When will a switch not replace a hub?

By dm3haggitt ·
We have 12 assorted cables (CAT3, CAT4, and CAT5) going to an
unmanaged 16 port hub. Documentation about the network is nil. Only
5 of the hub lights are lit. I recently tried replacing the hub with an
unmanaged 16 port switch capable of supporting CAT 3, 4, & 5. We put
the cables into the corresponding ports and rebooted all the systems.
Our network connections failed. At first all but one of the lights lit on
the switch. Then 3 more lights turned off. Why would a switch not
replace a hub. I imagine it has to do with more than one network,
addressing, or broadcasting (Captain Obvious).

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In Theory it shouldn't matter in this case

by HAL 9000 Moderator In reply to When will a switch not re ...

But in practise I would be looking at testing all the cables to make sure that they are all OK and working properly.

If they are I would imagine that you are actually seeing packet crashes and this is causing the unmanaged switch to stop working. As there is no documentation available you most likely have some Crossover cables somewhere in that group of cables which are now causing a problem with the new Hub.

Sorry but in this case if you can not pickup a similar Switch to the original and even that's no guarantee you are going to have to do some fault finding to see what is happening here. I would also document it as you go along so that the next time that a problem arises some poor fool doesn't need to spend days fault finding.


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What's at the other end of those cables?

by NickNielsen In reply to When will a switch not re ...

In addition to the possibility of having crossover cables in use, it's also entirely possible that there is a speed conflict. Are the devices attached to those cables capable of the speeds offered on the switch ports?

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What is working.

by dm3haggitt In reply to What's at the other end o ...

I replaced the original hub with a brand new hub of the
same model. The network is now working, and I have
been able to identify some of the cables and have labeled
them. The lights on the new hub have been revised and
tell me that there are some cables running at 10MB and
some at 100MB, and yes there are a few collisions. The
lights also say that less than 10% of the bandwidth is
being used. I think both of you are right. Two-thirds of
the cables go into the ceiling. I guess I may need to plug
each cable into a laptop and use ipconfig to get its ip
address? Thank you both.

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This might be easier

by NickNielsen In reply to What is working.

Plug your laptop into the hub, open a command window and enter the command ARP -a. This will return the associated IP and MAC addresses of all devices on the network. You can then Telnet to the hub and determine which of those devices are connected to which port.

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to map out unlabeled network

by jdclyde In reply to This might be easier

A way to map out the network, put the hub back in and get a partner to reach behind a pc, one at a time, and just keep plugging it in, unplugging it until you tell them to stop. You then go to the hub and look for the light that is going on and off.

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Thanks to jdclyde

by dm3haggitt In reply to This might be easier

Thanks is also helpful.

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Many thanks to NickNielsen for guiding the way to a RESOLUTION!

by dm3haggitt In reply to This might be easier

This is extremely helpful! It would have taken me forever to
discover this, if ever. Thank you so much!

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A switch needs to establish connections

by jdclyde In reply to When will a switch not re ...

where a Hub does not.

Remember that a Switch only forwards information on by the MAC address locally, and IP address remotely. If the cables are not able to handle the signal, the switch will not establish the connection.

What is the distance to the next device? Are you exceding the limit?

Are the devices at the end 10/100? If so, try to set them to 100.

I would STRONGLY recommend running new cat5e cables or going wireless if that is not practical.

Are systems on this segment using static IP addresses or DHCP? IF DHCP, try to go static.

My guess is it is the old cables causing your problems though.

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Downstream Hubs?

by blazinride In reply to A switch needs to establi ...

I think its also the crossover cables, if your switch doesn't have auto-mdx on all the ports, but also see if you have any downstream hubs. I recently had an issue where i replaced a companies backbone 24 port hub with a 24 port switch 10/100, and the downstream hub could no longer connect. If you have a downstream hub check that possibility, if not, im an idiot.

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It that were the case

by jdclyde In reply to Downstream Hubs?

would it really be dependent on if there is a downstream hub? (j/k)

As he has NOT told us what the brand and model of the old or new equipment is, it is hard to tell what is going on here.

I DID run into this problem last summer. The run was too long, but using a hub at each end as a repeater it "worked". When I upgraded to a switch to cut down on network noise, the switch would not connect. (This was a network I inherited) Ran a scan and found the run was 645 feet. Yeah, that might have had something to do with it..... Put in an ethernet extender and it now works like a champ.

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