Are you looking for an L2 test or L3?
By L2, I mean a Service Activation Test along the lines of ITU Y.1564. That is, the end-to-end service is L2 and you want to loopback the Ethernet frames transmitted by the JDSU traffic generator exactly as they are back to the receiver of the JDSU. I believe the ME3800 supports the external ("facility") loopback on non-routed ports, whereas I do not believe the A9K does (I believe other XR products like NCS 540/5700 do support Ethernet loopbacks).
If you only care about the L3 delivery of the UDP payload of the RFC 2544 packets, then it is a simple router-on-a-stick configuration for both the ME3800 and A9K using routed (L3) port configs. That is, a VLAN/subinterface configured on the router to receive the traffic from the JDSU and another VLAN/subinterface to forward the received traffic back to the JDSU over the same physical port. This requires the JDSU 2544 test suite to be able to transmit and receive test traffic with different IP addresses and VLAN tags on the same port.
It is possible to perform an SAT-like test at L3 where the JDSU sends and receives traffic with only a single IP address and VLAN, but this requires the JDSU to be able to craft test traffic packets using its own IP address as the destination, but with the destination MAC address of the router. Upon receiving the Ethernet frame with its own MAC address, the router would L3 forward it back out over the same VLAN (if VLAN tagged) back to the JDSU as this is its normal L3 routing function -- no special router config needed, as all the trickery happens in the test set. I know that Spirent's Qscope is capable of doing this, but I do not know about the JDSU.
Disclaimer: I am long in CSCO
