BPDU Guard & BPDU Filter (STP Toolkit) | CCNA 200-301 Day 21 (part 2)

Introduction

This tutorial will guide you through the concepts of BPDU Guard and BPDU Filter, essential features of the Spanning Tree Protocol (STP) toolkit. Understanding these features is crucial for maintaining a robust and secure network, particularly in preventing network loops and ensuring stability in your Cisco environment.

Step 1: Understanding PortFast and BPDUs

• PortFast: This feature allows switch ports to transition from a blocking to a forwarding state instantly. It is typically used for ports connected to end devices like PCs or printers.
• Bridge Protocol Data Units (BPDUs): These are messages exchanged across the switches in a network to maintain the STP topology. When PortFast is enabled, BPDUs should not be received on these ports.

Practical Tip

Always apply PortFast only on access ports to avoid introducing loops in your network.

Step 2: Identifying the Problem with BPDU Guard

• Issue: When a PortFast-enabled port receives a BPDU, it can cause network instability. This is particularly problematic in environments where end-user devices are connected.
• Impact: Without protection, a device connecting to a PortFast port that sends BPDUs could lead to a network loop.

Step 3: Implementing BPDU Guard as a Solution

• Purpose: BPDU Guard disables the port if BPDUs are detected on a PortFast-enabled interface, protecting the network from possible loops.

Configuration Steps for BPDU Guard

1. Access the switch CLI.
2. Enter global configuration mode:

   configure terminal
   

3. Enable BPDU Guard globally:

   spanning-tree portfast bpduguard default
   

4. To apply it on a specific interface, enter interface configuration mode:

   interface [interface-id]
   

5. Enable BPDU Guard:

   spanning-tree bpduguard enable
   

Common Pitfall

• Ensure that BPDU Guard is only applied to PortFast-enabled interfaces to avoid unintended port shutdowns.

Step 4: Understanding ErrDisable State

• ErrDisable: This state occurs when a port is put into a disabled state due to a detected error, such as receiving a BPDU on a PortFast port.
• Impact: The port will remain disabled until it is manually re-enabled or until a recovery procedure is initiated.

Step 5: Recovering from ErrDisable

1. Identify the err-disabled port using the command:

   show interface status
   

2. To re-enable the port manually:

   interface [interface-id]
   shutdown
   no shutdown
   

Practical Tip

Consider setting up automatic recovery from ErrDisable using:

 errdisable recovery cause bpduguard
 errdisable recovery interval [seconds]

Step 6: Understanding the Problem with BPDU Filter

• Issue: BPDU Filter prevents BPDUs from being sent or received on a port, which can lead to issues similar to those caused by a lack of BPDU Guard.
• Impact: If a switch port that should receive BPDUs is configured with BPDU Filter, it may not participate correctly in STP.

Step 7: Implementing BPDU Filter as a Solution

• Purpose: Use BPDU Filter on ports where you are confident that no BPDUs will be received, such as on access ports connected to end devices.

Configuration Steps for BPDU Filter

1. Access the switch CLI.
2. Enter global configuration mode:

   configure terminal
   

3. Enable BPDU Filter globally:

   spanning-tree portfast bpdufilter default
   

4. To apply it on a specific interface, enter interface configuration mode:

   interface [interface-id]
   

5. Enable BPDU Filter:

   spanning-tree bpdufilter enable
   

Conclusion

In this tutorial, we covered the essential features of BPDU Guard and BPDU Filter, including their purposes, configurations, and common pitfalls. Implementing these features will help maintain a stable and secure network environment. For further learning, consider practicing with Cisco Packet Tracer or similar tools to reinforce your understanding of STP and its components.