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:

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