[j-nsp] CoS and ingress traffic with DSCP markings

[John Neiberger]

I ran into an issue yesterday that confused me, which seems to be a
weekly occurrence lately regarding Juniper CoS.. We had an interface
that was receiving traffic marked as EF. The interface only had the
default CoS configuration. For some reason, the traffic was arriving
at the destination marked as CS0. After I applied the CoS group to the
interface, which included classifiers, the packets started arriving at
the destination as EF like they were supposed to be.

I don't understand why a lack of CoS config would reset DSCP markings
for traffic that is already marked when it hits the router. Could it
be that since there were no ingress classifiers, the traffic was not
put into a forwarding class, so the rewrite rules on egress re-marked
it? That just occurred to me. I'm going to go check the rewrites rules
we have applied on egress to see if that is what was happening. I was
under the bad assumption that traffic already marked would traverse
the router unchanged.

Thanks,
John