3024 + 3248 Gbic Blocking States

First thing I would look at is the settings for speed/duplex on the PowerConnect 3248.  The 3024, when the GBIC is enabled, is only able to operate at 1000Full duplex, it will not autonegotiate.

The 3248, however, can use autonegotiation for speed/duplex on it's fiber connections, and that's the default setting for the switch.  Try disabling autonegotiation on the 3248, and hard-setting the speed/duplex to 1000Full, and see if that helps.

If you continue to have problems, the next thing to look at would be fiber cabling - in most cases where I've seen a port flapping like you've described, it's a physical layer issue, and usually attributed to either a faulty cable or device somewhere in the chain.  If the switches are connected via an existing fiber run (as opposed to a direct SC-LC patch cable), you might want to try eliminating the fiber run, if possible, from the equation.  Try different cables and GBICs on both ends to confirm it's not a faulty fiber module.

Hi,

    I did as you suggested on the 3024's -  changed the Gbics, fibre patches etc but no difference - I tried turning off Spanning Tree on the 3248, that seemed to cause a broadcast across the whole network crashing the network - ended up having to reboot all switches to recover

It did however activate the fibre port again.

Any ideas what would have caused the broadcast, also what are the recommended settings in terms of spanning tree etc - are there any guidelines?

Thanks

Ian

Ah.... I think I know exactly what the issue is, then.

This isn't a problem with the fiber link - you have a loop in your physical network.  Spanning Tree put the fiber port into blocking mode because, for whatever reason, it doesn't think that is the best path to the spanning tree Root Bridge.

The broadcast storm would have occurred due to the loop in the network - once you disabled STP, ANY broadcasts will replicate themselves through any redundant loops in the topology - which can (and often will) kill performance, and in extreme cases crash the network.  The 3248 has a measure of broadcast storm control, but the 3024s do not - and even then, storm control only kicks in when the threshhold is exceeded.

For spanning tree, the first thing you want to do is re-enable it (I imagine this has already been done).  Then, you want to find what device is acting as the Spanning Tree Root Bridge - If you look under Spanning Tree > Global Settings on the 3248 or 3024, it should tell you the Bridge ID for the Root Bridge, and the Bridge ID of the switch you are currently managing.  Once you find the switch where both values match - you've found the Root. 

Ideally, the root should be the core of your network segment.  If it isn't, this could be a possible cause for the problem you're seeing.  Often, this is where we recommend building a topology diagram of your network, and map out what cables go to what devices.  This will not only give you the proper physical layout of everything, it will also show you where you have loops.  If the root bridge for STP is *not* where it should be, then you probably need to look at lowering the bridge priority on the switch that should be the root, so that it takes over properly  (Root Bridge is determined by the Bridge Priority value(Default is 32768 on most switches), and the lowest MAC address.)

If the topology is good, and your Root Bridge is where it's supposed to be, then the last thing you might want to start checking are path cost values (this is handled under Spanning Tree > Port Settings).  The default settings should be as follows:

10 Mbps link — half duplex:  2,000,000; full duplex:  1,000,000; trunk:  500,000
100 Mbps link — half duplex:  200,000; full duplex:  100,000; trunk:  50,000
1000 Mbps link — full duplex:  10,000; trunk:  5,000

For more information on path cost and Spanning Tree settings on the 3248, I recommend the user's guide addendum for the PowerConnect 3248 for some basic information and configuration assistance.  You can find the document at the following link:  http://premiersupport.dell.com/docs/network/common/en/addendum/K0111bk1.pdf

Let us know if this helps, or if you require further assistance.