Top 15 VTP Pruning Interview Questions

Here are the top 15 VTP pruning interview questions, designed to test a candidate’s understanding of the feature, its benefits, and troubleshooting.

List of VTP Pruning Interview Questions

1.What is Pruning in networking?

In networking, pruning refers to the process of limiting or controlling the traffic that is sent across certain links to improve network efficiency and reduce unnecessary data transmission.

2. What is VTP pruning, and why is it used?

VTP pruning or VLAN Trunking Protocol Pruning is a feature that reduces unnecessary broadcast, multicast, and unknown unicast traffic on trunk links by only forwarding this traffic to switches that have active ports in the target VLAN. It helps optimize bandwidth by preventing unnecessary traffic from being sent across links that don’t need it.

3. How does VTP pruning improve network performance?

VTP pruning improves network performance by minimizing the unnecessary propagation of broadcast, multicast, and unknown unicast traffic on VLANs that aren’t required on a particular trunk. This reduces bandwidth consumption and limits broadcast domain traffic to only the switches that require it.

4. What types of VLAN traffic are affected by VTP pruning?

VTP pruning affects broadcast, multicast, and unknown unicast traffic. It doesn’t affect unicast traffic where the destination MAC address is known.

5. Which VLANs cannot be pruned using VTP pruning?

VLAN 1 and VLANs 1002 through 1005 (used for legacy technologies like FDDI and Token Ring) cannot be pruned. VLAN 1 is typically used for management traffic and should not be pruned by design.

6. Can you explain how to enable VTP pruning on a Cisco switch?

Yes. To enable VTP pruning:

1. Ensure the switch is in VTP server mode (show vtp status to verify).
2. Enter global configuration mode: configure terminal.
3. Enable pruning with the command: vtp pruning.
4. Verify pruning is enabled using show vtp status and checking for VTP Pruning Mode: Enabled.

7. How can you verify which VLANs are being pruned on a specific trunk link?

You can verify VLAN pruning on a trunk link using the command:

switch# show interfaces trunk

This command shows the status of VLANs on trunk ports, including which VLANs are being forwarded or pruned.

8. What is the difference between VTP pruning and manual pruning?

VTP pruning is an automatic process controlled by the VTP server that dynamically prunes VLANs based on active hosts. Manual pruning, on the other hand, involves administratively removing VLANs from trunk ports using the command:

switch(config-if)# switchport trunk allowed vlan remove <vlan_id>

Manual pruning offers more control but requires administrative overhead.

9. What are common issues that can arise with VTP pruning, and how would you troubleshoot them?

Common issues include VLANs not being pruned as expected, VLANs that should be pruned but aren’t, and devices losing connectivity. Troubleshooting steps include verifying VTP pruning is enabled, checking VTP versions across switches, ensuring no active devices are using the VLAN on the trunk link, and confirming the VTP domain configuration is consistent.

Related: Troubleshooting VTP Pruning Issues

10. What could cause VTP pruning to not function correctly on a network?

Reasons could include:

• VTP pruning not being enabled on the VTP server.
• VTP version mismatch across switches.
• VTP domain inconsistencies.
• VLANs being actively used on the trunk link.
• The VLANs in question being non-prunable (e.g., VLAN 1 or VLANs 1002-1005).

11. What are the limitations of VTP pruning?

 Limitations include:

• VTP pruning does not work on VLANs 1 and 1002–1005.
• It may not work in environments with different VTP versions or mismatched configurations.
• It is only useful for broadcast, multicast, and unknown unicast traffic; unicast traffic to known destinations is unaffected.

12.How does VTP pruning impact spanning-tree operations for VLANs?

VTP pruning does not affect the spanning-tree topology. STP BPDUs are still sent for all VLANs to maintain loop-free paths. Pruning only stops unnecessary data traffic, not control plane traffic like STP BPDUs.

13.Describe a scenario where VTP pruning could cause network connectivity issues.

If VTP pruning is enabled and a VLAN is incorrectly pruned from a trunk link that services hosts in that VLAN, those hosts will lose connectivity because their broadcast and unknown unicast traffic will not reach the necessary switches.

14.Is VTP pruning enabled by default, and how do you disable it if necessary?

No, VTP pruning is not enabled by default on Cisco switches. It must be manually enabled on the VTP server switch using the following command:

switch(config)# vtp pruning

Once enabled on the VTP server, VTP pruning will propagate the configuration to all switches in the same VTP domain. However, even after enabling VTP pruning, it only takes effect on VLANs that are eligible for pruning (excluding VLAN 1 and VLANs 1002–1005).

15.What best practices should be followed when implementing VTP pruning in a network?

• Only enable VTP pruning on the VTP server switch.
• Ensure all switches are in the same VTP domain and running the same VTP version.
• Regularly back up VLAN configurations to prevent data loss.
• Monitor network traffic to confirm that essential VLANs are not being pruned unintentionally.
• Consider using VTP transparent mode for greater control in large or complex networks.

These questions assess knowledge of VTP pruning, its configuration, benefits, and common issues encountered in real-world scenarios.

ABOUT THE AUTHOR

Rashmi Bhardwaj

I am here to share my knowledge and experience in the field of networking with the goal being – “The more you share, the more you learn.”

I am a biotechnologist by qualification and a Network Enthusiast by interest. I developed interest in networking being in the company of a passionate Network Professional, my husband.

I am a strong believer of the fact that “learning is a constant process of discovering yourself.”
– Rashmi Bhardwaj (Author/Editor)