• Arista Spanning Tree User Manual : Docs

• Spanning Tree GFG : Article on GFG

• PPT made by me on Canva : PPT

Spanning Tree Protocol (STP) is a network protocol designed to prevent loops in Ethernet networks. It works by identifying a single path between network devices and blocking any redundant paths. Here’s how it operates:

1. Bridge Election:

   • STP begins by electing a Root Bridge. The switch with the lowest Bridge ID (a combination of its priority and MAC address) becomes the Root Bridge. This switch serves as the central point for all STP operations.

2. Port Role Assignment:

   • Once the Root Bridge is established, STP assigns port roles to each switch based on their distance from the Root Bridge:

     • Root Port: The port on a non-root bridge that has the lowest cost path to the Root Bridge.

     • Designated Port: The port on a switch that has the lowest cost path to the Root Bridge for that segment. It is responsible for forwarding traffic towards the Root Bridge.

     • Blocked Port: Any port that is not a Root or Designated port is placed in a blocking state to prevent loops.

3. BPDU Exchange:

   • Switches exchange Bridge Protocol Data Units (BPDUs) to share information about their respective roles and the network topology. BPDUs contain information such as the Root Bridge ID and the cost to reach it.

4. Convergence:

   • STP will converge, meaning it will stabilize the network, after the initial setup. If a link goes down or a new switch is added, STP recalculates the topology to maintain a loop-free network. It transitions ports between states (Blocking, Listening, Learning, and Forwarding) based on the current topology and network events.

5. Timers:

   • STP uses timers (Hello Time, Forward Delay, and Max Age) to manage the flow of BPDUs and to ensure timely updates to the network topology.

Spanning Tree Protocol (STP) has undergone several enhancements over the years to improve network efficiency, convergence times, and scalability. Here are some key enhancements:

1. Rapid Spanning Tree Protocol (RSTP):

   • Introduced in IEEE 802.1w, RSTP significantly reduces the convergence time of STP from seconds to milliseconds. It allows for quicker recovery from link failures by using port states that enable immediate transitions to forwarding without waiting for timers.

2. Multiple Spanning Tree Protocol (MSTP):

   • Defined in IEEE 802.1s, MSTP allows multiple spanning tree instances to exist within a single network. This means that different VLANs can have different spanning trees, which optimizes the network by reducing the number of blocked ports and improving load balancing.

3. Per VLAN Spanning Tree (PVST):

   • PVST, developed by Cisco, enables a separate spanning tree for each VLAN. This provides better utilization of available bandwidth and helps in the distribution of traffic across multiple links.

4. Enhanced STP (ESTP):

   • ESTP introduces enhancements to the original STP by optimizing the way BPDUs are processed and allowing for faster convergence through improved algorithms.

5. Bridge Assurance:

   • This feature ensures that the designated ports on a bridge are operational. If a designated port fails, the other switches will quickly react to re-establish the loop-free topology.

6. Loop Guard:

   • Loop Guard helps to prevent the formation of loops by placing a port in a loop-inconsistent state if it stops receiving BPDUs. This protects the network from accidental loops caused by misconfigurations or failures.

7. Root Guard:

   • Root Guard is used to protect the Root Bridge by preventing other switches from becoming the Root Bridge on a particular port, ensuring that only the designated switch remains the Root.

8. BPDU Filtering and BPDU Guard:

   • BPDU Filtering allows switches to ignore incoming BPDUs on specific ports, while BPDU Guard disables a port if BPDUs are received, protecting the network from misconfigurations.