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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
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Chapter 18 Configuring Private VLANs Understanding Private VLANs
Primary and secondary VLANs have these characteristics:
• Primary VLAN—A private VLAN has only one primary VLAN. Every port in a private VLAN is a
member of the primary VLAN. The primary VLAN carries unidirectional traffic downstream from
the promiscuous ports to the (isolated and community) host ports and to other promiscuous ports.
• Isolated VLAN —A private VLAN has only one isolated VLAN. An isolated VLAN is a secondary
VLAN that carries unidirectional traffic upstream from the hosts toward the promiscuous ports and
the gateway.
• Community VLAN—A community VLAN is a secondary VLAN that carries upstream traffic from
the community ports to the promiscuous port gateways and to other host ports in the same
community. You can configure multiple community VLANs in a private VLAN.
A promiscuous port can serve only one primary VLAN, one isolated VLAN, and multiple community
VLANs. Layer 3 gateways are typically connected to the switch through a promiscuous port. With a
promiscuous port, you can connect a wide range of devices as access points to a private VLAN. For
example, you can use a promiscuous port to monitor or back up all the private-VLAN servers from an
administration workstation.
In a switched environment, you can assign an individual private VLAN 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.
You can use private VLANs to control access to end stations in these ways:
• Configure selected interfaces connected to end stations as isolated ports to prevent any
communication at Layer 2. For example, if the end stations are servers, this configuration prevents
Layer 2 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.
You can extend private VLANs across multiple devices by trunking the primary, isolated, and
community VLANs to other devices that support private VLANs. To maintain the security of your
private-VLAN configuration and to avoid other use of the VLANs configured as private VLANs,
configure private VLANs on all intermediate devices, including devices that have no private-VLAN
ports.
IP Addressing Scheme with Private VLANs
Assigning a separate VLAN to each customer creates an inefficient IP addressing scheme:
• Assigning a block of addresses to a customer VLAN can result in unused IP addresses.
• If the number of devices in the VLAN increases, the number of assigned address might not be large
enough to accommodate them.
These problems are reduced by using private VLANs, where all members in the private VLAN share a
common address space, which is allocated to the primary VLAN. Hosts are connected to secondary
VLANs, and the DHCP server assigns them IP addresses from the block of addresses allocated to the
primary VLAN. Subsequent IP addresses can be assigned to customer devices in different secondary
VLANs, but in the same primary VLAN. When new devices are added, the DHCP server assigns them
the next available address from a large pool of subnet addresses.