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BPDU and block port

Unanswered Question

Three switch chained together physically to form a loop network as this:A(1/1)-B(1/1)B(1/2)-C(1/2)C(1/1)-A(1-2). A(1/2)is in block mode and C is root bridge.

1.)Will A(1/2) receive BPDU from C? My understand is after convergence BPDU is always sent from root bridge to the down stream's root port, and so on.So will A(1/2) receive BPDU?

2) If link between B(1/2)-C(1/2) goes down, after 20ms(maxage), A goes into listening mode.Will A send BPDU trying to announce itself is a root bridge? Obiously C is still root bridge this moment.If A(1/2) was receiving BPDU via the blocked port all the time then I think A should not send BPDU. I am reading the Cisco LAN switch book(Kevin hammilton), however at this point I am a little bit lost.Thanks a lot.

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baileja Sat, 02/07/2004 - 23:32
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BPDU's are always sent downstream orginiating from the Root Bridge. A(1/2) will continue to hear BPDU's so that switch A will know that that port can change state if A(1/1) is not functioning. In the book you are reading, take a look at chapter 6 Figure 6-9. This demonstrates it. If you have the same older version of the book as me, it will be on page 170.


baileja Mon, 02/09/2004 - 15:03
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All BPDU's orginate from catC, so you will hear one directly from catC and from the port coming from catB

OK. One more thing I am not clear is that on that book page 252 says if link A and B goes down, C won't receive BPDU from B for 20s until reaching max age. Somewhere else in this book says then C'1/1 goes to listening mode and send BPDU to announce itself is root. What I am confused is if A's 1/1 still keep receiving BPDU from A's 1/1. How should it anounce itself is root?

baileja Tue, 02/10/2004 - 05:23
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The way spanning tree works is that when a switch first comes on line, it will send out BPDU's announcing itself as root with the priority of 32768 (i think thats the right number, I dont have my book handy right now). It will continue to announce itself as root until it hears a better offer of lets say 32767. Now when it hears that BPDU, it knows there is a root bridge already elected and will discontinue the BPDU's. Now if any link were to go down that this switch is uplinked to, the election process will begin to find a new root bridge (if no configurations have been made than this should be the switch with the lowest mac address).


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