I am trying to use my ASA 5585-X as a pure sniffer where it sends all traffic to the SFR module without having to use the policy map redirect method.

I am following this doc:
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When I enter the below command at interface level it's all good:
traffic-forward sfr monitor-only

However doing a "show run" doesn't list that command and even though traffic is received on the physical port it does not get processed.
Does it matter which ASA ports I am using?

So it looks like this is a bug in the code for the ASA - I am on 9.3.2 and entering the command "traffic-forward sfr monitor-only" does not actually apply to the configuration.

When I do "show run" the command does not appear under the interface config. It turns out this is fixed in ASA code 9.5.2

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I upgraded one of my spare boxes to 9.5.2 and now I see the "traffic-forward" command applied in the configuration. Next step will be to send SPAN traffic to make sure it actually works.

Just sent SPAN traffic to the 5585-X and it works fine - FirePOWER can identify the app traffic without issues.