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Spirent TestCenter: Why are there drop counts when Tx-Rx=0?

Usually drop count can increment if there is more than one Rx port receiving traffic (f.e. failover tests). Say you have Port1 (Tx) and Port 2 (Rx) and Port 3 (Rx), as traffic is sent from P1>P2 say 100 frames with sequence #1-100, then switches to P1>P3 with another 100 frames sequence #101-200, then back to P1>P2 with another 100 frames sequence #201-300. So Total Tx-Rx=0 because P1 Tx 300 frames and P2/P3 Rx 300 frames so there is zero frame loss BUT drop count will be 100 frames because the sequence #’s received on P2 are missing between 101-200 so our analyzer will increment 100 frames as dropped. Drop count is determined by sequence numbers in our Spirent signature field and Tx-Rx is an EOT calculated at the end of test. The key to remember is if Tx-Rx=0 then the number of (physical) frames being sent and received are equal so there are ZERO frame loss which is a good thing.

Product : Routing,L2-3,Windows GUI,IP