Why are batches of users losing LAN access?

Hi
Hope someone can help with this one. We're a single site office with a number of rooms cabled into the comms room. Rooms pretty much use a switch each.

Had very few problems until last week.

- Last week a batch of users lost access to the LAN, that batch was around 8 users in one room. The remaining 15 users in that room were unaffected. While investigating the problem I temporarily connected them via a known good connection using an 8 port netgear switch to restore access.

Symptoms: connected to the network, but unable to renew IP address. Only 'limited' connectivity was reported by XP pro. It wasn't PC specific because known good laptops had the same problem when plugged into this connection.

We checked the cabling using a tester - this was fine, and subsequently traced it back to the comms room. At this point I was unsure what the problem was, so decided to replace the 24 port 10/100/1000 netgear switch they were using out of hours (we have a spare). I haven't done this yet, because...

- The next day another room lost connectivity. They were going through a similar netgear, only this one was a 10/100 so slightly older. At this point I decided to power down and then power up this second switch. All connections subsequently came straight back online. No problems since.

Architecture:
Each room's switch is plugged into a Layer 2 gigabit master switch. Servers and NAS boxes are plugged into the master. Total number of switches is 4 (layer 2, 2x24port netgears, and 1x 48 port netgear). We also have our VoIP traffic on the same LAN, which  goes through a separate subnet (255.255.0.0). We have approximately 100 workstations and 40 IP phones. 2000 domain.

Question;
Any advice on where I could look to start tracing the cause of the problem? The only 'event' that coincided with both outages was the set-up of a brand new Dell laptop. I dismissed this though, we do this frequently and I can't see where it would cause a problem?

Thanks in advance,
Chris.
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ChrisCranieAsked:
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briancassinCommented:
It may be possible if this happened after adding a new laptop that there is a duplicate MAC address but I doubt this.

Did you check the servers DHCP and event logs to see if there were any reported problems ?

Have you tried setting up packet monitoring VIA Ethereal/ Wireshark ?

I would get Wireshark and then mirror one of the switch ports that was not working if you currently have a switch which ports aren't  working definately monitor them.

Are you using Spanning Tree on your network ?
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ChrisCranieAuthor Commented:
- Nothing reported in the servers event logs
- There's a warning icon in DHCP, but I believe that's because we're down to 9 free IP addresses (this is reported in the event logs)
- Haven't done any packet monitoring yet. On my project list, but heavily under-resourced so I'll take your advice and look into this.
- Spanning tree. Can't confirm this or not, how can I tell? Our config has simply been to put servers into the layer 2 switch, and uplink from the layer 2 to the room-specific switches

The only other thing I can say is that OWA doesn't run from an IP address internally (e.g. http://192.168.0.2/exchange) but it does run fine from the servername (http://servername/exchange).
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briancassinCommented:
only having 9 free IP addresses is cutting it close...

Try manually assigning an IP address to one of the computers on the switch that that the computers are not getting addresses. Whatever IP address you choose you will have to modify your DHCP scope to exclude that address. After that PC has a static IP assigned to it traceroute and ping back to the server. See if it can complete successfully if so I doubt your switches are the problem.

If you ping the OWA server by IP address does it respond ? If you ping by name does it respond please post the results. OWA may be a seperate problem all together.

Are your switches managed or unmanaged ?
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ChrisCranieAuthor Commented:
Thanks.
- I was planning on extending the scope range. We're using 192.168.0.100-200 only at the moment.
- IP address. Will try setting one statically and let you know the results shortly.
- Two are managed, two not. They've worked out of the box though, no custom config has been done
- Results from ping successful, both below;

C:\>ping 2ksvr

Pinging 2ksvr.fairtrade.org.uk [192.168.0.2] with 32 bytes of data:

Reply from 192.168.0.2: bytes=32 time<1ms TTL=128
Reply from 192.168.0.2: bytes=32 time<1ms TTL=128
Reply from 192.168.0.2: bytes=32 time<1ms TTL=128
Reply from 192.168.0.2: bytes=32 time<1ms TTL=128

Ping statistics for 192.168.0.2:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 0ms, Maximum = 0ms, Average = 0ms

C:\>ping 192.168.0.2

Pinging 192.168.0.2 with 32 bytes of data:

Reply from 192.168.0.2: bytes=32 time<1ms TTL=128
Reply from 192.168.0.2: bytes=32 time<1ms TTL=128
Reply from 192.168.0.2: bytes=32 time<1ms TTL=128
Reply from 192.168.0.2: bytes=32 time<1ms TTL=128

Ping statistics for 192.168.0.2:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 0ms, Maximum = 0ms, Average = 0ms

C:\>
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ChrisCranieAuthor Commented:
Update;
- IP address lease scope increased now, 20 free IP addresses
- Tried setting a static IP with a problem connection. The "limited connectivity" warning disappeared, but I was unable to ping a server or gain access to network resources. As soon as the cable was plugged into an active switch, the static IP worked fine..

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briancassinCommented:
Your going to have to go into the management of the switches. Check the port configuration and see what they are currently set for you may need to back down the speed or adjust the duplex mode.

Ultimately you may have to reset the switch to factory defaults.
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ChrisCranieAuthor Commented:
Ok, will try. May take a few days though so will post shortly...
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ChrisCranieAuthor Commented:
Sorry about the delay replying. Busy helpdesk at the moment.

Ok - after checking the switches in question actually aren't managed, I have two other managed switches but the ones with problems are both Netgear JGS524 (http://www.netgear.com/Products/Switches/UnmanagedSwitches/JGS524.aspx)

This hasn't happened again, but rebooting both switches has solved the problem. We're at capacity in any case, so I've purchased a new linksys managed switch to cope with future load.

Is it the case that switches have to be restarted like this? First time I've had to do it in 3 years here.

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briancassinCommented:
Sometimes yes because there internal tables become corrupt. Can happen for many reasons. Also you may have had a bad NIC on the network that caused issues. In managed switches generally you can force a rebuild of there tables. You can also control many other things such as speed duplexing etc... You can also see the current status, for example you could have a bad nic on a pc that is only communicating at 10mbps but the switch is at 100mbps or the card is communicating full duplex and the switch is set for half. These types of issues can cause problems with the switch.

I have had to power cycle many of the unmaged personal switches after a period of time due to problems like this. This is why managed is better you have more control over the switch and can see what is going on unmanaged you are at the mercy of the switch deciding what is best.
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ChrisCranieAuthor Commented:
Really helpful comments, thanks. Chris.
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briancassinCommented:
Thank you, glad I could help
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