Private VLAN

Private VLAN, also known as port isolation, is a technique in computer networking where a VLAN contains switch ports that are restricted such that they can only communicate with a given "uplink". The restricted ports are called "private ports". Each private VLAN typically contains many private ports, and a single uplink. The uplink will typically be a port (or link aggregation group) connected to a router, firewall, server, provider network, or similar central resource.

The switch forwards all frames received from a private port to the uplink port, regardless of VLAN ID or destination MAC address. Frames received from an uplink port are forwarded in the normal way (i.e. to the port hosting the destination MAC address, or to all ports of the VLAN for broadcast frames or for unknown destination MAC addresses). As a result, direct peer-to-peer traffic between peers through the switch is blocked, and any such communication must go through the uplink. While private VLANs provide isolation between peers at the data link layer, communication at higher layers may still be possible depending on further network configuration.

A typical application for a private VLAN is a hotel or Ethernet to the home network where each room or apartment has a port for Internet access. Similar port isolation is used in Ethernet-based ADSL DSLAMs. Allowing direct data link layer communication between customer nodes would expose the local network to various security attacks, such as ARP spoofing, as well as increasing the potential for damage due to misconfiguration.

Another application of private VLANs is to simplify IP address assignment. Ports can be isolated from each other at the data link layer (for security, performance, or other reasons), while belonging to the same IP subnet. In such a case direct communication between the IP hosts on the protected ports is only possible through the uplink connection by using MAC-Forced Forwarding or a similar Proxy ARP based solution.

Overview

Private VLAN Traffic Flow
Example of private VLAN port types on the switch

Private VLAN divides a VLAN (Primary) into sub-VLANs (Secondary) while keeping existing IP subnet and layer 3 configuration. A regular VLAN is a single broadcast domain, while private VLAN partitions one broadcast domain into multiple smaller broadcast subdomains.

There are mainly two types of ports in a Private VLAN: Promiscuous port (P-Port) and Host port. Host port further divides in two types  Isolated port (I-Port) and Community port (C-port).

Example scenario: a switch with VLAN 100, converted into a Private VLAN with one P-Port, two I-Ports in Isolated VLAN 101 (Secondary) and two community VLANs 102 and 103 (Secondary), with 2 ports in each. The switch has one uplink port (trunk), connected to another switch. The diagram shows this configuration graphically.

The following table shows the traffic which can flow between all these ports.

I-Port P-Port C1-Port C2-Port Uplink to Switch2
I-Port Deny Permit Deny Deny Permit
P-Port Permit Permit Permit Permit Permit
C1-Port Deny Permit Permit Deny Permit
C2-Port Deny Permit Deny Permit Permit
Uplink to Switch2 Permit/Deny Permit Permit Permit Permit

Traffic from an Uplink port to an Isolated port will be denied if it is in the Isolated VLAN. Traffic from an Uplink port to an isolated port will be permitted if it is in the primary VLAN.

Use cases

Network segregation

Private VLANs are used for network segregation when:

Secure hosting

Private VLANs in hosting operation allows segregation between customers with the following benefits:

Secure VDI

Isolated VLAN can be used to segregate VDI desktop one from the other, allowing filtering and inspection of desktop to desktop communication. Using a firewall will require a VLAN and a subnet per VDI desktop.

Backup network

On a backup network, there is no need for hosts to reach each other. Hosts should only reach their backup destination. Backup clients can be placed in one Isolated VLAN and the backup servers can be placed as promiscuous on the Primary VLAN, this will allow hosts to communicate only with the backup servers. [4]

Vendor support

Hardware switches

Software switches

Other private VLANaware products

See also

Related RFCs

References

Notes