Spanning Tree, Step by Step

About this interactive

From the lesson: spanning tree lets switches keep redundant links as backups while only one path between any two switches carries frames, so frames cannot loop at Layer 2. Every switch runs the same algorithm, and all of them reach the same answer. The terms the cards use: - **Designated port:** on each link, the port nearest the root bridge, which forwards onto that link. - **Root path cost:** the sum of the link costs from a switch to the root bridge. Faster links cost less. - **Alternate (blocked) port:** a port held in reserve that forwards nothing until it is needed. - **Root bridge:** the switch with the lowest bridge ID (priority first, then MAC address). - **Root port:** a non-root switch's port on its cheapest path to the root bridge. To order the cards, ask what each step needs to know already, and place it after the step that tells it.

What you'll learn

Put the steps of the spanning tree algorithm in order
Explain why each step depends on the one before it, and how a blocked port restores the path after a failure

Aligned to

Cisco CCNA

2.5 Interpret basic operations of Rapid PVST+ Spanning Tree Protocol

Key terms

Designated Port: In STP, the port on each network segment that provides the best path to the root bridge and is placed in a forwarding state. Each segment has exactly one designated port, which is selected based on the lowest cumulative path cost to the root.
Bridge ID: A unique identifier used in the Spanning Tree Protocol consisting of a configured priority value and the bridge's MAC address. The BID is used in STP elections to determine which switch becomes the root bridge.
Alternate Port: A switch port that is not elected as a root port or designated port and is placed in a blocking state to prevent loops, serving as a backup path.
Spanning Tree Algorithm: The original loop-prevention algorithm developed by Radia Perlman that forms the basis of IEEE 802.1D STP, computing a loop-free logical tree topology over a redundant switched network. STA elects a root bridge and places redundant links in a blocking state to prevent loops.
Root Path Cost: The cumulative cost calculated by a switch to reach the root bridge, based on the speed of each link along the path.
Root Bridge: The switch elected as the reference point for the Spanning Tree Protocol topology, determined by the switch with the lowest Bridge ID (priority + MAC address). All other switches calculate their best path to the root bridge and block redundant links to create a loop-free tree.
Bridge Protocol Data Unit: A data message exchanged between switches running Spanning Tree Protocol to detect network loops and elect a root bridge. BPDUs contain the sending switch's BID and path cost information used to build the loop-free spanning tree.
Root Port: The port on a non-root switch that provides the least-cost path to the root bridge.

Topics

Stp Designated Port Root Port Spanning Tree Algorithm Root Bridge Interactive Ordering

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