Having enabled NetFlow on both the ATM and Ethernet interfaces, a more complete picture of the traffic flows through this router was obtained. For example, in the route cache, an ICMP flow (ping echo request) arrived on interface ATM1/0.1 from IP address 10.1.10.1 destined for IP address 192.168.10.1 on interface Fa4/0. Because there was some visibility of flows on the Ethernet port, the ICMP echo reply, which arrived on interface Fa4/0 destined for interface AT1/0.1, could be seen.
With NetFlow enabled on all applicable interfaces, there was no need for the output of both routers (as in the Frame Relay example) to track the flow of IP packets.
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http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6555/ps6601/prod_white_paper0900aecd8045b422.pdf