STP convergence state: After a spanning-tree topology has converged, which statement accurately describes the state of switch/bridge ports?

Introduction / Context:
Spanning Tree Protocol converges by selecting a root bridge and then determining which links forward and which must be blocked to prevent loops. Understanding the steady-state port roles and states is essential for troubleshooting and design.

Given Data / Assumptions:

• Classic STP (802.1D) and RSTP (802.1w) converge to a loop-free tree.
• Ports have roles (root, designated, alternate/backup) and states (forwarding, blocking/discarding).
• Edge/PortFast ports transition rapidly but still obey loop-free principles.

Concept / Approach:
In a converged topology, exactly enough ports forward to maintain connectivity; redundant paths are placed in a non-forwarding state (blocking in 802.1D, discarding in 802.1w). Therefore, every non-administratively-down port is either forwarding or blocked/discarding.

Step-by-Step Solution:

Verification / Alternative check:
show spanning-tree output displays port roles and states; you will observe a mix of forwarding and discarding states in redundant topologies.

Why Other Options Are Wrong:

• “All ports forwarding” is false in a redundant design.
• “All ports root or designated” ignores non-designated/alternate roles.
• “Blocking or looping” uses a nonstandard term; ports do not “loop.”
• “Only edge ports forward” is inaccurate; trunk/root/designated ports also forward.

Common Pitfalls:
Equating roles with states; forgetting that RSTP uses “discarding” not “blocking,” though conceptually similar.

Final Answer:
All ports are in either the forwarding or blocking (alternate) state.