I am confused with the STP port states, how long a port will transistion from Block to Forwarding, from the Cisco LAN Switching it describes a port will wait for 15 sec before going into listening state and wait another 15 sec in the learning state so that a total 0f of 30 sec.
Then it go on to say that the forward delay controls both states of only 15 secs
Opps the text did not appear
from the Cisco LAN Switching Book
A variety of events (such as a bridge thinking it is the Root Bridge immediately after booting or an absence of BPDUs for certain period of time) can cause the bridge to transition into the Listening state. At this point, no user data is being passed—the port is sending and receiving BPDUs in an effort to determine the active topology. It is during the Listening state that the three initial convergence steps discussed in the previous section take place. Ports that lose the Designated Port election become non-Designated Ports and drop back to the Blocking state. Ports that remain Designated or Root Ports after 15 seconds (the default timer value) progress into the Learning state. This is another 15-second period where the bridge is still not passing user data frames
consider that with plain old stp it may take anywhere from 30-50 seconds to fully transition from blocking to forwarding... 15 and 15 comprise the listening and learning to forwarding transition, which can take the whole of 30 seconds... think of basic stp as a simple yardstick measure; there are a variety of methods to reduce convergence...
So the switch stay's in listening/learning for 30 seconds, The forward delay states "this is a single value which controls both states default to 15 secs"
So which is it 30 secs or 15 ?
i've done this with my lab, you can actually see the transition from lis to lrn on the switch using sh spann... while each transition state may not take all of 15 seconds each, it is a good barometer... remember that nothing is written in stone, and actual results may very depending on equipment... 30 to 50 total is the conventional wisdom with 15s each ascribed to lis and lrn states... not sure how much clearer this can be...