Which Cisco 802.1d Spanning Tree Protocol extension is used to stop BPDUs from being transmitted out a specific port, often on access ports that are connected to end hosts and should not participate in STP?

Introduction / Context:
In Cisco switching environments, several Spanning Tree Protocol enhancements are available to fine tune how BPDUs are sent and received. On some edge ports that connect only to hosts, administrators may want to prevent BPDUs from being sent or processed so that those ports do not cause unnecessary STP activity. The Cisco feature that stops BPDUs from being transmitted out a port under certain conditions is called BPDU Filter. This question focuses on correctly identifying that feature among other STP related mechanisms.

Given Data / Assumptions:

• You have an access port that connects to an end host and should not participate in STP exchanges.
• You want to prevent BPDUs from being transmitted out that port.
• You may also want to ignore incoming BPDUs on that port, depending on configuration mode.

Concept / Approach:
BPDU Filter is a Cisco extension that can be configured either globally or on a per interface basis. When applied to an interface, it can suppress the sending and in some cases the processing of BPDUs, effectively hiding the port from the STP topology. This is particularly useful on ports that are connected to devices that are never expected to participate in STP, such as end user PCs. It is different from BPDU Guard, which shuts down the port when a BPDU is received, and from Root Guard and Loop Guard, which protect the stability of the STP topology in other ways. The key clue in the question is that the feature stops BPDUs from being transmitted.

Step-by-Step Solution:
1. Identify that the main requirement is to stop BPDUs from being sent out a port. 2. Recall that BPDU Guard reacts to received BPDUs by err-disabling the port, rather than simply suppressing BPDU transmission. 3. Note that Root Guard prevents a port from becoming a root port, and Loop Guard protects against unidirectional link issues, neither of which directly stops BPDU transmission. 4. Recognize that BPDU Filter can be configured so that an interface neither sends nor processes BPDUs in normal operation. 5. Conclude that BPDU Filter is the Cisco 802.1d extension that stops BPDUs from being transmitted out a specific port.

Verification / Alternative check:
Cisco configuration guides show that the command spanning-tree bpdufilter enable under an interface enables this behavior. When active, the interface does not send BPDUs, and depending on mode, may also ignore incoming BPDUs, effectively removing the port from STP participation. This confirms that BPDU Filter is the feature described. In contrast, BPDU Guard is configured with spanning-tree bpduguard enable and causes a port shutdown on BPDU reception, which is a different behavior than just stopping BPDU output.

Why Other Options Are Wrong:
Option b: BPDU Guard is used to err-disable a port when BPDUs are received, not to silently stop BPDUs from being transmitted. Option c: Root Guard is applied to ports where you do not want a new root bridge to appear, and it places a port into a root inconsistent state on receiving certain BPDUs, but it does not stop BPDU transmission by itself. Option d: Loop Guard protects against a condition where BPDUs suddenly stop being received on non designated ports; it does not stop BPDUs from being sent. Option e: PortFast allows a port to transition quickly to forwarding state, but it does not in itself suppress BPDUs. It is often used together with BPDU Guard or BPDU Filter for edge ports.

Common Pitfalls:
Many students confuse BPDU Guard with BPDU Filter because both are associated with access ports and PortFast. The simple way to remember the difference is that Guard protects the network by shutting the port when BPDUs are seen, while Filter hides the port from STP by suppressing BPDUs. Misusing BPDU Filter can be dangerous because if a switch is accidentally connected to such a port, the lack of BPDUs can allow loops to form undetected. Therefore, network designers often prefer BPDU Guard as a safer default for user facing ports, while using BPDU Filter only in carefully controlled scenarios.

Final Answer:
The Cisco extension that stops BPDUs from being transmitted out a port is BPDU Filter.