Matt/ December 5, 2017

Earlier this year I was involved in a string of interviews for an open network engineer position. The questions and scenarios provided during the interviews were aimed for someone mid-level. One of the more basic-ish scenario questions I like to ask is the following:

Given a brand new switch, can you provide me the commands you would use to configure the first four ports for hosts in VLAN 15?

This question is always interesting because I get such a wide variety of responses. You can certainly filter out people quickly who have never touched a switch. Some people will start with conf t, while others just jump straight into setting the VLAN tag. Some people specify that they’ll use an interface range command, while others get confused and want to configure the ports as a trunk. In fact, during this year’s interviews I had quite a significant number of people who completed the scenario by providing the commands to configure a trunk port, instead of static access. One thing that struck me as noteworthy is that the people who did this also provided the commands they needed to change the default native VLAN.

For a vast majority of networking devices (maybe all of them), the default/native VLAN for a trunk port is VLAN 1. This is not the best configuration for reasons we’ll get into a bit later – but unfortunately it needs to be manually changed. In every interview where the candidate suggested this be changed, I followed up with asking why. I asked so that I can find out two things: How well the individual is at explaining concepts, and whether or not this is something they just do because they were taught or if they actually understand the logic behind it. Surprisingly, a vast majority of the candidates could only provide the reasoning that “Well, VLAN 1 is bad” – but they couldn’t elaborate on why.

So why is VLAN 1 bad to use?

Technically, VLAN 1 itself isn’t the problem. The concept of a default VLAN allows for someone to attack a network by taking advantage of how switches use a default VLAN. Since VLAN 1 is typically set as the default for most vendors, then it becomes a well-known configuration for attackers to abuse. If every vendor had set the default native VLAN to 52, then we would still run into the exact same problem except the ‘bad’ VLAN would be 52.

So let’s back up just a little here and explain a bit of background. The concept of a VLAN is a segmented logical network. Rather than requiring a different piece of physical hardware to keep hosts separate, we can assign them into different virtual LAN segments. This is accomplished by ‘tagging’ each Ethernet packet with a VLAN ID. Internally, the switch code does not allow packets that contain one VLAN ID tag to be sent to hosts configured with a different VLAN ID tag.

Let’s look at a few practical examples of how this works. When you configure a host port, you would configure the switch with the appropriate VLAN ID tag that the host should be assigned to. The actual server may know nothing about the VLAN it is in, but the switch knows to inject that VLAN tag into the Ethernet headers of every packet received from that server. For the rest of that packet’s life on the network, every switch reads that VLAN tag to assist in forwarding decisions. Trunk ports are commonly used between switches, and these ports are enabled to carry multiple VLAN ID tags. The problem here is that each switch is expecting that the packets it receives already contain a VLAN tag in the Ethernet header.

So whats the problem with the default native VLAN? This is a special type of VLAN in which the switch never attaches a VLAN tag to the headers. Whenever we connect two switches via a trunk port, we are likely configuring multiple VLANs that need to cross that link. However, the switches themselves still need to communicate directly with eachother (for protocol negotiations/spanning-tree), and each switch isn’t necessarily going to know what VLAN the other wants to use for this management traffic. This is where the concept of a native VLAN comes in. For every configured trunk link, each switch has a default native VLAN for which it expects to receive packets with no VLAN tag headers. Even though normal network traffic crossing a trunk link is going to require a VLAN tag in the headers, the switch-to-switch control-plane communication is sent with no header present.

This is where VLAN 1 becomes a problem because of the native VLANs are processed/interpreted by the switch. Whenever a switch receives a packet which contains the VLAN ID set to the native VLAN, it knows that packet doesn’t need to contain a VLAN header – so it removes the VLAN ID header.

How could this be exploited? Well for example, let’s say that we had a network where every developer PC was using a trunk port and permitted to use VLAN tags. This might be so they could run local VMs for testing that connect to both the DEV and QA networks. If we leave the default native VLAN as 1, then a malicious developer could exploit this to gain access to another segment. This is accomplished by using a software package to double-tag an Ethernet packet with two separate VLAN ID headers. The first VLAN tag header will be set to the native VLAN (VLAN 1), and the second header will be set to the target VLAN – let’s say we use VLAN 20 for the accounting network. When the switch receives the packet across the trunk link, it will read the Ethernet headers. When it processes the first VLAN tag and sees that it matches the default native VLAN, the switch strips this header – which then leaves the second VLAN tag header for VLAN 20. When the switch goes to forward this traffic to it’s destination, the remaining VLAN header will allow the packet to bypass any security measures and be directly forwarded on VLAN 20.

Well what if we don’t provide end users with a trunk port? Could they still execute this type of attack? Remember that the default switchport configuration allows for dynamic negotiation – where the connecting computer can tell the switch whether or not it needs an access port or a trunk port.

If VLAN 1 is well-known as the default, what should the native VLAN be set to?

Anything you like! The key thing is that it should be a VLAN which has no access to any network resources – so it should be a VLAN with no hosts and gateway. I usually don’t even create the VLAN on the switch itself, therefore immediately black-holing all traffic sent to that VLAN. 

How else can this be prevented?

Always statically set your end user ports to switchport mode access, and enable switchport nonegotiate. This will prevent the switch from allowing port negotiations, which prevents a user from tricking the switch into assigning a trunk port.

Newer Cisco switches also support a global configuration command: vlan dot1q tag native. This command will force the switch to require VLAN tags to be present on packets in the native VLAN. See more detail here. This will need to be configured on all switches in your network to be effective.


Do you change the native VLAN in your network? Or is it not a big security concern for you? Comment below!

About Matt

Cisco certified since 2007 with a wide variety of IT and networking experiences. Just looking to share a bit of my own knowledge and experiences – the type of things I wish I would have known when I started my career.

2 Comments

  1. “Remember that the default switchport configuration allows for dynamic negotiation – where the connecting computer can tell the switch whether or not it needs an access port or a trunk port.”

    What is the point of double-tagging when you could just access the vlan directly because you are already in this vlan?

    1. Hey there,
      The issue is that double-tagging would allow someone to access a VLAN they wouldn’t otherwise have access to. Even with dynamic port negotiation, an admin could configure ‘switchport trunk allowed vlan’ to restrict VLAN access. If the port negotiated as access, then this command would have no effect – but if it negotiated as a trunk, then it would apply it’s VLAN filtering.

      Let’s say we had a switch with VLANs 4, 7, and 10 – then we configure a port for dynamic negotiation, but only permit allowed VLANS 4 and 7. If we still use a well-known native vlan, then theoretically a user could double tag a packet as VLAN1 and VLAN 10 – which would allow them to hop onto VLAN 10.

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