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Re: spanning tree - looping basic question

 
 
stephen
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      10-01-2006
<(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> Hi friends,
>
> I have some trouble understanding a basic loop detection scenario in
> switched networks.
>
> The topology is as follows:
> Host A
> |
> Segment A
> | (1/1) | (1/2)
> Switch A Switch B
> | (2/1) | (2/2)
> Segment B
> |
> Host B
>
> Now if Host A on Segment A wants to communicate with Host B of Segment
> B, the packet goes to Switch A.


"segment A" is a collision domain or similar - so it is also recieved by
host B

Switch A adds entry for Host A in its
> MAC address table and broadcasts the frame to all its Switch ports
> except the port 1/1.


without spanning tree B will do the same, so 2 copies arrive at host B.

Now Switch B hears the broadcast from Switch A and
> learns address of Host A through its port 2/2.


it doesnt have to be a broadcast.

at this point switch B think host A is on segment B - because it "saw" a
packet with host A mac address. Switch A is transparent - which means it
cannot put a marker in the packet to show the packet was just forwarded by a
switch.....

B is now confused since it has seen indications of host A on 2 different
ports with a few mSec or uSec between them

either B will complain, shut down a port or 2 and break the loop, or merrily
forward the copy back towards host A. Meanwhile switch A is doing exactly
the same with the copy via B.

so - 2 switches in parallel act as a packet replicator, and will spawn
copies until they hit some sort of limit - with modern switch hardware, that
is often the bandwidth on the attached LANs.

Now it broadcasts on its
> port 1/2 which is heard by Switch A on 1/1. Now A again broadcasts
> through 2/1 and the loop continues.


at this point you are confusing unicast and broadcast packets.

My questions are:
>
> 1. If B responds to the broadcast, then does the loop stop?


No

> 2. If Switch A and Switch B listen to each other's broadcasts, then how
> are they exactly connected? Are they connected to each other directly?


your diagram implies a layer 1 ethernet topology for each segment - so a
co-ax segment, or some sort of repeater.

> I am not able to understand how would they be physically connected to
> hear each other's broadcasts?


in practice it wouldnt matter if each connection is a direct cable between 2
ports and the host is connected separately to the loop

the key issues are that you can trace a loop between ports on switches,
something generates a packet to kick off replication, and that you dont have
anything to stop the entire loop from forwarding packets in circles.
>
> Thanks a lot
> Gautam
>

--
Regards

http://www.velocityreviews.com/forums/(E-Mail Removed) - replace xyz with ntl


 
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