Devices getting disconnected unless there is an unmanaged switch

[Horazul]

Hi.

I am not a network expert, so it is possible that I don't use the correct terminology... Sorry for that.

I am having a very weird issue with some of my devices (access control door control units) connected to a core switch, they get randomly disconnected from the network at the same time, the ports are up and ok, but devices  are not recognized and MAC are lost.

But if there is some unmanaged switch/router in between that does not happen even when using same ports.

Devices----Core switch: door control units get disconnected from time to time
Devices----Unmanaged switch----Same port in the core switch: connection is not lost (i have a couple of door control units using this kind of setup and while the others get disconnected, these remains intact)

Network engineers tell me that they are not able to find any issue with these ports, but the problem happen a couple of times a week.

What do you could be happening here? 

Best regards

[DanielP211]

Hello,

Its really bad practice to connect end devices to the CORE. That being said, could you show the logs of the core? Does the port flapp at that time?

BR

[Horazul]

Hi Daniel!

These are the kind of the thing that I said I could be mistaken, my bad!, when I said core switch I mean the final managed switches that belongs to the corporate network, which I think it is formed by the core, distribution and end layers, all of these are managed units.

When we put a "dumb" switch in the middle of the door control unit and the managed switch, the issue disappears.

I am going to try to get those logs!

Thanks for your answer!

Best regards

[MHM Cisco World]

Dumb SW that not run STP can make traffic flow.

I think STP blk the traffic 

[Leo Laohoo]

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@Horazul wrote:
When we put a "dumb" switch in the middle of the door control unit and the managed switch, the issue disappears.

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Because the hub only looks at two pairs of copper. 

If the door control unit has a faulty cable, the hub won't see it and the link will always stay up.