which stp port role is adopted by a switch port if there is no other port with a lower cost to the root bridge?
In Spanning Tree Protocol (STP), a switch port adopts the root port role if no other port on that switch has a lower cost to the root bridge.
STP Port Roles Explained
STP prevents loops in Ethernet networks by assigning specific roles to switch ports based on their path cost to the root bridge—the central reference point elected by the lowest Bridge ID. Each non-root switch selects exactly one root port : the port with the lowest cumulative path cost to reach the root bridge. If multiple ports tie for the lowest cost, tiebreakers like port priority or port ID decide, but the port with no lower-cost alternative becomes the root port. This port stays in the forwarding state to send traffic upstream toward the root.
Why Root Port, Not Designated?
- Root Port : Chosen per switch (non-root switches only). It has the lowest cost to root bridge on that switch. Forwards toward root; no competition from lower-cost ports on the same switch.
- Designated Port : Chosen per LAN segment. The switch with the lowest cost to root on that segment makes its port designated; others block. Applies across switches, not within one.
- Key distinction: Question specifies "no other port with a lower cost" (implying on the same switch), confirming root port. Some sources confuse this with designated (e.g., segment view), but standard STP (IEEE 802.1D) defines root port here.
Role| Selection Criteria| State| Example Cost Scenario
---|---|---|---
Root Port| Lowest cost to root on the switch 1| Forwarding| Port A: cost
4; Port B: cost 19 → A is root 1
Designated Port| Lowest cost from switches on segment 7| Forwarding|
Switch X cost 10 beats Switch Y's 19 on link 7
Blocking/Alternate| Higher cost alternatives 3| Blocking/Listening|
Backup paths, ready if root fails 2
Real-World Context
Imagine three switches: Root Bridge (A), Switch B (ports to A: cost 4; to C: cost 19), Switch C. On B, port to A becomes root port—no lower option. Port to C? Likely designated if B's root cost (4) < C's. Recent CCNA forums (2024-2026) highlight this in exams; mislabeling as "designated" trips students since designated is segment-based.
"The root port is the port with the lowest cost to reach the root bridge." – CCNA exam reference
TL;DR: Root port. Bottom note: Information gathered from public forums or data available on the internet and portrayed here.