A private VLAN domain has only one primary VLAN. Each port in a private VLAN domain is a member of the primary VLAN; the primary VLAN is the entire private VLAN domain.

Secondary VLANs provide isolation between ports within the same private VLAN domain. The following two types are secondary VLANs within a primary VLAN:

• Isolated VLANs—Ports within an isolated VLAN cannot communicate directly with each other at the Layer 2 level.

• Community VLANs—Ports within a community VLAN can communicate with each other but cannot communicate with ports in other community VLANs or in any isolated VLANs at the Layer 2 level.

The three types of PVLAN ports are as follows:

• Promiscuous port—A promiscuous port belongs to the primary VLAN. The promiscuous port can communicate with all interfaces, including the community and isolated host ports, that belong to those secondary VLANs associated to the promiscuous port and associated with the primary VLAN. You can have several promiscuous ports in a primary VLAN. Each promiscuous port can have several secondary VLANs or no secondary VLANs that are associated to that port. You can associate a secondary VLAN to more than one promiscuous port, as long as the promiscuous port and secondary VLANs are within the same primary VLAN. You may want to do this for load-balancing or redundancy purposes. You can also have secondary VLANs that are not associated to any promiscuous port.

  A promiscuous port can be configured either as an access port or as a trunk port.

• Isolated port—An isolated port is a host port that belongs to an isolated secondary VLAN. This port has complete isolation from other ports within the same PVLAN domain, except that it can communicate with associated promiscuous ports. PVLANs block all traffic to isolated ports except traffic from promiscuous ports. Traffic received from an isolated port is forwarded only to promiscuous ports. You can have more than one isolated port in a specified isolated VLAN. Each port is completely isolated from all other ports in the isolated VLAN.

  An isolated port can be configured as either an access port or a trunk port.

• Community port—A community port is a host port that belongs to a community secondary VLAN. Community ports communicate with other ports in the same community VLAN and with associated promiscuous ports. These interfaces are isolated from all other interfaces in other communities and from all isolated ports within the PVLAN domain.

  A community port must be configured as an access port. A community VLAN must not be enabled on an isolated trunk.

Note	A trunk port on the Fabric Extender (FEX) can be either a FEX trunk port or a FEX isolated trunk port.

Note	Because trunks can support the VLANs that carry traffic between promiscuous, isolated, and community ports, the isolated and community port traffic might enter or leave the switch through a trunk interface.

Primary VLANs and the two types of secondary VLANs (isolated and community) have these characteristics:

• Primary VLAN— The primary VLAN carries traffic from the promiscuous ports to the host ports, both isolated and community, and to other promiscuous ports.

• Isolated VLAN —An isolated VLAN is a secondary VLAN that carries unidirectional traffic upstream from the hosts toward the promiscuous ports. You can configure only one isolated VLAN in a PVLAN domain. An isolated VLAN can have several isolated ports. The traffic from each isolated port also remains completely separate.

• Community VLAN—A community VLAN is a secondary VLAN that carries upstream traffic from the community ports to the promiscuous port and to other host ports in the same community. You can configure multiple community VLANs in a PVLAN domain. The ports within one community can communicate, but these ports cannot communicate with ports in any other community or isolated VLAN in the private VLAN.

The following figure shows the traffic flows within a PVLAN, along with the types of VLANs and types of ports.

Figure 2. Private VLAN Traffic Flows

Note	The PVLAN traffic flows are unidirectional from the host ports to the promiscuous ports. Traffic received on primary VLAN enforces no separation and forwarding is done as in a normal VLAN.

A promiscuous access port can serve only one primary VLAN and multiple secondary VLANs (community and isolated VLANs). A promiscuous trunk port can carry traffic for several primary VLANs. Multiple secondary VLANs under a given primary VLAN can be mapped to promiscuous trunk ports. With a promiscuous port, you can connect a wide range of devices as access points to a PVLAN. For example, you can use a promiscuous port to monitor or back up all the PVLAN servers from an administration workstation.

In a switched environment, you can assign an individual PVLAN and associated IP subnet to each individual or common group of end stations. The end stations need to communicate only with a default gateway to communicate outside the private VLAN.

A promiscuous trunk port can carry traffic for several primary VLANs. Multiple secondary VLANs under a given primary VLAN can be mapped to a promiscuous trunk port. Traffic on the promiscuous port is received and transmitted with a primary VLAN tag.

An isolated trunk port can carry traffic for multiple isolated PVLANs. Traffic for a community VLAN is not carried by isolated trunk ports. Traffic on isolated trunk ports is received and transmitted with an isolated VLAN tag. Isolated trunk ports are intended to be connected to host servers.

To support isolated PVLAN ports on a Cisco Nexus Fabric Extender, the Cisco Nexus device must prevent communication between the isolated ports on the FEX; all forwarding occurs through the switch.

Caution

You must disable all the FEX isolated trunk ports before configuring PVLANs on the FEX trunk ports. If the FEX isolated trunk ports and the FEX trunk ports are both enabled, unwanted network traffic might occur.

For unicast traffic, you can prevent such a communication without any side effects.

For multicast traffic, the FEX provides replication of the frames. To prevent communication between isolated PVLAN ports on the FEX, the switch prevents multicast frames from being sent back through the fabric ports. This restriction prevents communication between an isolated VLAN and a promiscuous port on the FEX. However, as host interfaces are not intended to be connected to another switch or router, you cannot enable a promiscuous port on a FEX.

Broadcast traffic from ports in a private VLAN flows in the following ways:

• The broadcast traffic flows from a promiscuous port to all ports in the primary VLAN (which includes all the ports in the community and isolated VLANs). This broadcast traffic is distributed to all ports within the primary VLAN, including those ports that are not configured with private VLAN parameters.

• The broadcast traffic from an isolated port is distributed only to those promiscuous ports in the primary VLAN that are associated to that isolated port.

• The broadcast traffic from community ports is distributed to all ports within the port’s community and to all promiscuous ports that are associated to the community port. The broadcast packets are not distributed to any other communities within the primary VLAN or to any isolated ports.

You can use PVLANs to control access to end stations as follows:

• Configure selected interfaces connected to end stations as isolated ports to prevent any communication. For example, if the end stations are servers, this configuration prevents communication between the servers.

• Configure interfaces connected to default gateways and selected end stations (for example, backup servers) as promiscuous ports to allow all end stations access to a default gateway.