spanning tree - looping basic question
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Posted by on October 1, 2006, 12:37 pm
Please log in for more thread options 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. 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. Now Switch B hears the broadcast from Switch A and learns address of Host A through its port 2/2. 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. My questions are: 1. If B responds to the broadcast, then does the loop stop? 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? I am not able to understand how would they be physically connected to hear each other's broadcasts? Thanks a lot Gautam | |||||||||||||
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Posted by stephen on October 1, 2006, 5:28 pm
Please log in for more thread options "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. >
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> Thanks a lot > Gautam > Regards stephen_hope@xyzworld.com - replace xyz with ntl | |||||||||||||
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>
> 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.