traffic Marking

[Tariqemad]

Hi, i'm working on traffic marking idea . let we say we have 3 router ( A , B , C ) .. router C connect to internet . and there is BGP peer between the routers.. let i have Subnet (x) and this subnet marked with DSCP 28 in Router A . So if the traffic flow like below 

src ip : x    >>>> dst. ip :any      ( Upstream traffic from Router A  to internet though Router C) 

src ip :any  >>> dst ip :x    ( downstream traffic from internet  to Router A via Router C ..  Here Router C should check what was the DSCP value for prefix X  when the prefix X was the source , then set the same value of DSCP to this downstream packet .. )  

So, is there method in Router C can do like this process ? 

Kind Regards, 

[shaps]

If I've understand what you are looking to achieve is to have markings honoured throughout the network.
Anything within your own infrastructure and to some degree over an mpls provider can be treat accordinly, however anything traversing the internet will have the value re-written to best effort.

[Tariqemad]

my goal any upstream traffic out from my router with DSCP value to the internet via Router C .. Router C set the the same DSCP for the downstream traffic to the same subnet .. something like this ,, 

[Joseph W. Doherty]

". . . however anything traversing the internet will have the value re-written to best effort. "

My experience has been Internet devices generally don't rewrite the ToS, i.e. they generally just ignore it and treat all traffic BE.

[Joseph W. Doherty]

If router C will set DSCP values based on statically defined (internal) destination prefixes, yes no problem. If you want router C to "reflect" whatever DSCP value went the other way (outbound), i.e. some form of dynamic remarking, matching what was seen earlier, that, as far as I know, wouldn't be possible.

[Tariqemad]

If i have customer connected to router A .. with subnet 1.1.1.1/28 .. and i need give this subnet priority then other traffic. So in router A i will mark this subnet with DSCP value 28..  So when the traffic upstream ( i.e from Router A to internet via Router C ) Router C should see this subnet which DSCP value has. Then  after the traffic comeback (i.e downstream from internet to Router A via Router C to the destination  1.1.1.1/28 ) then router C should set the DSCP to this destination equal to 28  .. 

All this to enable me to set Qos between Router B and Router A .. So Can Router C do this operation or not Thanks in advance 

[Joseph W. Doherty]

If router C is just using looking at destination traffic, inbound from Internet, bound to 1.1.1.1/28, yes it should be able to set the ToS to whatever is desired.

[Tariqemad]

So , how i can do this ,, thanks in advance 

[Joseph W. Doherty]

You define an ACL to match the network.

You define a class-map to use the ACL.

You define a policy-map class to use the class-map and set the desired ToS marking.

You could use such a policy map either for an interface's ingress or egress policy (in your case ingress on Internet interface or egress interface to router B).