Re: spanning tree - looping basic question

Discussion in 'Cisco' started by stephen, Oct 1, 2006.

  1. stephen

    stephen Guest

    <> wrote in message
    > 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

    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?


    > 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


    - replace xyz with ntl
    stephen, Oct 1, 2006
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