loopback interface TX counter double count (2x actual packets)

[lizheng243]

Bug Summary

In VPP 19.01 and later versions, the TX counter of loopback interface is double-counted, showing 2x the actual number of sent packets.

Root Cause

The loopback interface's forwarding path increments the TX counter twice for a single packet:

1. First count: All interfaces (including loopback) pass through the universal interface-output node , where vlib_increment_combined_counter is called to increment the TX counter once.
2. Second count: After interface-output, the loopback packet enters the simulated_ethernet_interface_tx, which calls vlib_increment_combined_counter again to increment the TX counter a second time。