I'm having a little trouble with the theory of operation with LoopGuard.
It seems to me that if LoopGuard is enabled, redundancy is lost. For
instance, consider 3 switches (A, B, and C) connected in a full layer 2
mesh like a triangle, with A and B comprising a collapsed core, A is
root, and C is access with LoopGuard enabled on both non-designated
ports as follows:
Segment1
A-----------B
\ /
Segment 2 \ / Segment 3
\ /
\ /
\ /
C
My current understanding:
In normal operation, B would be the designated port on Segment 3, and
transmit BPDU's, and C's port on Segment 3 would be placed in Blocking
mode.
However, if the link between A and B went down, C would become the
designated bridge on Segment 3, while B's port on segment 3 would become
a root port and no longer transmit BPDUs. Once BPDUs are no longer
received on C's Segment 3 Port, LoopGuard would put it in
Loop-Inconsistent state, and B is isolated. B then assumes it is the
root, starts sending BPDUs on Segment 3, and then STP converges again on
Segment 3 with C having designated port, B having a root port, and the
process repeats.
I'm obviously missing something here, can somebody clue me in?
Thanks!
Doug
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