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Trying to upgrade to new switches but cannot move my DNS servers without bringing down the network!

Posted on 2008-11-18
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Last Modified: 2012-05-05
We currently have 10 unmanaged switches on our network which are HP Procurve 2124 (100mbs). We have purchased 10 HP Procurve 1800 24G gigabit swithces with which to replace them.

- We can plugin the new switches and connect them to the existing switches with no problem.
- We can then move client connections (PC's, Laptops, Printers and the like) over to these new switches without a problem.
- We can move standard servers (non DC) over to the new switches.

Our problem is that if we try to move either of out two DNS servers over to the new switches all hell breaks loose! To start with on boot up the server hangs on "Preparing Network Connections" before finally failing and allowing the log on. We then find that not only can this particular DNS server not browse or ping any other resource but that every server and every computer on the network suffers the same fate instantly which essentially crashes the entire network.

If we pull the network cables out an stick them back into the old switches, within minutes everything is back to normal.

At first I thought that the problem was a faulty switch however this cannot be the case on all 10 of them!

For refereence we have out switches set up in a cascading pattern.

All suggestions would be appreciated.
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Question by:flowit
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by:edster9999
ID: 22983044
It is a network problem.
Check the following...

1. Do you form a loop anywhere in the system ?  Where one pc is plugged into two switches and they are connected together or switches connected to switches that connect back to the original ones ?  If so you have made a loop and the traffic is going round in circles.  Break the loop and it will clear itself.

2. IP addresses.  Have you assigned ips to the switches ?  Or put the new switches in a different range.  The IP on the servers has to match.

3. Link speeds.  I have seen this problem happen where the switch is set to auto negotiate for with speed (10/100/1000) or duplex (full/half) and so is the nic card.
Pick the settings and lock one or better still both onto that setting.

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by:flowit
ID: 22983688
Thanks for the response.

1. I dont belive that we have a loop anywhere. We do have a few servers that have bridged network connections and therfore two cables but each cable going to a different switch. This hasnt caused us any problems on our existing switches.

2. The new switches are still on the default IP address's that HP have them on which is 192.168.2.10. This is a different range to the one our network uses which is 10.x.x.x - I can see why they should go onto the same range however we dont have any problems plugging clients or non-DC servers into these new switches on the default IP address and whats more all 10 switches are on the same default address.

3. I will give the auto negotiate issue a go and see what happens.

Thanks.
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by:edster9999
ID: 22983847
you should not be plugging in servers if the switch has a different IP range.  I'm going for number 2 as the cure.
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mikebernhardt earned 2000 total points
ID: 22999340
If your servers are bridging, you may be creating a spanning tree loop when you move them to the new switches, if the new switches are not stopping the loop. A spanning tree loop will kill your network by overwhelming the switches.

Try plugging just one of the server connections into the new switches, and leave the other cable temporarily disconnected. If it works OK, that's your problem. Turn off bridging on the servers if it's on. Come up with a different way to dual-home your servers, using active-standby teamed NICs instead of bridging.
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by:mikebernhardt
ID: 22999349
I am sure #2 is NOT your cure, by the way. The servers could care less about the management IP of your switches.
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by:edster9999
ID: 23001858
It can impact it if your switch is part of a network (not just a separate management IP range) then the things plugged in have to be on same range.
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by:mikebernhardt
ID: 23006793
No, they don't. It's just that the 2 IP ranges won't be able to talk to each other without a router that can route multiple subnets on the same vlan. And if the IP address of the switch is also supposed to be the default gateway address then of course nothing will be able to leave the subnet.

But everything that is plugged into the same vlan on the same switch should continue to talk to each other just fine. And that seems to be the case since the clients all move without a problem. Only when he moves the servers does he have a problem.

So as I said, the test is to only connect one of the server NICs to the new switch and leave the other NIC down. If that works, then you need to fix the NIC teaming configuration and TURN OFF BRIDGING ON THE SERVERS if it is on- that will definitely cause big problems for you.
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