STP Convergence (if something changes/link fails):

Each Non-Root Switch relies on RB's Hello BPDU. If however they fail to receive the Hellos or Hellos lists different details, then Switch starts the convergence process.

• Convergence Process requires the use of 3 Timers:

  STP Timers:

  Timer	Default Value	Description
  Hello	2 seconds	Frequency between RB Hello
  Max-Age	10 Times Hello (20 seconds)	Convergence process starts after Max-Age timer expires
  Forwarding Delay	15 secs	Delay of 15 seconds in Listening State & 15 Seconds in Learning State after Blocking till Forwarding State

• If a link fails then by default a Switch receives BPDU every 2 seconds. Then Switch will wait for 20 seconds (as the max age timer is 10 Hellos (20 seconds)) & then put the port in Listening & Learning State (again starting the election process).

• Convergence Process will be completed in 50 seconds as it takes 50 seconds to bring the port up from Blocking to Forwarding Mode. If However the Blocking port & the Forwarding port are on the same link then Switch will understand it's running port went down & will skip the 20 seconds Max-Age timer & immediately start the Forwarding Delay Timer (Listening + Learning State)

  Reference:

• CCNA 200-301 OCG, Volume 1, Pg. 226- Wendell Odom.

• https://vimeo.com/ondemand/ccna200301/397383402?autoplay=1