How STP works?

Previously we understood STP's main job is to Block a port on the loop to break the Loop & to do that STP puts the port either in Forwarding mode or in the Blocking mode in.

• Initially STP's STA (Spanning Tree Algorithm) creates a (spanning) Tree structure of ports that are in the forwarding State to & from each Ethernet link just like the structure of a tree from the base to it's each leaf.

• That means STP only works on deciding which interface to put in the forwarding State & then simply put the rest in a Blocking State.

• As all the L2 switches does the same L2 functions hence there must be some criteria just for the sake of formality or for the sake of election.

• 3 Criteria to put an interface in a Forwarding Mode:

• Elect a RB:

• Elect One Root Port on each Non RB:

• Elect One/Multiple Designated Ports on each Non RB:

  Root Port	Designated Port
  Least path cost from Non-RB to reach the RB	Lowest STP Cost
  Single port on each Non-Root Bridge	Single/Multiple Port on every Switch
  No Root Ports on the RB	All ports of the RB are Designated Ports
  Root Port's opposite end will always be a Designated Port	Designated Port's opposite end will either be a Designated/Non-Designated

• Remaining Ports will automatically be elected as Non-Designated Ports & STP will put them in the Blocking State to avoid Loop.

• Failed interfaces (interfaces with no cable installed) or administratively shutdown interfaces are placed in disabled state not to be confused with the shut down state)

Reference:

• CCNA 200-301 OCG, Volume 1, Pg. 216-217, 223 - Wendell Odom.

• https://www.omnisecu.com/cisco-certified-network-associate-ccna/difference-between-root-port-and-designated-port.php#:~:text=The Root Port is the,Root Bridge (Root Switch).&text=There can be ONLY one,on a Bridge (Switch).

• https://community.cisco.com/t5/switching/stp-root-port-vs-designated-port/td-p/1517842

• https://vimeo.com/ondemand/ccna200301/397383326?autoplay=1