[j-nsp] <COS Query> While imposing MPLS labels what value is set on EXP bits

[Mark Tinka]

On 1/Mar/15 19:14, Amarjeet Singh wrote:
> Folks - I've query on COS in MPLS VPN environment.
>
> While receiving Packets ingress from CE, let's say TOS field in IP header
> is set to IPP-4.
> And i've a MF that is classifying IPP-4 into FC-4 + PLP-LOW.
> + There is MPLS Rewrite rule on egress interface that is matching
> FC-4+PLP-low and setting EXP bits to 4 (100)  while relaying packet to next
> hop.
>
> EXP rewrite happens only on MPLS encapsulated packets before that packet
> should be MPLS encapsulated first.
>
>  While receiving IP packets coming from CE and imposing 2 Label stack on it
> (Transport/VPN label) what value is set on EXP bits of Labels? Does the TOS
> value in IP header of IP packet is copied to MPLS EXP bits?
>
> How does the MPLS classification happens to set FC+PLP on MPLS packets
> before hitting MPLS EXP rewrite rule on egress?

Most vendors, by default, will copy the first 3 bits of the DSCP value
into the EXP field, if there is no explicit remarking of the EXP field
on ingress.

Since IPP is 3 bits only, the IPP bits get copied to the EXP field verbatim.

I'm not sure how this works if you're remarking on egress (classic
Junos). We remark on ingress, separately for DSCP and EXP.

Mark.