3.4 5 Packet Tracer Configure Trunks: Exact Answer & Steps

How to Configure Trunks in Packet Tracer: The 3.4.5 Lab Walkthrough

If you're working through Cisco's CCNA material, you've probably hit lab 3.Here's the thing — 4. Still, 5 — the one where you configure trunk links between switches. And if you're anything like most students, something clicked the first time you saw VLANs work across multiple switches. That's the moment networking starts feeling real.

Configuring trunks in Packet Tracer isn't just about memorizing commands. It's about understanding how VLAN traffic actually travels between switches without getting stripped of its tag. Once you get it, a whole layer of network design makes sense.

Let's break down what trunking actually is, why it matters, and exactly how to configure it in Packet Tracer — step by step.

What Is a Trunk Link in Networking?

Here's the simplest way to think about it: a trunk link is a connection between two switches that carries traffic for multiple VLANs at once.

When you have a single VLAN on one switch, regular access ports work fine. But once you create VLANs on different switches — say VLAN 10 for Sales on both Switch A and Switch B — you need a way for that traffic to travel between the two switches without losing its VLAN identity. The details matter here.

That's what a trunk does. 1Q encapsulation) so the receiving switch knows which VLAN that traffic belongs to. Practically speaking, it tags each frame with a VLAN ID (using 802. Consider this: without trunking, VLANs stay isolated to a single switch. With trunking, they span across your entire network.

802.1Q vs. ISL

You might see both mentioned in older Cisco material. ISL is Cisco's old proprietary protocol — you'll rarely touch it in modern labs. 1Q is the industry standard, and that's what Packet Tracer uses by default. 802.It inserts a 4-byte tag into the Ethernet frame, between the source and type/length fields.

The native VLAN concept matters here too. By default, untagged traffic on a trunk link gets assigned to VLAN 1 (the native VLAN). Just make sure both ends of the trunk agree on which VLAN is native, or you'll get some confusing connectivity issues.

Why Trunk Configuration Matters

Here's why this lab matters beyond the exam. Which means in real networks, you almost never have just one switch. And you almost never want every device on the same broadcast domain.

Trunk links are the backbone of any multi-switch VLAN deployment. Without them, you can't have the same VLAN spread across multiple switches — which defeats the whole purpose of VLAN segmentation in a larger network.

In practice, trunk configuration is one of those skills you'll use over and over. Setting up guest networks, voice VLANs, management networks across buildings — all of it depends on working trunk links.

So yes, you're learning it for the exam. But you're also learning it for the job.

How to Configure Trunks in Packet Tracer (3.4.5 Lab)

Let's walk through the actual configuration. I'll assume you've already got your basic topology built with two switches and some PCs in different VLANs. If not, set that up first — two switches connected by a crossover cable (or any interface, really), with at least two PCs on each switch in different VLANs.

Step 1: Verify Your VLANs Exist

Before you configure trunks, make sure you've actually created the VLANs you want to trunk. On both switches:

Switch> enable
Switch# configure terminal
Switch(config)# vlan 10
Switch(config-vlan)# name Sales
Switch(config-vlan)# exit
Switch(config)# vlan 20
Switch(config-vlan)# name Engineering
Switch(config-vlan)# exit

Do this on both switches. The VLAN numbers and names don't have to match perfectly across switches for the trunk to work, but it makes debugging way easier if they do.

Step 2: Assign Access Ports to VLANs

Now put your PC connections into the appropriate VLANs. Let's say PC1 on Switch A should be in VLAN 10:

Switch(config)# interface fastethernet 0/1
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 10

Repeat this for all your access ports, assigning them to the right VLANs.

Step 3: Configure the Trunk Port

This is the main event. On the link connecting your two switches:

On Switch A:

Switch(config)# interface gigabitethernet 0/1
Switch(config-if)# switchport mode trunk

That's it. Seriously. Packet Tracer defaults to 802.1Q, so you don't even need to specify the encapsulation in most cases.

Now do the exact same thing on Switch B:

Switch(config)# interface gigabitethernet 0/1
Switch(config-if)# switchport mode trunk

Step 4: Verify the Trunk is Working

A few commands confirm everything is working:

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Switch# show interfaces trunk

This shows you which ports are trunking, what VLANs are allowed, and the encapsulation type.

Switch# show vlan brief

This confirms your VLANs are there and your access ports are assigned correctly.

Switch# show interface gigabitethernet 0/1 switchport

This shows you the mode, encapsulation, and native VLAN for that specific interface.

If you see "trunk" under Mode and "Dynamic desirable" or "trunk" under Administrative Mode, you're good. If both ends show trunk, your VLANs should now span both switches.

Common Mistakes Students Make

A few things trip people up in this lab:

Mismatched native VLANs. If one side of the trunk has switchport trunk native vlan 99 and the other is left at default (VLAN 1), you'll get native VLAN mismatch errors. It's one of the first things to check when trunking isn't working.

Forgetting to create the VLANs on both switches. You create VLAN 10 on Switch A, trunk to Switch B, but never create VLAN 10 on Switch B. The trunk will come up, but the VLAN won't exist on the second switch to assign ports to.

Using the wrong interface. Double-check which ports you're actually connecting between switches. It's easy to pick the wrong gigabit interface in a larger topology.

Assuming the trunk automatically passes all VLANs. By default, a trunk passes all VLANs. But you can restrict which VLANs are allowed with switchport trunk allowed vlan — just make sure you know if you've restricted it accidentally.

Practical Tips for Success

A few things that'll save you time:

• Use do show running-config instead of exiting config mode every time you want to verify something. It keeps you in the configuration terminal but lets you run show commands.
• Label your topology. In Packet Tracer, you can add notes or rename devices. It sounds trivial, but when you're troubleshooting, knowing which switch is which matters.
• Test with ping. After configuring the trunk, ping from a PC in VLAN 10 on Switch A to a PC in VLAN 10 on Switch B. If it works, your trunk is doing its job.
• Read the error messages. Cisco's error messages aren't always friendly, but "VLAN 10 not allowed on trunk" tells you exactly what's wrong.

FAQ

How do I fix "Native VLAN mismatch" errors?

Check both ends with show interface switchport. Make sure both trunk ports have the same native VLAN. If they don't, set them to match:

Switch(config-if)# switchport trunk native vlan 10

Can I use different VLAN numbers on each switch and still trunk?

Yes, technically the VLAN tag carries the number from the sending switch. But this is a terrible idea in practice. Keep your VLAN numbers consistent across the network — it makes everything easier to troubleshoot.

What if my trunk shows "not trunking"?

First, verify both ends are set to switchport mode trunk. Now, if one side is set to dynamic auto or dynamic desirable, it might not negotiate up. Hard-set both sides to trunk for the lab.

Does Packet Tracer support DTP?

Yes, but for the 3.4.Day to day, 5 lab, just hard-code both sides to trunk mode. It's more reliable and ensures you understand what's happening rather than relying on negotiation.

What happens if I don't configure a native VLAN?

It defaults to VLAN 1. That's fine for most labs, but in production networks, you usually want to change the native VLAN to something else (like 999) to avoid potential VLAN hopping attacks.

The Bottom Line

Trunk configuration in Packet Tracer is straightforward once you've done it a couple times. On the flip side, the key is understanding why you're tagging frames and what happens when traffic crosses that link. The commands are simple — the concept is what matters.

Get comfortable with show interfaces trunk and show vlan brief. You'll use those two commands in every real-world switch deployment you touch.