[j-nsp] Question on SCU/DSU

[cc loo]

Hey folks,

I have some problems understanding SCU/DSU so some clarification would help
here !

I'm trying to do some policy-based-routing base on source prefixes.

So when a packet enters my router, it would like to tag it with a class
(local,transit-customers,upstream). Then i would like to send it to another
routing-instance (default route it to a proxy actually), base on the class
tagged


I have some configs here

### this is to tag packets to see what kind of customers
policy-statement identify-prefixes {
    term 1 {
        from {
            protocol [ ospf static direct local ];   ### my access customers
        }
        then {
            destination-class dcu-ospf;
            source-class scu-ospf;
            accept;
        }
    }
    term 2 {
        from {
            protocol bgp;
            community [ 12345:1304 12345:1305 12345:1307 12345:1308
12345:1400 ];  ### my transit customers
        }
        then {
            destination-class dcu-bgp;
            source-class scu-bgp;
            accept;
        }
    }
    term 3 {
        from protocol bgp;
        then {
            destination-class dcu-all-others;   ### anything else
            source-class scu-all-others;
            accept;
        }
    }
}


Now i read the official docs that i have to enable a input and a output
interface. (access interface and upstream interface)
But i don't quite understand the direction of the interface.

What i'm trying to find out is what class a packet belongs to when it enters
the route. Base on that i'll inspect the packet's class to decide if i want
to forward it to the proxy or not.
Hope someone can shed some light on this, its giving me heaps of headache.
The more i read the more confusing it gets