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TOPIC: Layer 2 broadcast storm (Need clarification please)

Layer 2 broadcast storm (Need clarification please) 9 years 4 months ago #22504

  • shamrock
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Good afternoon to all!
I have a question regarding redundant links between switches and
broadcast storms.
if you consider the following diagram (from www.howstuffworks.com):


I can understand that when node b wants to send a frame to node a, switch A will send a broadcast out all ports (except the one that the frame was received on) through segment A and segment C (so far so good!).Switches B and C will send a broadcast over segment B because as of yet, they are unaware of Node a's mac address.

Now, am I right in saying that:
Node a will receive the broadcast but also, the broadcast from switch B will be forwarded to switch C and vice versa,broadcast from switch C will be forwarded to switch B.Subsequently, both switch B and C will forward the broadcast to switch A via segment A and C respectively?Also does node A reply to switches B and C saying "That frame was destined for me"?

And finally, since switches B and C now know what port to forward any frames out on for Node A (and yes the broadcast from switches B and C is now at Switch A because of the loop) does this mean that when switch A forwards the broadcast on again through segment A and C for the second time (looping), the broadcast dies when it reaches Switches B and C since they now know what Node A's mac address is and on what port to reach it?So in actual fact, will Node A receive more than one copy of the same frame?

I hope this last question makes sense!
Many thanks for you help gurus and regards.
Shamrock.
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Re: Layer 2 broadcast storm (Need clarification please) 9 years 4 months ago #22505

  • toddwoo
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This is where something called STP (spanning tree protocol) comes into play. STP makes sure that only 1 path to a destination is used.

STP is a pretty big subject.. a little too big for me to go over.. BUT...

http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/sw_ntman/cwsimain/cwsi2/cwsiug2/vlan2/stpapp.htm

Gives you a pretty good idea about it.
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Re: Layer 2 broadcast storm (Need clarification please) 9 years 4 months ago #22573

  • shamrock
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Thanks for that, good link!
One question left:
Do broadcast storms run indefinitely around a
Layer 2 network?Is there a point where a broadcast storm
dies, such as some sort of Time To Live with a frame?
thanks!!!
Regards from Ireland,
Shamrock
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Re: Layer 2 broadcast storm (Need clarification please) 9 years 4 months ago #22583

  • toddwoo
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This is where my knowledge gets a little thin.

A broadcast storm can go on forever.. or until your networks stops in its tracks. A broadcast storm can be an absolute BEAR to deal with.

A layer 2 "frame" has no TTL (time to live) However on layer 3 "packet" has one. But thats only updated by routers... But thats misleading and going in that direction is probably confusing.

If you have any broadcast storms you need to get rid of them, you can't just wait for them to "settle down". No mechanic involved is sufficient to relieve one that you have.. You simply must prevent them.
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Re: Layer 2 broadcast storm (Need clarification please) 9 years 4 months ago #22586

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Thats great, thank you for that info.
Just another question relating to Layer 2 switches.
When you telnet onto a switch and ping another switch
using the ping command, Im trying to establish how this
can be possible on a layer 2 device as they deal with Hardware
addresses not logical layer 3 addresses.
Im a little confused as to how a layer two device could
even do this as ARP is a layer 3 protocol that IP would use
to translate logical addresses to hardware addresses.
But what utilities does a switch have?
Thanks again for the help,
much appreciated!
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Re: Layer 2 broadcast storm (Need clarification please) 9 years 4 months ago #22587

  • skepticals
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To my knowledge, the IP addresses that switches are assigned are for management reasons only. The switches at that point are only acting as an IP host on the network.
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