STP Election and Operation Process | Generated by AI
Question: What is the Spanning Tree Protocol (STP) election and operation process, and why is the provided summary confusing or hard to understand?
Answer:
Spanning Tree Protocol (STP) is a Layer 2 protocol that prevents loops in Ethernet networks with redundant links by creating a loop-free logical topology. The provided summary is mostly accurate but can be confusing due to oversimplification and minor inaccuracies (e.g., all switches don’t continuously send BPDUs every 2 seconds claiming to be root—only during initial convergence or changes). Here’s a clear, step-by-step explanation of the classic IEEE 802.1D STP process:
1. Initial State and BPDU Exchange
- When switches boot up or new links come online, every switch initially assumes it is the Root Bridge.
- Each switch sends Configuration BPDUs (Bridge Protocol Data Units) out all active ports. These BPDUs contain:
- The sender’s Bridge ID (BID): Priority (default 32768) + MAC address (lower is better).
- The assumed Root Bridge ID (initially its own BID).
- Path cost to the root (initially 0).
- Sender’s port details.
- BPDUs are sent every Hello Time (default 2 seconds), but only the elected Root Bridge continues sending them regularly once convergence is complete. Non-root switches relay superior BPDUs.
2. Superior vs. Inferior BPDUs
- A superior BPDU has better (lower) information:
- Lower Root BID, or
- Same Root BID but lower path cost to root, or
- Same root and cost but lower sender BID, or
- Same above but lower sender port priority/ID.
- When a switch receives a superior BPDU on a port:
- It stops claiming to be root.
- It updates its own stored root information.
- It forwards (relays) the superior information out its other ports (with updated cost).
- Inferior BPDUs are ignored or used to confirm designated roles.
This exchange converges the network until all switches agree on the same Root Bridge.
3. Root Bridge Election
- The switch with the lowest BID becomes the Root Bridge.
- BID = Priority (configurable, multiples of 4096) + MAC address.
- Tiebreaker: Lowest MAC address if priorities are equal.
- The Root Bridge has all its ports as Designated Ports (forwarding).
4. Port Role Election on Non-Root Switches
After root election:
- Root Port (one per non-root switch): The port with the lowest cumulative path cost to the Root Bridge.
- Tiebreakers: Lowest sender BID → lowest sender port ID.
- Root Ports forward traffic toward the root.
- Designated Port (one per network segment/LAN link): The port sending the best (superior) BPDU on that segment.
- The switch closest to the root (or with lowest BID if tie) wins.
- Designated Ports forward traffic away from the root.
- Non-Designated (Blocked) Ports: All remaining ports that would create loops.
- They listen for BPDUs but do not forward data frames (blocking state).
All Root and Designated Ports eventually forward; blocked ports stay blocked.
5. Topology Change Notification (TCN) Process
- When a topology change occurs (e.g., link failure, port up/down affecting forwarding):
- The detecting non-root switch sends a TCN BPDU out its Root Port.
- Upstream switches acknowledge with TCA (in next BPDU) and relay the TCN toward the root.
- The Root Bridge receives the TCN, sets the Topology Change (TC) flag in its Configuration BPDUs, and floods them network-wide.
- All switches receiving TC-flagged BPDUs shorten their MAC address table aging time from 300 seconds to Forward Delay (default 15 seconds).
- This flushes stale MAC entries quickly, allowing faster relearning after the change.
The summary you provided matches this closely, but point 1 is slightly misleading—ongoing BPDUs are mainly from the root (relayed by others), not all switches constantly claiming root status.
Here are visual diagrams to help understand:
References:
- https://www.networkacademy.io/ccna/spanning-tree/how-stp-works
- https://www.geeksforgeeks.org/root-bridge-election-in-spanning-tree-protocol/
- https://networklessons.com/spanning-tree/spanning-tree-topology-change-notification-tcn
- https://www.networkacademy.io/ccna/spanning-tree/stp-topology-changes