[j-nsp] CoS Marking/Rewrite Theory

[Derick Winkworth]

It matches also on PLP, not just forwarding-class..

Since PLP is just one bit... why not use it is as a flag for rewriting? 

'1' = rewrite
'0' = no rewrite


I think you can effectively do that....



Chris Evans wrote:
> 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
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