3.2.2. Spanning Tree Protocol

Redundant links prevent single points of failure—but they create Layer 2 loops. Without STP, a broadcast frame travels both paths, creating copies that travel both paths, exponentially multiplying until the network crashes (broadcast storm).

STP (Spanning Tree Protocol): Prevents loops by blocking redundant paths while keeping them available for failover.

802.1D Port States:

1. Blocking: Not forwarding frames; listening to BPDUs to learn topology
2. Listening: Processing BPDUs, participating in root bridge election
3. Learning: Building MAC table but still not forwarding
4. Forwarding: Normal operation—learning and forwarding
5. Disabled: Administratively shut down

Port Roles:

• Root Port: Best path to root bridge (one per non-root switch)
• Designated Port: Best path for a segment to reach root
• Blocked/Alternate: Backup path, kept in standby

MTU and Jumbo Frames:

• MTU (Maximum Transmission Unit): Largest frame size allowed (standard Ethernet: 1500 bytes)
• Jumbo frames: Larger MTU (typically 9000 bytes) for reduced overhead on storage/backup traffic
• Must be configured consistently across entire path—MTU mismatch causes fragmentation or drops