Configuring SCU on the Output Interface
Since VPN traffic enters the egress router through the VPN loopback tunnel interface, you still need to determine the exit interface for this traffic. To complete your SCU configuration, configure the output version of source class usage on the exit interface of your egress router:
[edit interfaces]
at-1/1/0 {
unit 0 {
family inet {
accounting {
source-class-usage {
output;
}
}
}
}
}