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Broadcast storm

What is a broadcast storm and what causes it?

Is it the same thing as a switching loop?  In my lab (I have redundancy in between my switches), I shut off STP on both switches.  Suddenly the link lights blinked rapidly. Was this a broadcast storm?
Thanks
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pseudocyberCommented:
Check this out:  http://en.wikipedia.org/wiki/Broadcast_Storm

If you "have redundancy" in between switches, and you turn off STP, you have created a switching loop.  The switches are usually sending something out - when there's a loop, you'll get a bunch of problems, but the basic switching problem is that Switch A wants to know where PC1 is - so it floods all ports with a packet for PC1 - this flood crosses ports 1 and 2 over to switch 2.  Switch B makes a note of Switch A's MAC in its table, however it notes that SwitchA can be accessed through two ports 1 and 2.  Switch B doesn't know where PC1 is, so it floods to all ports - including ports 1 and 2.  Switch A sees the traffic from Switch B, coming in through ports 1 and 2 and makes entries in its forwarding table for SwitchB on ports 1 and 2 but it still doesn't know where PC1 is, so it floods to all ports ...

You can see where the loop is, right?

You cannot have a loop.  You can have a physical loop, if you're running Spanning Tree.  If you want to get crazy, you can have two VLANs, two Spanning Tree groups, and play around with having redundant connections in different vlans and spanning tree groups so that there is no loop.

You can also "bond" connections together virtually - using Cisco Fast Ether Channel or Nortel Multi Link Trunking or something similar.  Then, the two connections are seen as one with double the bandwidth for load sharing.
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dissolvedAuthor Commented:
Thanks, so I was correct. What I described was indeed a broadcast storm?

>>If you want to get crazy, you can have two VLANs, two Spanning Tree groups, and play around with having redundant connections in different vlans and spanning tree groups so that there is no loop.>>

I actually do have mutliple VLANs spanning my two switches. There is an instance of STP for each VLAN right?  
Please elaborate on "having redundant connections in different VLANs and spanning -tree groups so there is no loop."

This sounds very interesting.
Thanks!

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pseudocyberCommented:
It's a little beyond my scope of expertise - I haven't done that much work with spanning tree.  I've just read that it's possible.
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dissolvedAuthor Commented:
sounds good man. Thanks for the explanation on the switching loop. I've read about how it's "supposed" to occur. But your explanation was the first time I really understood HOW it occurs
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pseudocyberCommented:
No prob.  Thanks.
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