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

[John Neiberger]

On Thu, Jan 23, 2014 at 2:41 AM, Alexandre Snarskii <snar at snar.spb.ru> wrote:
> 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).

Ah, that's exactly it. That explains the mystery. I'm pretty sure I
get it now, but I thought that same thing last week.  :)

Thanks to everyone who helped me get this straightened out in my head.

John