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Netflow statistics w/ layer 3 switching

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Guest_imported

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Jan 1, 1970
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Hi there!

I am currently writing a program for my employer (an ISP) in order to allow IP accounting/billing. Basically, it is an interface from Cisco NetFlow to the billing software.

The problem is as follows: it is required to distinguish between multiple interfaces that the traffic passes. They use a combination of a router AND a switch to do all the Internet backbone routing (I assume this is some sort of layer 3 switching solution). Only the first packet of each flow is forwarded by the router, further packets are delegated to the switch.
NetFlow data exported by the router contains the interface number. That`s fine and I can easily extract that information.
However, the switch obviously gets all data from it´s interface to the router and hence it is not possible to get the original interface (the interface on the router, which is relevant for the flow) number from the NetFlow data exported by the switch.
So, on the switch it is not possible to distinguish what interface on the router the traffic went over.

Does anyone know how to solve that problem? Is there a way to tell the router to pass on interface information for each flow to the switch? Or is it possible to configure the router and switch so as to let the router do all NetFlow exporting (including data from the switch)?

Thanks in advance for any help!
Markus
 
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