Routing

Hello Experts

 

I am doing BA classifier depending upon the dscp code points. When the traffic leaves the JUNOS device then the marking will be preserved or it would be altered (assuming I did not apply any rewrite-rule).

 

As I tested with EX switch, there is not default rewrite rules on the interface so definitely it will not change the marking and preserve it but what about other JUNOS devices like MX/J/SRX?

 

Thanks 

Hello,

If no rewrite rule, then no rewrite.

Thanks,

Ato

Hello Ato

 

But could you please let me know the default behaviour of JUNOS?

 

Thanks 

Junos by default does not perform any rewrite on existing headers.

By default, Junos only writes codepoints of new headers (e.g. new IP headers added in a GRE tunnel, or new MPLS headers added during MPLS label push or swap, etc...).

If it helps, please consider accepting solution + kudos 🙂

Thanks

The default behavior is a little different in an MPLS interface. The JUNOS applies a default exp rewrite rule on all mpls interfaces. This has to do something with the exp bits in the MPLS header and not with the DSCP bits in the IPV4 header.

 

 
Router>show class-of-service interface ae1  
Physical interface: ae1, Index: 129
Queues supported: 8, Queues in use: 4
  Scheduler map: <default>, Index: 2
  Congestion-notification: Disabled
 
  Logical interface: ae1.0, Index: 326
    Object                  Name                   Type                    Index
    Rewrite                 exp-default            exp (mpls-any)             33 >>>>>>>>>>>>>>>>>>>>>>>>
    Classifier              dscp-ipv6-compatibility dscp-ipv6                  9
    Classifier              exp-default            exp                        10
    Classifier              ipprec-compatibility   ip                         13

Just for completeness, in Trio chipset, there is no default EXP rewrite rule (there was a cosmetic PR displaying it but it wasn't true). By default in Trio, the new MPLS headers get their EXP copied from prior EXP or TOS.

True, the exp default rewrite is kind of a platform driven, other platforms like M,T,PTX, MX(DPCs) still hold on to the default exp rewrite irrespective of the JUNOS version.

 

following is an output from the PTX router PFE.

 

SNGFPC0(router vty)# show cos ifl-entry 98
CoS IFL IDX: 98
Flags: 0x0
CoS Flags: 0x0
COS NODE: ifl index: 98
         parent count: 0
Features for ifl:: et-0/0/4.0, idx 98: IFL has 4 features
  Ingress classifier, type DSCP type 1 table id 32267 flags 0
  Egress rewrite, table type DSCP rewrite type 0  proto 0 table_id 14931 flags 0
  Ingress classifier, type EXP type 2 table id 16160 flags 0
  Egress rewrite, table type EXP rewrite type 2  proto 0 table_id 64738 flags 0 >>>>>>>>>>>>>>>>>>>default
 
SNGFPC0(router vty)#