I had a similar-ish issue a few months back. I know the fix doesn’t make the most sense, so please bear with me. The short version is we discovered legacy SFPs on the network. As soon as we replaced them with the usual model, then it all went back to normal. The longer version is a power outage lasted longer than our backup generators could support. When it all came back online, our speeds were different - good upload speeds but poor download speeds. My full-stack Meraki setup let me reproduce the issue on various MX and MS firmware versions. Also, I mostly skipped checking the individual links, because I had identical SM fiber SFPs all around with no CRC errors (or similar). However, after extensive searching, my team located only one pair of outdated MM fiber SFPs on a single link connecting our MXs and a core switch. The issue vanished the moment we replaced it with identical SM SFPs. The reason I say this doesn’t make the most sense is two-fold. First, the problem was observed while I was directly plugged into the MX - and not a switch using the MM fiber link. Second, the problem only appeared after a power outage caused a complete power cycle of the network appliances. (I see it as a problem that was lying in wait, then appear after a reboot.)
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