Private vlan’s

This blogpost is about private vlan’s. For  my CCIE study I looked into PVLAN’s. First some terminology. When working with PVLAN’s words like primary vlan, secondary vlan, promiscuous port, isolated port and community port have to sound familiar. If not check the following summary:

  • Primary VLAN: Original vlan, used to forward the actual traffic
  • Secondary VLAN: is configured with one of the following types Isolated or Community
  • Isolated port: A switch port configured as Isolated is only able to communicate with the promiscuous port. It is not able to reach any other destination!
  • Community port: A switchport configured as Community is able to communicate with a server in the same community vlan and the promiscuous port. A community port is not able to communicate with an isolated port.

Below a drawing which describes the above summary.


In the configuration below two laptops are configured in an isolated vlan. Therefor they won’t be able to communicate with each other, but they are able to communicate with the promiscuous port.



In the next configuration the two laptops are configured in a community vlan. because of that they are able to ping each other.



As you probably noticed, the only configuration change is that the private-vlan host-association changed.

Private vlan’s are very useful to large ISP, because with isolated ports it’s possible to use the same subnets. There is virtually no limit to the number of customers and only one firewall is needed for a lot of customers. The only limiting option is the throughput of the used firewall!


** to build this topology I used a Cisco 3560 switch and a Cisco ASA 5505 firewall


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