What occurs if a BPDU is received on a port enabled with BPDU Filter?

What occurs if a BPDU is received on a port enabled with BPDU Filter?

• The port is placed in an error-disabled state.
• The port is immediately disabled.
• The BPDU is ignored and the port remains functional.
• The port is placed into the loop-inconsistant state.

Answer: The BPDU is ignored and the port remains functional.

---

Understanding BPDU Filtering in Spanning Tree Protocol (STP)

BPDU (Bridge Protocol Data Unit) filtering is a feature in networking that controls the behavior of BPDU processing on a switch port. It plays a crucial role in how Spanning Tree Protocol (STP) functions and interacts with network topology changes.

1. What is BPDU Filtering?

BPDU filtering is a mechanism that prevents STP BPDUs from being sent or received on a particular port. The purpose of this feature is to prevent the exchange of STP messages, effectively isolating a port from participating in the spanning-tree topology.

STP utilizes BPDUs to detect network loops and manage redundant paths dynamically. By filtering BPDUs, administrators can prevent a port from being affected by STP operations, ensuring that it behaves as if STP were disabled.

2. Effects of BPDU Filtering on a Port

When BPDU filtering is enabled on a port, the following behaviors occur:

1. The switch does not send BPDUs on the port.

   • This prevents the port from advertising its presence in the STP topology.
   • Other switches in the network will not consider this port while computing the spanning-tree topology.

2. The switch does not process received BPDUs.

   • If a BPDU is received on the port, it is simply ignored.
   • The port continues to function as a standard access or trunk port without STP involvement.

3. The port remains active and does not enter a blocking, listening, or forwarding state dictated by STP.

   • This behavior ensures that the port continues forwarding traffic without any STP-related interruptions.

3. Scenarios Where BPDU Filtering is Used

BPDU filtering is commonly applied in specific network designs where spanning-tree convergence is not needed or should be avoided:

1. On Edge Ports (Access Ports) in PortFast Mode:

   • BPDU filtering is often enabled on ports connected to end devices like computers or printers.
   • This prevents unnecessary BPDU processing and speeds up port activation.

2. On Links to Non-STP Aware Devices:

   • Some network devices do not support STP.
   • Enabling BPDU filtering prevents these devices from mistakenly participating in STP.

3. On Layer 3 Routed Ports:

   • Ports configured as Layer 3 interfaces do not participate in STP.
   • BPDU filtering ensures no accidental STP interactions occur.

4. In Service Provider Networks (ISP Edge Ports):

   • ISPs may disable STP on customer-facing ports to prevent customer devices from affecting their spanning-tree topology.

4. Difference Between Global BPDU Filtering and Per-Port BPDU Filtering

BPDU filtering can be applied globally or per-port, and the behavior varies depending on the implementation.

1. Global BPDU Filtering (Enabled via spanning-tree portfast bpdufilter default)

   • Affects all PortFast-enabled ports.
   • The switch stops sending BPDUs.
   • If a BPDU is received, the port automatically loses its PortFast status and resumes normal STP operation.

2. Per-Port BPDU Filtering (spanning-tree bpdufilter enable)

   • Completely disables BPDU processing on the port.
   • The port does not send or receive BPDUs under any circumstances.
   • The port remains in the forwarding state regardless of network changes.

The key difference is that globally enabled BPDU filtering still allows the switch to react to received BPDUs, whereas per-port BPDU filtering completely disables BPDU handling.

5. Consequences of BPDU Filtering

While BPDU filtering has advantages, it also comes with risks:

1. Risk of Loops

   • Disabling STP on a port removes loop prevention mechanisms.
   • If a topology change creates a loop, BPDU filtering prevents STP from detecting and breaking it.

2. Loss of STP Protection

   • BPDU filtering prevents a switch from detecting and reacting to topology changes.
   • If a redundant link is added, it may cause broadcast storms.

3. Unexpected Behavior in Misconfigured Networks

   • If BPDU filtering is accidentally enabled on a trunk or uplink port, the switch may fail to participate in STP, causing disruptions.

6. Why is the Correct Answer “The BPDU is ignored and the port remains functional”?

Let’s analyze the incorrect answer choices and why they do not apply:

• “The port is placed in an error-disabled state.”

  • This does not happen because BPDU filtering does not trigger an error-disabled state.
  • Ports enter an error-disabled state due to security violations, like BPDU Guard violations, not BPDU filtering.

• “The port is immediately disabled.”

  • A port with BPDU filtering remains operational.
  • It does not disable itself upon receiving a BPDU.

• “The port is placed into the loop-inconsistent state.”

  • The loop-inconsistent state occurs with features like Root Guard when a BPDU is unexpectedly received.
  • BPDU filtering does not cause this condition.

Since BPDU filtering explicitly ignores received BPDUs, the correct answer is:

“The BPDU is ignored and the port remains functional.”