[c-nsp] PFC QOS question about 802.1Q trunks

[Mark Austen]

Just copied out of the Cisco doco:


*Ingress Classification and Marking at Trust CoS LAN Ports*

You should configure LAN ports to trust CoS only if they receive traffic
that carries valid Layer 2 CoS.

When an ISL frame enters the switch through a trusted ingress LAN port,
PFC QoS accepts the three least significant bits in the User field as a CoS
value. When an 802.1Q frame enters the switch through a trusted ingress LAN
port, PFC QoS accepts the User Priority bits as a CoS value. PFC QoS Layer 2
remarking marks all traffic received in untagged frames with the ingress
port CoS value.

On ports configured to trust CoS, PFC QoS does the following:

•PFC QoS maps the received CoS value in tagged trust CoS traffic to the
initial internal DSCP value.

*•PFC QoS maps the ingress port CoS value applied to untagged trusted
traffic to the initial internal DSCP value.*
•PFC QoS enables the CoS-based ingress queues and thresholds to provide
congestion avoidance. See the "Understanding Port-Based Queue Types"
section<http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SXF/native/configuration/guide/qos.html#wp1665946>for
more information about ingress queues and thresholds.

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SXF/native/configuration/guide/qos.html#wp1705197

*Configuring the Ingress LAN Port CoS Value*
http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SXF/native/configuration/guide/qos.html#wp1727961


-Mark

2009/5/20 Geoffrey Pendery <geoff at pendery.net>

> I can't answer your question about the default DSCP value without
> lab-ing it, but as to the second part: yes, it's generally a good
> practice to either pick a bogus/dummy VLAN for your native, or to
> apply the command "vlan dot1q tag native" to force it to apply a tag
> even on the native VLAN.
>
> If you need the native VLAN untagged, like to present a potential
> trunk port as an access port for untagged hosts, then "trust cos"
> might be inappropriate.
>
> Like you I would expect the DSCP to default to zero, but maybe someone
> else has the answer...
>
>
> -Geoff
>
>
> On Tue, May 19, 2009 at 11:20 AM, Matthew Huff <mhuff at ox.com> wrote:
> > I have a question about QOS trust between two 6509 switches connected via
> a L2 802.1Q trunk with multiple VLANs. If the port is set to "trust cos",
> what does the internal DSCP value get set for native frames since their
> isn't a COS field? I would assume the internal DSCP value would be set to
> the default or normally zero. Do most people then set the native VLAN to a
> unused VLAN so that native packets have internal DSCP values set? Or do most
> use "trust dscp"? If so, what do people use "trust cos" for?
> >
> >
> >
> > ----
> > Matthew Huff       | One Manhattanville Rd
> > OTA Management LLC | Purchase, NY 10577
> > http://www.ox.com  | Phone: 914-460-4039
> > aim: matthewbhuff  | Fax:   914-460-4139
> >
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