Spanning Tree Protocol (STP) is a Layer 2 protocol that prevents network loops in Ethernet networks with redundant paths. It creates a loop-free logical topology by selectively blocking certain redundant paths while maintaining backup connections for fault tolerance.

STP operates through a distributed algorithm that:

• Elects a root bridge (the reference point for all path calculations)
• Determines the least-cost path from each switch to the root bridge
• Blocks redundant paths that could cause loops

// Simplified STP state machine logic
switch(stp_state) {
  case DISABLED:
    port_down_actions();
    break;
  case BLOCKING:
    drop_non_bpdu_frames();
    break;
  case LISTENING:
    process_bpdus();
    break;
  case LEARNING:
    build_mac_table();
    break;
  case FORWARDING:
    forward_frames();
    break;
}

Here's how STP configuration might look on a Cisco switch:

Switch(config)# spanning-tree vlan 1 priority 4096
Switch(config)# spanning-tree mode rapid-pvst
Switch(config)# interface fastEthernet 0/1
Switch(config-if)# spanning-tree portfast

When troubleshooting STP problems, these commands are invaluable:

show spanning-tree summary
show spanning-tree vlan 1
debug spanning-tree events

Remember that STP problems often manifest as unexplained network slowdowns or intermittent connectivity issues rather than complete outages.

Spanning Tree Protocol (STP) is a layer 2 protocol that prevents broadcast storms by logically disabling redundant paths in Ethernet networks while maintaining fault tolerance. The protocol creates a loop-free logical topology by blocking certain ports, automatically activating backup paths when primary links fail.

The protocol works through a distributed algorithm where bridges exchange BPDUs (Bridge Protocol Data Units) to:

1. Elect a root bridge (the reference point for all paths)
2. Determine root ports (the best path to the root)
3. Select designated ports (which forward traffic)
4. Block alternate paths (creating a loop-free tree)

Here's how STP configuration typically looks on Cisco switches:

Switch(config)# spanning-tree vlan 1 priority 4096
Switch(config)# spanning-tree mode rapid-pvst
Switch(config)# interface GigabitEthernet0/1
Switch(config-if)# spanning-tree portfast

Modern networks use enhanced versions of STP:

• RSTP (802.1w): Faster convergence (1-2 seconds vs 30-50 seconds)
• MSTP (802.1s): Maps multiple VLANs to a single spanning tree instance
• PVST+: Cisco's per-VLAN spanning tree implementation

Common problems and their solutions:

With software-defined networking, STP's role is evolving. Many modern solutions use alternative loop prevention mechanisms like:

// Example SDN controller rule to prevent loops
flow = {
    'priority': 40000,
    'match': {'in_port': 1},
    'actions': [{'type': 'OUTPUT', 'port': 'FLOOD'}],
    'flags': ['NO_PACKET_IN']
}