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When will a switch not replace a hub?

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25 replies

by dm3haggitt

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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).

25 replies

by hal 9000

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.

Col

by nicknielsen

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?

by dm3haggitt

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.

by nicknielsen

Plug your laptop into the hub, open a command window and enter the command [b]ARP -a[/b]. 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.

by jdclyde

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.

by blazinride

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.

by jdclyde

would it really be dependent on if there is a downstream hub? :D (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.

by dm3haggitt

It will be interesting to see if there are downstream hubs. I
don't think there are any. I haven't seen any, but your never
know what people hide under thier desk. Thanks for the tip.

by hal 9000

It pays to go looking for unauthorised Wireless Access Points that may have been added.

I've seen a few of these where some staff member thinks that they can get some free Internet Time and leave the LAN wide open for attack because the WiFi Hub isn't locked down or have any security at all on it.

I've seen several mounted in Suspended Ceilings as well as under desks. I used to use a NB with a WiFi access point on it but now I just use a WiFi seeker as they are easier to carry around and far cheaper.

Col

by dm3haggitt

I was failing to realize that switches need to make
connections. Thanks!
The dimensions of the building are 150 x 200 x 25 (1 story).
The most, if not all, of the cables run between 2 ajoining
rooms. The longest cable I know of in the building is 175ft
and it is not connected to the hub.

by jdclyde

even if we do trip all over each other trying to give the best answer! :D

Is everything working now?

by jdclyde

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.

by dm3haggitt

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

by dm3haggitt

Original hub:
3Com OfficeConnect Dual Speed (10/100) 16 Port Hub
Replacement hub that works:
Same as original only brand new w/ revised diagnosic
lights

1st switch that didn't work:
Netgear FS116 w/ auto uplink (CAT5 only)
2nd switch that didn't work:
D-link DSS+16 w/ auto upling (CAT3, 4, and 5 supported)

by dm3haggitt

Never occurred to me someone with the savy to use an
access point would compromise a secured site. I better step
up that security training. I was kind of the only person
available for a task that I find myself growing into.

by dm3haggitt

A user called in because their WIRELESS router wasn't working. This is a major offense. We supposedly had a secure site. Everything is wired by design. A user installing their own wireless access point to reach to the next building is probably poorly secured if at all. This access point is shut down and the user will be without Internet till proper arrangements can be made.

by jdclyde

Users only understand one thing, "I want to do .....".

The WHY of NOT doing something like that never occurs to them. If they are made aware of the "WHY", as well as what can (and should) happen to them if they break the policy, it saves you a lot of effort.

This is where walking around with either a laptop or WiFi detector every now and then is a good idea.

[i]I notice you have marked just about everyone helpful but me? :(

by dm3haggitt

I agree. 'Why' would be help in stating policy. Unfortunately, they have not given me any authority. They do not have an IT position. They are reactive. I have talked to them about this and my fears about backups and security.

About the thumbs up; I accidently gave it to myself when I intended to give it to you. I haven changed that to two thumbs up to you because you deserve them. How do I remove the one I gave myself? Thanks for noticing.

by hal 9000

The easy solution is to report this user and allow them to go their own way for breaching security. In any Secure Environment this type of thing is grounds for immediate dismissal without the hope of any legal recourse to the courts for breaching the Computer Usage Policy.

However if you don't have a Company wide Computer Usage policy in place that could be some real problems. What you need to do in this case is get the person in charge of the place and after you enable the WiFi point walk outside and log into the LAN and go directly to his computer and start looking through his files while he's/she's standing there. It's a fairly good way to show the holes in a system and press home the point that someone has opened up the system to intrusion and you can not be sure what has been stolen. When I personally showed just how open the system was to someone I hacked their accounting package and started to look at who was being paid what on a NB in no way connected to their LAN. This caused the immediate removal of the WiFi Access point as I was outside the companies grounds when I showed them just how vulnerable that they had allowed the LAN to become.

There is always the possibility that this person got approval to insert the WiFi point so you'll need to get the Powers That Be to change the approval process and if they insist on keeping the WiFi access point you'll need to lock it down quite hard and setup the remote NB to be the only computer able to read the WiFi hub.

Oh as far as I know if you make a mistake and give yourself a [b]Thumbs Up[/b] you can not remove it so you'll just have to live with it. :)

Col

by dm3haggitt

Again, you guys understand where I am coming from and have very helpful advice. I really appreciate it!

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