There are several ways to do this:
1). Go to Detailed Stream Results/Advanced Sequencing, find out which stream is dropping traffic. Let’s say Stream ID 400999 shows drops. Then go to Stream Preview, find Stream ID 400999, you can see which BGP route attached to that stream ID is dropping traffic.
2). Use Interesting Streams Resulst and setup a filter condition like Dropped Count (Frames) > 0. Any streams with drops will show up in this results then you can check the Rx Port or Stream ID to trace back the BGP route like above step using Stream Preview.
3). Use Create Dynamic/Query View to choose Stream ID, StreamBlock name, Dropped Count (Frames), and IPv4 Destination.
4) Refer to FAQ12321 - How to determine Loss on a Flow in Spirent TestCenter that has the same Stream ID as other Flows?