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 <[email protected]> > 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 <[email protected]> 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 > > > > _______________________________________________ > > cisco-nsp mailing list [email protected] > > https://puck.nether.net/mailman/listinfo/cisco-nsp > > archive at http://puck.nether.net/pipermail/cisco-nsp/ > > > _______________________________________________ > cisco-nsp mailing list [email protected] > https://puck.nether.net/mailman/listinfo/cisco-nsp > archive at http://puck.nether.net/pipermail/cisco-nsp/ > _______________________________________________ cisco-nsp mailing list [email protected] https://puck.nether.net/mailman/listinfo/cisco-nsp archive at http://puck.nether.net/pipermail/cisco-nsp/
