Introduction to the show int trunk Command
The show int trunk command is a crucial tool for network engineers, especially those working with Cisco switches. This command provides real-time insights into the status of trunk ports, which are responsible for carrying VLAN (Virtual Local Area Network) traffic across different switches in a network. Trunk ports are essential for communication between switches, as they allow multiple VLANs to traverse a single physical link.
As network infrastructures become more complex, the role of trunk ports grows significantly. Ensuring that these ports are correctly configured and operational is key to maintaining a smooth network operation. By using the show int trunk command, you can quickly check the operational status of trunk interfaces, identify any VLAN configuration mismatches, and troubleshoot connectivity issues across your network.
Why Is show int trunk Important?
The show int trunk command provides valuable information that is critical for maintaining and troubleshooting Cisco-based networks. Here are some reasons why this command is vital:
- Trunk Configuration Verification
The command lets you verify that trunking is enabled on interfaces and shows the VLANs that are allowed on the trunk. - Native VLAN Validation
It helps you ensure that the native VLAN is correctly set and does not cause any mismatches across the network. - Troubleshooting VLAN Communication
If devices in the same VLAN cannot communicate, using this command can help you identify trunk misconfigurations or VLAN pruning issues. - Detecting VLAN Leaks
A misconfigured trunk can lead to unwanted VLANs passing through the link, leading to security and performance problems. The show int trunk command helps detect such issues.
How to Use show int trunk
The show int trunk command is used in Cisco switches to display the status of trunking on interfaces. Here’s the basic syntax:
bash
Switch# show int trunk
Running this command provides a summary of trunk ports, including information about allowed VLANs, the trunking encapsulation method (usually 802.1Q), and the trunking status. The command is particularly useful when diagnosing issues with trunked VLANs, checking if specific VLANs are allowed, or ensuring that trunk ports are operational.
Sample Output from show int trunk
When you enter the command, you’ll typically see the following output format:
bash
Port Mode Encapsulation Status Native vlan
Gi0/1 on 802.1q trunking 1
Port Vlans allowed on trunk
Gi0/1 1-1005
Port Vlans allowed and active in management domain
Gi0/1 10,20,30
Port Vlans in spanning tree forwarding state and not pruned
Gi0/1 10,20,30
Key Fields in the Output
- Port: Displays the interface name (e.g., Gi0/1).
- Mode: The trunking mode of the interface, which can be "on", "desirable", "auto", etc.
- Encapsulation: Shows the trunking encapsulation method (usually 802.1Q or ISL).
- Status: Indicates whether the port is actively trunking or not.
- Native VLAN: Displays the native VLAN ID for the trunk port.
- VLANs Allowed on Trunk: Lists the VLANs that are allowed to pass through the trunk link.
- VLANs in Spanning Tree Forwarding State: Displays the VLANs that are not blocked by Spanning Tree Protocol (STP).
Common Issues Identified Using show int trunk
Issue 1: Trunk Port Is Not Active
Problem: A trunk port is not active, and VLANs are not being carried between switches.
Solution: Use the show int trunk command to verify that the port is indeed configured for trunking. If it’s in a non-trunking mode, change the port mode to trunk using the following command:
bash
Switch(config)# interface Gi0/1
Switch(config-if)# switchport mode trunk
Re-run the show int trunk command to confirm the port is now trunking.
Issue 2: VLANs Are Not Propagating
Problem: A specific VLAN is not propagating across the trunk link.
Solution: Check the Vlans allowed on trunk output in the command results. If the VLAN is missing from the allowed list, use the following command to permit the VLAN:
bash
Switch(config)# interface Gi0/1
Switch(config-if)# switchport trunk allowed vlan add 100
Issue 3: Native VLAN Mismatch
Problem: A native VLAN mismatch between two trunking switches causes traffic to be dropped.
Solution: Use the show int trunk command to check the native VLAN on both ends of the trunk. Ensure both ends have the same native VLAN set.
bash
Switch(config)# interface Gi0/1
Switch(config-if)# switchport trunk native vlan 1
Issue 4: VLAN Pruning
Problem: Unwanted VLANs are being passed through a trunk.
Solution: Use the show int trunk command to identify which VLANs are being allowed on the trunk. You can prune unwanted VLANs using the following command:
bash
Switch(config)# interface Gi0/1
Switch(config-if)# switchport trunk allowed vlan remove 100
Best Practices for Using show int trunk
1. Regular Audits
Regularly run the show int trunk command to check for any configuration drift or potential misconfigurations. This helps avoid issues before they escalate.
2. Limit Allowed VLANs
Be selective about the VLANs you allow on trunk ports. Avoid the default 1-1005 range unless absolutely necessary to reduce unnecessary traffic.
3. Monitor Native VLAN Consistency
Ensure that the native VLAN is consistent across all trunk links to avoid native VLAN mismatches.
4. VLAN Pruning
Prune unused VLANs to optimize bandwidth and reduce the risk of security breaches.
Conclusion
The show int trunk command is an essential diagnostic tool for network engineers working with Cisco switches. It provides invaluable insights into trunk port status, VLAN configurations, and native VLAN settings. Mastery of this command is vital for troubleshooting network connectivity, verifying VLAN propagation, and ensuring proper trunking configuration across your network.
For anyone studying for networking certifications or looking to improve their understanding of trunking in Cisco networks, incorporating this command into your toolkit is indispensable. Use Exam Prep Dumps and Study Guide material to reinforce your knowledge and stay ahead in your networking career.
Sample Questions and Answers (MCQs)
Here are a few sample multiple-choice questions (MCQs) based on the show int trunk command. These questions can be useful in your Exam Prep Dumps and Study Guide material.
Question 1:
What does the show int trunk command display?
A) The operational status of access ports
B) The trunking status and VLAN information on trunk ports
C) The IP address of the switch
D) The spanning tree topology information
Answer: B) The trunking status and VLAN information on trunk ports
Question 2:
Which of the following is NOT a possible output of the show int trunk command?
A) Trunking mode
B) Allowed VLANs
C) Native VLAN
D) Switch model
Answer: D) Switch model
Question 3:
You notice a "Native VLAN mismatch" error. Which of the following commands will resolve the issue?
A) switchport trunk allowed vlan 1
B) switchport trunk native vlan [vlan-id]
C) show vlan brief
D) show interface status
Answer: B) switchport trunk native vlan [vlan-id]
Question 4:
What does the show int trunk command help you verify?
A) Router configurations
B) VLAN allowed status on trunk links
C) Layer 2 forwarding table
D) Access port configurations
Answer: B) VLAN allowed status on trunk links
