> For the complete documentation index, see [llms.txt](https://mudassirs46.gitbook.io/network-fundamentals/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mudassirs46.gitbook.io/network-fundamentals/stp-convergence-if-something-changes-link-fails.md).

# 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** ](https://app.gitbook.com/@mudassirs46/s/network-fundamentals/~/drafts/-MRZ8l67L5MHnaQIEh9W/stp-spanning-tree-protocol)**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>


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