Errors using span interface on if_bridge(4)

[hiren panchasara]

Alright. I am having a "mind blown" moment right now.

While reporting this checksum error issue, I always had ix3 (the
culprit interface) being monitored via tcpdump in other tmux session.
Something gotten into me and I stopped monitoring it and everything
was kosher after that. Not a single checksum error!

Just to be sure, I kldunloaded/loaded if_ixgbe (as I've built that as
a module) to reset all the stat counters and ran my tests again using
both iperf3 and netperf - and saw same behavior. If I start tcpdump on
interface again, the dev.ix.3.mac_stats.checksum_errs starts going up.
Stopping tcpdump would stop the counter from incrementing.

Why is that happening? I have no clue. How/why is tcpdump affecting
this interface traffic stats in such a way?
Just as a recap, bridge0 has ix1 as a member and ix2 as a span
interface. ix2 and ix3 are connected back to back so that I can
monitor traffic coming to bridge0 on ix3 (as ix2 will forward it all
being a span interface).

cheers,
Hiren