[j-nsp] CoS Marking/Rewrite Theory

[Chris Evans]

First of all please forgive me if I cause confusion on this and let me know
if I can clarify things more..

I come from a Cisco world and am learning JUNOS. I have a question in
regards to CoS markings on packets. In Cisco devices I can modify Layer2 or
Layer3 CoS header information INGRESS an interface. From my reading in
Juniper Devices you can only write that information EGRESS an interface and
it comes from the 'rewrite-map'.

With Juniper devices you apply an input firewall filter that matches the
traffic and then you define it to a forwarding class. Traffic is then
forwarded through the device and once it reaches its egress interface using
the rewrite-map it marks the packet CoS information based on the
forwarding-class the packet was defined to. Also as we know, if filters
aren't applied to force traffic forwarding classification the 'classifier'
map is used to correlate the CoS markings to forwarding classes by default.
We also know that if a rewrite-map isn't defined the traffic passes out and
interface unmodified.


Here's my question. Say I have a router with 3 interfaces, 2 interfaces are
input and 1 output. Interface #1 and #2 are input and #3 would be output. On
interface #1 I want to mark the traffic as its currently unmarked and I want
it marked to DSCP EF(46). I have to apply the firewall filter and define
this traffic into the expedited forwarding class. To make traffic egress of
the router have this marking I have to also apply the dscp rewrite-map on
interface #3. On interface #2 the traffic is already marked to DSCP43. As I
do not have a firewall filter applied, the default classifer map kicks in
and maps the DSCP 43 traffic to expedited forwarding class as well. Once
this traffic exits the router out of interface #3, the rewrite map that had
to be defined for interface #1 will rewrite this traffic to DSCP 46,
overwriting my original markets. Now I cannot differentiate the traffic
further on in the network.


I see this is as a big limitation. Are there workarounds that I'm missing?


Thanks

BuckWeet