Introduction to Spanning Tree Protocol (STP)

In current computer networks, it is very important to avoid loop formation and to maintain redundancy. The Spanning Tree Protocol (STP) is a network protocol that is aimed at preventing formation of loops in Ethernet networks. Created by Radia Perlman and standardized as IEEE 802. 1D, STP adopts a logical topology for Ethernet network with no more than one active path between any two nodes of the network.

How STP Works

This is how the STP operates; it selects the root bridge, assigns each bridge with the shortest distance from the root bridge and then blocks all paths which would lead to the formation of loops. Here’s a step-by-step breakdown of how STP functions:Below is a brief description on how the STP operates:

1. Root Bridge Election: STP starts with the election of a root bridge, which basically acts as a central known point in the network. The bridge which has the lowest value of Bridge ID, which consists of a priority value and the device’s MAC address, is designated as the root bridge.
2. Path Cost Calculation: Every switch determines the cost to the root bridge from the path. Dependent on the actual speed of the network links, path cost is defined as the next variable. Hence the lower the speed the higher the cost.
3. Port Roles Assignment:

• Root Port: The port on a switch with the lowest path cost to the root bridge.
• Designated Port: The port on a network segment with the lowest path cost to the root bridge. There is one designated port per segment.
• Blocked Port: Any port that is neither a root port nor a designated port is placed in a blocked state to prevent loops.

1. Bridge Protocol Data Units (BPDUs): Switches transmit BPDUs to inform each other about cost of the path and bridges’ identification. This assists in keeping the loop free topology.
2. Port States: PORTS pass through various states namely: blocking state , listening state, Learning state, forwarding state and disabled state. These states dictate if a port is permitted to either transmit or receive traffic when the topology of a network is being changed.

Configuring STP

STP is configured in network switches by enabling the protocol as well as potentially adjusting, the bridge priority, the path costs, etc. Below are the basic steps to configure STP on a Cisco switch:Below are the basic steps to configure STP on a Cisco switch:

Step 1: Enable STP

STP is typically enabled by default on Cisco switches. However, if it is disabled, you can enable it with the following command:

switch(config)# spanning-tree vlan <vlan-id>

Step 2: Configure Bridge Priority

The default bridge priority is 32768. To influence which switch becomes the root bridge, adjust the priority value (lower values have higher priority):

switch(config)# spanning-tree vlan <vlan-id> priority <priority-value>

Step 3: Configure Port Path Cost

You can manually set the path cost of a port to influence STP’s path selection:

switch(config-if)# spanning-tree cost <cost-value>

Step 4: Configure Portfast

For ports connected to end devices (not other switches), enabling Portfast can speed up network convergence:

switch(config-if)# spanning-tree portfast

Example Configuration

Consider a network with three switches: Switch A, Switch B, and Switch C. We want to configure STP to ensure Switch A becomes the root bridge.

1. Enable STP on all switches (usually enabled by default):

SwitchA(config)# spanning-tree vlan 1
SwitchB(config)# spanning-tree vlan 1
SwitchC(config)# spanning-tree vlan 1

2. Set the bridge priority for Switch A to make it the root bridge:

SwitchA(config)# spanning-tree vlan 1 priority 4096

3. Verify the root bridge selection (on Switch B and Switch C):

SwitchB# show spanning-tree vlan 1
SwitchC# show spanning-tree vlan 1

You should see Switch A’s Bridge ID as the root bridge.

4. Adjust port costs if necessary:

If Switch B has two paths to the root bridge (one directly to Switch A and one via Switch C), we can adjust the path cost on Switch B to prefer the direct link:

SwitchB(config-if)# interface GigabitEthernet0/1
SwitchB(config-if)# spanning-tree cost 10

SwitchB(config-if)# interface GigabitEthernet0/2
SwitchB(config-if)# spanning-tree cost 20

5. Enable Portfast on edge ports (ports connected to end devices):

SwitchA(config-if)# interface FastEthernet0/1
SwitchA(config-if)# spanning-tree portfast

SwitchB(config-if)# interface FastEthernet0/1
SwitchB(config-if)# spanning-tree portfast

Advanced STP Configurations

In larger and more complex networks, advanced STP configurations may be necessary. These include:

1. Rapid Spanning Tree Protocol (RSTP): An enhancement to STP that provides faster convergence. It is defined in IEEE 802.1w.
2. Multiple Spanning Tree Protocol (MSTP): Allows multiple VLANs to be mapped to a single spanning tree instance, optimizing resource usage and improving scalability. It is defined in IEEE 802.1s.
3. Per-VLAN Spanning Tree Plus (PVST+): Cisco’s proprietary extension of STP that runs a separate instance of STP for each VLAN, providing load balancing and improved fault tolerance.

Conclusion

The STP is an essential part in network design since it provides an Ethernet network redundancy, at the same time avoiding formation of loops. Thus, having proper cognition and management of STP allows network managers to design fault-tolerant, optimized and efficient networks. Portfolio configurations such as RSTP, MSTP, and PVST+ provide added functions and versatility to STP, making it a crucial weapon in modern day network administration.