When configuring a Cisco switch infrastructure, security is a top priority. One common recommendation for hardening a Cisco switch is to assign unused ports to a blackhole VLAN. But why should this be done? And how does it help improve security?

To understand the reasoning behind this recommendation, let's take a closer look at the different types of VLANs and their roles in a network. A native VLAN is typically used as the default VLAN for a switch, while other VLANs are used for specific purposes such as data, voice, or management.

When a port is not assigned to a VLAN, it can still receive traffic from the control plane, which allows it to be accessed remotely. This is why assigning unused ports to a blackhole VLAN is recommended - it prevents anyone who gains access to the switch from using it to get beyond the switch without going through an IVR (Intelligent Virtual Router) and vetting the traffic.

Another benefit of assigning unused ports to a blackhole VLAN is that it forces any untagged traffic to be tagged, which helps prevent VLAN hopping/double encapsulation attacks. By tagging all traffic, even if it's not allowed on the trunk, an attacker would need to go through the IVR and vet the traffic before accessing the network.

Additionally, assigning unused ports to a blackhole VLAN can help prevent someone who is placed in the default VLAN from gaining access to the management network or SVI (Switched Virtual Interface) without passing through an IVR. This provides an additional layer of security by ensuring that all traffic must be vetted before it reaches the management network.

In summary, assigning unused ports to a blackhole VLAN helps improve security by preventing unauthorized access to the switch, forcing untagged traffic to be tagged, and providing an additional layer of security for the management network. By following this best practice, you can help harden your Cisco switch infrastructure and reduce the risk of security breaches.

Assign ports to blackhole vlan