Why Vlan 1 is so insecure ?

[Roger.Nature]

Hello guys,

I am trying to understand why the Vlan 1 is so insecure. Any help would be appreciate it!

Thanks in advance

[Reza Sharifi · Accepted solution]

Hi,

Have a look at this link:

https://networkengineering.stackexchange.com/questions/32737/why-should-the-native-vlan-never-be-used/32740

HTH

[Seb Rupik · Accepted solution]

Hi there,

Keep in mind two facts about default configuration on cisco switches. By default all switchports are in VLAN1 and on a trunk link VLAN1 is the native VLAN.

So this means that a device connected to a default switchport on one switch could communicate at Layer2 to a device somewhere else on your network which is also on VLAN1. Essentially you have a broadcast domain which you have not explicitly designed. From a security perspective we like networks which are explicitly defined, that way we know what traffic to expect….to a certain degree!

To mitigate this problem there are two fixes. The first is to place all unused switchports in a VLAN other than 1. Whatever this VLAN ID, it should not be trunked off the switch.

Combine this with defining the native VLAN ID on your trunk links to a number other than 1 (at both ends of the link). This ensures VLAN1 is actually tagged on the trunk link.

Both of these steps help to break up and give visibility to the VLAN1 broadcast domain.

…and remember VLAN1 is used by control traffic, so don’t try blocking, just don’t try mixing user traffic in with it.

Cheers,

Seb.

[Roger.Nature]

Thank you so much Seb for taking the time to provide that response. Very helpful !!

[luis_cordova · Accepted solution]

Hi @Roger.Nature ,

The Vlan1 itself is not insecure, even many still occupy it.

The insecure thing is that everyone knows that it is the Vlan by default.

And as already mentioned, you can be a victim of attacks by this vulnerability. Therefore it is recommended not to use it to switching/routing end devices.

It is also recommended to leave disabled ports (shutdown) that are not occupying.

Regards

[Roger.Nature]

Thank you so much!! I really appreciate it !!!