #2 in which a campus traffic
has plenty of bandwidth but can experience 'micro bursts' of traffic when
majority of interfaces are actively transmitting/receiving. MLS QoS will
forward priority queue and appropriately buffer streams to service priority
queue first when configured. It will also starve other queues aside from
priority queue at times of congestion. I suggest reading the whole section
where the above chart was referenced:
http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/15.0_2_se/configuration/guide/swqos.html#wp1372578and
come back with any further questions.

Regards,

Marc Edwards
CCIE #38259

On Wed, Apr 10, 2013 at 5:15 AM, sameer khan <khanzadap_at_hotmail.com> wrote:

> Thanks for answering.
> But what i am confused is, lets say for example a packet comes with cos
> marking in 3560 switch, now as per what i have read on the different blogs
> is
> that cos-dscp table will be consulted and cos will be changed to dscp and
> when
> is leaving the switch it will be changed back to cos from dscp-cos table.
> So I dont know why cos is getting converted to dscp is it because all the
> prioritization or what stuff you want to do is done on dscp, as i am not
> clear
> in the thought process of marking internally or you can say i want to know
> the
> thought process of a switch and router.
> Secondly is cos->dscp marking is only how switches work or switches and
> router
> are same.
>
> Thanks
> > Date: Wed, 10 Apr 2013 10:21:23 +0300
> > Subject: Re: Qos Clarification
> > From: immrccie_at_gmail.com
> > To: khanzadap_at_hotmail.com
> > CC: ccielab_at_groupstudy.com
> >
> > for 1) if switch is receiving frames on a trunk link then COS will
> be
> > considered , (experts can explain better how behavior changes with
> > mutation maps / MLS settings on swith)
> > on routed port on 3560 DSCP will be considered.
> >
> >
> > On Wed, Apr 10, 2013 at 10:16 AM, sameer khan <khanzadap_at_hotmail.com>
> wrote:
> >
> > > Hey guys,
> > >
> > > I need clarification on QOS, i always get confused about two things. If
> > > any of
> > > you can please clarify this to me.
> > >
> > > 1) A packet is marked with Cos and DSCP, what marking will be
> considered
> > > internal by a 3560 & Routers. So basically how is router prioritizing
> the
> > > packets internally.
> > > 2) How do i calculated this, for example configure this
> shaping/policing
> to
> > > drop 3 ping packets and allow only 2 packets.
> > >
> > > Thanks
> > >
> > >
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Received on Wed Apr 10 2013 - 06:41:43 ART