AI-Mesh Ethernet backhaul SLOW

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I've got a problem with an Ethernet backhaul. I've got an Asus RT-AC86U main router on the 3rd floor, and two RT-AC68 mesh nodes on the other two floors, all running rMerlin 386.1 (latest release). I have Cat-5 ethernet cables, 50' (15 meters) long, from the main router to the two nodes, no switches in either line. The 2nd floor node shows a one gigabit connection, no problem, and is happy using the Ethernet as backhaul, but the 1st floor node only registers 100 Mbit and opts for 5GHz wifi for the backhaul instead of the Ethernet cable. I just swapped out the Cat-5 cable to the first floor for a brand new Cat-6, thinking there might be an issue with that cable, and no change. I swapped the two nodes, and the problem stayed with the 1st floor location, so it's not the node hardware either. I'm baffled, appreciate any ideas. Thank you!

 

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Switch ports at the router. Possibly a bad port or dirty.

 

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Switch ports at the router. Possibly a bad port or dirty.

Thanks, but I've done that, swapped ports around, a couple of times.

 

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RESOLVED -- Update: I bought a gift foot SHIELDED Cat-7 cable, ran that alongside the other cables, and plugged it in. For the first time I got 1 gig ethernet to the node. I have no idea what might have been screwing up the un-shielded cables, but I have to assume something is radiating noise.