From: William Chen (kwchen@netvigator.com)
Date: Wed Apr 28 2004 - 13:05:52 GMT-3
Just want to add something to this point:
>
> if the ingress port is trusted (dscp or ip prec), it does NOT
matter
> what the cos value is or even if the cos value does not exists, the dscp
or
> ip prec will be used to derive the int dscp.
If the frame is IP packet, the DSCP/Prec is trusted.
However, if the frame is non-IP packet, the COS is trusted. If the frame
don't have cos, the default cos is used.
Check DocCD for confirmation.
- William Chen
----- Original Message -----
From: "Bob Sinclair" <bsinclair@netmasterclass.net>
To: "ccie2be" <ccie2be@nyc.rr.com>; "Ng, Kim Seng David (David)"
<ksng@avaya.com>; <ccielab@groupstudy.com>
Sent: Wednesday, April 28, 2004 11:28 PM
Subject: Re: Cat3550 QOS confirmation
> Tim,
>
> I tested my previous conclusions on my box and got the result reported, so
I
> am pretty confident in my conclusion that the command -mls qos cos X- is
> ineffective without the command -mls qos trust cos- but I am always
willing
> to learn.
>
> I will grant you that the following quote is in the config guide:
>
> -QoS assigns the CoS value specified with the mls qos cos interface
> configuration command to untagged frames received on trusted and untrusted
> ports.-
>
> Which certainly seems definitive and in accord with your view. I just
have
> not seen it act that way in practice. And I note that all configs I have
> seen that rely on -mls qos cos X- also have the command -mls qos trust
> cos-
>
> This is definitely confusing, and I certainly take your point. I just
have
> to go with what I have seen.
>
> Comments inline below:
>
> Bob Sinclair
> CCIE #10427, CISSP, MCSE
> www.netmasterclass.net
>
>
> ----- Original Message -----
> From: "ccie2be" <ccie2be@nyc.rr.com>
> To: "Bob Sinclair" <bsin@cox.net>; "Ng, Kim Seng David (David)"
> <ksng@avaya.com>; <ccielab@groupstudy.com>
> Sent: Wednesday, April 28, 2004 9:43 AM
> Subject: Re: Cat3550 QOS confirmation
>
>
> > Hi Bob,
> >
> > Are you 100% positive you didn't mis-speak? Because what you're saying
> > doesn't make sense to me. Here's the problem I have with what you said.
I
> > tried to confirm my thinking with both the Flannagan book and the Cat
3550
> > Config Guide but couldn't with 100% confidence. Maybe I terribly
> > misunderstand this port trust concept but here's my current
understanding.
> >
> > 1) When a frame comes into a Cat port and mls qos is enabled, by def,
all
> > ports are UNTRUSTED, so
>
> True
>
> >
> > if the ingress port is cos trusted & the frame has a cos value,
> that
> > cos value is used to derive the internal dscp value
>
> True
>
> >
> > if the ingress port is cos trusted & the frame does NOT have a
cos
> > value (untagged frames), the def cos of 0 is assigned & used to derive
> dscp.
>
> True
>
> >
> > if the ingress port is trusted (dscp or ip prec), it does NOT
> matter
> > what the cos value is or even if the cos value does not exists, the dscp
> or
> > ip prec will be used to derive the int dscp.
>
> True, I believe.
>
> >
> > if the ingress is NOT trusted, it doesn't matter what the cos,
ip
> > prec, or dscp value is, the default cos value is used to derive internal
> > dscp
>
> I think this may be the bone of contention. It appears to me that if mls
> qos is enabled and the port is not trusted, then CoS is zero, IP prec is
> zero and DSCP is zero. Not set to the "default value for untagged frames"
>
> >
> > 2) The default cos value is, by default, = 0, but can be changed
>
> True, if a frame is untagged, and the port is trusted, the CoS will be set
> to the default 0.
>
> >
> > Please let me know if anything I've said above is NOT correct. Also,
> please
> > see comments in-line. Thanks
> >
> > ----- Original Message -----
> > From: "Bob Sinclair" <bsin@cox.net>
> > To: "Ng, Kim Seng David (David)" <ksng@avaya.com>;
> <ccielab@groupstudy.com>
> > Sent: Wednesday, April 28, 2004 8:19 AM
> > Subject: Re: Cat3550 QOS confirmation
> >
> >
> > > David,
> > >
> > > The Cisco Catalyst QOS book by Flannagan suggests pretty strongly that
> the
> > > default CoS is not applied unless you also trust COS on that port. I
> > > confirmed this on my lab 3550 running 12.1(19)EA1.
> >
> > **************************
> > What cos is used when the port is NOT trusted? It seems to me there are
> > only 2 possibilities for determining the cos a) using the cos in
incoming
> > frame or b)assigning a default cos. Am I mistaken about this?
> >
> > **************************
> > >
> > > It is necessary to trust CoS before the default will be applied. I
> tried
> > > this on access port, but would expect the same result on a trunk or
> voice
> > > vlan.
> > >
> > > HTH,
> > >
> > > Bob Sinclair
> > > CCIE #10427, CISSP, MCSE
> > > www.netmasterclass.net
> > >
> > >
> > > ----- Original Message -----
> > > From: "Ng, Kim Seng David (David)" <ksng@avaya.com>
> > > To: <ccielab@groupstudy.com>
> > > Sent: Wednesday, April 28, 2004 5:31 AM
> > > Subject: Cat3550 QOS confirmation
> > >
> > >
> > > > Hello group,
> > > >
> > > > I would like to seek confirmation whether my understanding of the
> > > following is correct. The following two configurations will produce
the
> > same
> > > results i.e. the ingress data COS will be assigned "1" and ingress
voice
> > COS
> > > (regardless of the original value of COS) will be overwritten to "1"
> also.
> > > >
> > > > Config 1:
> > > > ======
> > > > mls qos
> > > > !
> > > > interface FastEthernet0/1
> > > > switchport access vlan 20
> > > > switchport voice vlan 100
> > > > mls qos cos 1
> > > >
> > > > Explaination: Since by default the cat3550 ports are not trusted,
all
> > > ingress COS values will be overwritten to the default COS which is
> > > configured as "1" here. As for untagged ingress data, it will acquire
> > > default COS of "1".
> > > >
> > > > Config 2:
> > > > ======
> > > > mls qos
> > > > !
> > > > interface FastEthernet0/1
> > > > switchport access vlan 20
> > > > switchport voice vlan 100
> > > > mls qos cos 1
> > > > mls qos trust cos
> > > > mls qos cos override
> > > >
> > > > Explaination: Even though I configured COS trusting here, the
> "override"
> > > command will still overwrite all ingress COS to "1". As for untagged
> > ingress
> > > data, it will acquire default COS of "1".
> > > >
> > > > What I am trying to get here is that I do not need to have the
> > "override"
> > > command to overwrite the COS value if the port is already in an
> > "untrusted"
> > > state. Is my understanding correct?
> > > >
> > > > Thanks in advance
> > > > David
> > > >
> > > >
> _______________________________________________________________________
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> > >
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