Configuring VLAN Trunk Links Between Switches

Q: How would you configure a trunk link between two switches while ensuring that only specific VLANs are allowed on that trunk?

VLAN Configuration and Inter-VLAN Routing
Senior level question

Share on:

Explore all the latest VLAN Configuration and Inter-VLAN Routing interview questions and answers

Explore

Most Recent & up-to date

100% Actual interview focused

Create Interview

Create VLAN Configuration and Inter-VLAN Routing interview for FREE!

Configuring trunk links between switches is a critical aspect of network management, especially when working with VLANs (Virtual Local Area Networks). Trunking enables the transfer of multiple VLANs over a single physical connection, which is essential for efficient bandwidth utilization in modern networks. Proper configuration of trunk links ensures that only specific VLANs have access, enhancing both security and performance. When preparing for a networking role, understanding VLAN trunking is essential.

VLANs separate broadcast domains in a network, improving traffic management and organization. The IEEE 802.1Q standard is widely used for VLAN tagging, allowing different VLANs to co-exist on a single trunk link. Each VLAN is assigned a unique identifier, ensuring data packets remain tagged correctly throughout the network. During an interview, candidates might be asked to explain how to configure trunk links while restricting VLAN access.

This involves specifying allowed VLANs, which prevents unnecessary traffic from flowing through the trunk. Network professionals often utilize commands to define allowed VLANs on switches, which strengthens security by minimizing exposure to unauthorized VLANs. Additionally, familiarity with switch models and their specific commands is crucial since different manufacturers, such as Cisco or Juniper, may have unique command syntax. Knowing how to troubleshoot trunk links is equally important.

Common issues include misconfigurations or mismatched settings on either end of the trunk line, which can lead to VLANs not passing correctly. For candidates, it's advisable to practice hands-on scenarios in a lab environment where you can simulate trunk configurations and VLAN management. This experience will not only enhance your troubleshooting skills but also build your confidence when discussing VLAN trunking in interviews. Understanding these concepts thoroughly can set you apart as a candidate, showcasing your expertise in network infrastructure and design..

To configure a trunk link between two switches while ensuring that only specific VLANs are allowed on that trunk, I would follow these steps:

1. Set the switch ports to trunk mode: On both switches, I would configure the appropriate interfaces to operate in trunk mode, allowing them to carry traffic for multiple VLANs. The command for this on Cisco devices would be:
```
SwitchA(config)# interface GigabitEthernet0/1
SwitchA(config-if)# switchport mode trunk
SwitchA(config-if)# switchport trunk allowed vlan
```

2. Specify allowed VLANs: To restrict which VLANs are allowed on the trunk, I would use the `switchport trunk allowed vlan` command followed by the VLAN numbers that should be permitted. For example, if I want to allow VLANs 10, 20, and 30, the command would be:
```
SwitchA(config-if)# switchport trunk allowed vlan 10,20,30
```

3. Configure the same settings on the other switch: I would then replicate these configurations on the corresponding trunk port of the other switch (SwitchB) to ensure they match. The configuration would be identical:
```
SwitchB(config)# interface GigabitEthernet0/1
SwitchB(config-if)# switchport mode trunk
SwitchB(config-if)# switchport trunk allowed vlan 10,20,30
```

4. Verify the configuration: After configuring the trunk, I would verify that the VLANs are correctly configured using the command:
```
SwitchA# show interfaces trunk
```
This should show the allowed VLANs on the trunk link.

5. Testing: Finally, I would test to ensure that devices on the specified VLANs can communicate across the trunk and that devices on VLANs not specified cannot communicate.

By using the `switchport trunk allowed vlan` command, I ensure only specified VLANs traverse the trunk, enhancing network security and efficiency.