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

Geoffrey Pendery geoff at pendery.net
Tue May 19 12:35:49 EDT 2009

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...


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?
> ----
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