What are Private VLANs: Configuration on Cisco Switches

As a network engineer, you have a powerful and valuable tool at your disposal - Private VLANs (PVLANs) in Cisco switches. With PVLANs, you can enhance the security and efficiency of your network by dividing traditional VLANs into primary, secondary, and isolated ports.

When implementing PVLANs, you gain greater control over how devices communicate within VLANs. You can designate specific ports as primary, secondary, or isolated, allowing for a more granular and secure network environment.

In this article, we will learn about Private VLANs and understand how to configure Private VLAN in Cisco switches.

To further your expertise in configuring and managing PVLANs, obtaining Cisco certifications such as the Cisco Certified Network Associate (CCNA) or Cisco Certified Network Professional (CCNP) can be invaluable. These certifications provide in-depth knowledge of networking concepts, including VLAN configurations and security best practices.

What is Private VLAN?

Private VLAN (PVLAN) is a specialized type of Virtual Local Area Network (VLAN) that enhances network security and traffic management by allowing multiple isolated subdomains within a single VLAN. Unlike traditional VLANs, where all devices can communicate with each other, PVLANs restrict communication between devices connected to the same physical link.

One of the main and important benefits of PVLANs is that the devices within the same VLANs can be isolated preventing them not to communicate with each other. This isolation boosts network security, safeguarding sensitive or critical devices from potential threats.

Moreover, PVLANs empower you to specify which ports devices can use to communicate. By designating certain ports as the only means of communication between devices, you optimize network resources and reduce unnecessary traffic.

Especially in large-scale Ethernet environments with multiple interconnected switches, PVLANs become an essential tool for maintaining a secure and efficient network infrastructure.

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To understand the concept of private VLANs better, let's take a scenario of three Cisco Switches connected via a link and here we'll understand how to implement private VLANs which allow isolations within the same VLANs and other communications between primary and secondary VLANs.

Scenario Overview:

Let's consider a scenario in which a company has three Cisco switches connected. The goal is to configure Private VLANs to enhance security and restrict communication between certain devices within the VLANs.

Topology Diagram:

VLAN Configuration:

First, we need to configure the VLANs on each Cisco switch. Assume we have the following VLANs:

● VLAN 10: Servers VLAN (Primary VLAN)

● VLAN 20: IoT Devices VLAN (Secondary VLAN)

● VLAN 30: Guest Devices VLAN (Secondary VLAN)

PVLAN Communication:

In this scenario, the Private VLANs are configured as follows:

Communication between isolated ports (e.g., GigabitEthernet1/0/4 on Switch A, GigabitEthernet1/0/4 on Switch B, and GigabitEthernet1/0/4 on Switch C) is blocked. This isolation enhances security and prevents direct communication between end-user devices.

Communication between isolated ports and the promiscuous port (GigabitEthernet1/0/3 on all switches) is allowed. This enables connectivity to the resources in the primary VLAN (VLAN 10), such as servers and gateways.

Communication between devices in the secondary VLANs (VLAN 20 and VLAN 30) is blocked. Secondary VLANs operate in isolation from each other.

Communication between devices within the same secondary VLAN (e.g., between two IoT devices in VLAN 20 on Switch A) is allowed, as long as they are connected to the promiscuous port (GigabitEthernet1/0/3).

How to Configure Private VLAN:

Next, we'll set up the Private VLANs on each switch to enhance security and restrict communication.

Switch A Configuration:

Switch B Configuration:

Switch C Configuration:

PVLAN Configuration on Switch A:

Now, we'll configure the Private VLANs on Switch A to enhance security.

PVLAN Configuration on Switch B:

PVLAN Configuration on Switch C:

Private VLAN and Configuration - Conclusion

Private VLANs (PVLANs) offer network engineers a powerful tool to enhance security and optimize network resources in large-scale Ethernet environments. By segmenting traditional VLANs into primary, secondary, and isolated ports, PVLANs provide granular control over communication within VLANs.

In the example scenario with three interconnected Cisco switches, we configured PVLANs to enhance security, restricting communication between certain devices and allowing communication only through designated ports. By implementing PVLANs, organizations can achieve improved network security and better utilize network resources in multi-switch environments.