Switch Can't Get Link but MR74 Does

[Twitch]

Hey gang - I had a really bizarre situation yesterday that defied explanation, and I'm wondering if anyone has experienced something similar.

I recently installed a new MS210-24P as an MDF switch. This switch connects directly to the MX and out to the dashboard without any problems. This switch also has a cable that runs across the shop to an IDF.

When I attempted to install a second MS210-24P at this IDF I could not get a link light on this switch. Thinking that the cable run may have an issue - poor termination, break in the cable, etc - I connected the same cable to an MR74 which promptly powered-on, reached-out to the dashboard and received its configuration, and connected wireless clients to the network and the Internet without any problems. The connection was, in fact, excellent, which ruled-out any issues with the cable run and termination.

The same IDF switch did the same when connected directly to the MDF switch with a patch cable - it worked fine, link lights and all.

Has anyone else seen a situation like this? Or has our network just taken a trip into Bizarroland? Why would one device connect without any problems but another device connected to the same cable can't connect at all?

I have seen issues like the in the past, many moons ago, when dealing with 568A vs 568B or straight-through vs crossover, but nothing in recent memory.

Thanks.

Twitch

[BrandonS]

Maybe the MR came up as a repeater?  I have seen this happen with bad cables.  It can get power, but not network.

[RouterGuy]

If your cabling is proven then it's a config issue on one side or the other. Is the MS running a vanilla config or is it a legacy unit?  The test MR isn't going to be carrying some of the cruft that a legacy switch might - STP config, port-security, trunk settings, etc.

[Twitch]

Hey RouterGuy - vanilla config. The link between the two switches is a trunk with native VLAN 1 on both sides. No other VLANs. Nothing special configured on either switch at this point. Out-of-the-box and plugged-in, literally. I've just never seen an issue like this with a cable that works fine for another device, but not the one you're trying to get connected.

Also, both switches are MS210-24's, so no legacy equipment involved.

[cmr · Accepted solution]

I'd put a cable tester on the cable, you could have one pin shorted to another or oxidation on one of the pins, either way different devices will react differently.  The MR could have dropped down to 100Mb/s (dark green on core switch port in dashboard), but the MS might not be set to allow negotiation down below 1Gb/s.

I recently had an issue with some cabling I did where I had a short between pin 3 (B, so the positive data signal for one direction) and the shield that allowed an IP phone or AP to work, but only at 100Mb/s...

[Twitch]

Hey @cmr - I believe the jack at one end may have had some oxidation on it. It was on an exterior wall of a construction trailer inside the plant and it looked like it wasn't exactly in a dry location. After snipping both ends off and re-terminating, the cable worked fine for the switch.

Still weird that it worked fine for the AP but would not get link for the switch.

I'll chalk it up to things in IT that defy explanation.

Thanks everyone for the replies!!

Twitch

[cmr]

Excellent news @Twitch, glad you got it sorted.

I find these sort of bizarre situations are all what communities are about as someone somewhere might have had a relevant experience.