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Half Duplex issue


While I am no CCNA certified, I've been messing around and configuring Cisco switches for the past 6-7 years so I know my way around. I've been having an issue for the past few weeks that I can't figure out and I hope I can get some replies here that will help me out.

We recently moved our main distribution switch (previously a Cisco 4510R) to a new distribution switch (stack of 4x 3850'). The duplex issue began after the migration. Some of the trunks to some buildings are negociating at half duplex since we switched over the new stack. Previously, these buildings were connected in the 4510R with fixed MT-RJ connectors running at 100Mbps. At the building's end, some buildings are connected with Wireweks tranceivers to Cisco 2960 swiches while other buildings are connected to GLC-GE-100FX into Cisco 2960 X. For the migration to the stack of 3850', we bought some Cisco GLC-GE-100FX to replace the fixed MT-RJ connectors and thought it would work. It doesn't. The buildings are negociating at half-duplex with the GLC-GE-100FX. This happens either if there's a tranceiver at the building's end or an SFP. There is no duplex half in the config of the trunk, all is negociated auto. Forcing duplex full results in no connection.

We are now thinking about the fibers being an issue. We tried switching to other pairs, but still running with the same issue. Those fibers are Multi-Mode and probably 15+ years. Could it be the fiber quality ? Could it be that the MT-RJ connectors in the previous 4510R were more "powerful" than the new SFPs ?

Thanks for any comments.


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level 1
5 points · 9 days ago

Some 100BaseFX SFPs will not support auto-negotiation.

Try just setting it to 100 full on both sides and specify media type SFP where applicable.

100BaseFX will run a couple km on full duplex mode so I doubt it has anythingto do with the fibre.

level 1

You have to set speed 100 duplex full on both sides of the link. In the links with transceivers check for the same setting, probably some DIP switches.

level 2

I second this, may not be the issue, but it's where I'd look next. You'll need something tiny to change their position, like a sturdy paperclip. At least I do.

level 1

Also, try a different firmware. This could very easily be a bug.

level 1
Original Poster1 point · 12 hours ago

Just so you know, I am doing a followup on this as I was able to work on this last week.

Thanks to you, duplex full did the trick. I was however able to find another route with a newer mono fiber for one building which helped me to get up to Gigabit for this one. The others are running fine on 100Mb / duplex full.

There is one building that I was able to get duplex full, but I am still getting errors from time to time on the interfaces. If I slot a Gigabit SFP in the switches instead of a 100BaseFX, the link won't even come up. It's down/down and no light on the interfaces on both end. I guess there's something wrong with the fiber at that building. We're not equipped well enough to check the quality of the fiber so we'll get someone in to check it. Other pairs on the fiber didn't work either.

Thanks for your help guys. My original question was properly answered :)

level 1
CCIE1 point · 8 days ago

I believe the default duplex for 100mb is half unless you set it. Look up the ieee spec.

level 2

Nope. Default is auto. *If* autoneg fails, then peers should use half-duplex for compatibility with dumb hubs.

So question here is why in 2018 FastEthernet used between switches and why one of the sides do not respect autonegotiation.

level 3
CCIE1 point · 7 days ago

Read the spec.

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