A private VLAN 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 on a private port out the uplink port, regardless of VLAN ID or destination MAC address. Frames received on an uplink port are forwarded in the normal way (i.e., to the port hosting the destination MAC address, or to all VLAN ports for unknown destinations or broadcast frames). "Peer-to-peer" traffic is blocked. Note that while private VLANs provide isolation at the data link layer, communication at higher layers may still be possible.
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.
Other articles related to "private vlan":
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