From: ccie2be (ccie2be@nyc.rr.com)
Date: Wed Apr 28 2004 - 10:43:14 GMT-3
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
if the ingress port is cos trusted & the frame has a cos value, that
cos value is used to derive the internal dscp value
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.
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 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
2) The default cos value is, by default, = 0, but can be changed
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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