Managed Switch slows down Mac network, but not PC's

I manage a network with 36 PC's and 8 G5 Macs running Tiger or later.  Recently a managed PowerConnect 3348 switch was installed and configured with 3 VLAN's to keep department traffic seperate.  However whenever this switch is plugged into the network it immediately slows the Mac's network speed down to a crawl.  I thought it may have been the way the VLAN's were setup so I restored the factory defaults on the PowerConnect and plugged the switch back in and again it slowed the Mac's down.  As soon as the switch is removed from the network the Mac's network speed increases.  The PC's on the network are not effected by the switch.

Any help would be appreciated.
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If this switch is what takes care of all the 48 ports on the network, than your PC's auto-negotiate better with it than the G5 macs. Check the autonegotiation, on the ports the MAC's are plugging into, they may negotiate all the way down to 10/half.

Also check to see if spanning tree is not shutting down anything
atessinAuthor Commented:
I'll look into the autonegotiation settings, but there are only 4 things plugged into the switch, 1 is a uplink and the other 3 are go out to 3 seperate Gigabyte dummy switches.  Even before this recent configuration changes with the VLAN's if this 3348 switch was hooked up anywhere on the network it would slow the Macs down even if they were not directly connected to it.
Okay so you have 3 unmanaged switches Gig-E, they were connected together, then you decided to connect them all via a managed 3348?

Are the G5's all connected to one of those un-managed switches or different ones?

Did you break the original cross-connects betweeen the un-managed switches before deploying the 3348 otherwise you're causing a network loop.

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Please tell me what are the G5's plugged into, and what do you mean by anywhere on the network. You mean with a single cable or with ALL 4 cables connected?

Make sure Spanning Tree Protocol is turned OFF on the switch.
How are you determining that the Macs' speed decreases?
Are the Macs distributed randomly across the network, or are they all plugged into one switch?

You are making too many configuration changes all at once to have a precise indication of the problem, without further info gathering.

I suggest plugging the new switch into power.

Label and
clear ALL cables from ports on the new switch (no cable should be plugged into any port)

Then plugin one uplink cable,  and see if you can measure a decrease in Macs' speed.
If you can, then unplug that cable, and try the next one.

Wait long enough that you can be certain there is no adverse effect, before thinking about plugging in more uplink ports.

Eventually you should find which uplink cable is causing the problem.

Most likely you would be forming a loop in your network, or you may be creating a bottleneck..     if you are changing your network so that this new switch is in the path between the Macs and the rest of the network.

(i.e. perhaps a Gig port is only operating at 100 megabits...  you should be able to login to the managed switch and see if this is true, or if any line transmit/receive errors are showing up on the port.)

There may be a bad network cable, bad wiring of a cable, or an  unexpected loop you are creating along with other changes you are making when you try to attach this
new switch to the network.

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atessinAuthor Commented:
Yes, all the Mac's are plugged into the same unmanaged GB switch.  Yes all cross-connects between the unmanaged switches are removed and now each unmanaged switch has a single uplink coming from the 3348.

I have tried several different ports on the 3348 switch so I don't think it's going to be single bad port and/or cable causing the problem.  I'm also sure that there is not a loop anywhere in the network.

I will disable the Spanning Tree and see if it makes a difference tomorrow with the end users and report back...Thank you for the help.
If you have no loops in your network: spanning tree protocol has no effect on speed. There are many reasons to never so much as think about turning STP off.

Did you have all unmanaged switches connected using gigabit ethernet links before?

The powerconnect 3348  is  a  switch that has 48  FastEthernet ports and 2 combo ports The two combo ports are  single ports with a 1000Base-T  connector or optional mini-GBIC/SFP connector.

You have 3 unmanaged switches.

This means that at most  two of them can be connected with gigabit ethernet to the managed switch.
At least one of them must have a slower  10/100  link to the  managed switch.

This slower link can be a bottleneck if the traffic generated by PCs attached to another switch is excessive.

And all broadcast traffic has to cross the links between all switches, unless you have used VLAN divisions to optimize this.   Try installing wireshark on a laptop and monitoring for excessive broadcast traffic.

The Mac's network speed to  _what_ is being reduced...  hosts on the internet.. hosts on another switch..  how about to other Macs? Your users probably do not provide a detailed enough report to accurately troubleshoot this.   It is best to actually test the performance yourself  on  the PCs that are supposedly slow...

To determine whether the performance drop is imagined or not.
It may even be an issue independent of your switches.

The Mac users may be generating much more traffic than they should, causing a link to become congested.     I suggest exmaining counters on the managed switch to see how much traffic is being across the link.

According to Apple KB re ethernet speed:

Connect to an auto-negotiated Ethernet port

 For best networking results when using Ethernet, the Ethernet port which your Mac is connected to should be set to auto-negotiate. If you are not sure, ask your network administrator to verify this setting.  
Important: Ethernet network connection issues may indicate that the network switch is not set to auto-negotiate. If a switch's port is not auto-negotiating, your computer detects the appropriate speed but defaults to half-duplex mode. If the switch's port at the other end of the link is configured for full-duplex, a large number of late events can occur on the link. If it isn't possible to set the switch to auto-negotiate, then set the switch's port to half-duplex. For more information, see the documentation that came with the Ethernet switch.

 Disable spanning tree if directly connected to a switch  

Note: If you connect your computer directly to a configurable Ethernet switch, have your network administrator disable spanning tree on that port. Otherwise your computer may duplicate the AppleTalk address of another computer on the same network and both network connections may become unreliable.
The apple link is only relevant if you are actually plugging the Macs directly into the new managed switch.
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