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

[Alexandre Snarskii]

On Wed, Jan 22, 2014 at 09:20:36AM -0700, John Neiberger wrote:
> 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? 

When there are no explicit classifiers configured for interface, there 
are implicit "default ones" applied: 

snar at LAB.SPB> show class-of-service interface ge-1/0/0.13 detail 
  Logical interface: ge-1/0/0.13, Index: 336
Object                  Name                   Type                    Index
Classifier              ipprec-compatibility   ip                         13

and yes, this classifier maps EF (DSCP 101110 = IPPREC 101) traffic 
to BE forwarding class: 

snar at LAB.SPB> show class-of-service classifier name ipprec-compatibility   
Classifier: ipprec-compatibility, Code point type: inet-precedence, Index: 13
  Code point         Forwarding class                    Loss priority
  000                best-effort                         low         
  001                best-effort                         high        
  010                best-effort                         low         
  011                best-effort                         high        
  100                best-effort                         low         
  101                best-effort                         high        
  110                network-control                     low         
  111                network-control                     high        

so rewrite-rule configured on outbound interface will rewrite dscp/ipprec
to all-zeros (default for BE). 

> 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
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In theory, there is no difference between theory and practice. 
But, in practice, there is.