Hi every body!
Here is what my book says:
" RSTP does not do anything differently from stp on link-type shared links."
" RSTP does not improve convergence over link-typed shared connections"
But lost of bpdu is detected in 6 seconds ( 3*hello) unlike 20 seconds in stp, a port in discarding state connected to shared medium, will detect the loss of bpdu sooner(in 6 seconds). Is it not right?
Here is what my book says about rstp
" If the DP( designated port) fails, switch can quickly move the back up port from a discarding state to learning state and then forwarding state"
How long it takes before back up port transitions into forwarding state after DP fails on hared medium ?
My calculation is 15 seconds+ 6 seconds(3*hello.( for learning state) , so around 21 seconds . Which is in fact an improvement over stp where a blocked port will take 50 seconds to transition into forwading state.
If i am correct then my concept is in conflict with what the book says ( please read the excerpt).
An alternate port connected to shared medium, will detect loss of bpdu in 6 seconds, then spend only 15 seconds ( learning state before finally settles into forwading state.
a blocked port connected to shared medium, detects the loss of bpdu in 20 sec( max-age), then spends 30 sec( listening and learning ) before finally transitions into forwading state.
Again i see an improvement in convergence over shared mdium when rstp is used.
Thanks a lot!
Discarding = Listening ;-)
The final state for a designated port is forwarding. It starts in discarding (or listening) state for fwd_delay, then learning for fwd_delay before reaching forwarding.
You are right, the aging in 3xhello time will provide an enhancement over spanning tree. However, you still have 2xforward-delay, as both listening/learning stages still exist in RSTP on shared segments.
In fact, there is a further enhancement in the latest IEEE spec that allows going a little bit faster on shared segments, but Cisco does not implement it (it require cooperation from all the switches on the shared segment and would be difficult to enforce).