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Catalyst 3750 SwitchSoftware Configuration Guide
OL-8550-09
Chapter16 Configuring Private VLANs
Understanding Private VLANs
You should also see the “Secondary and Primary VLAN Configuration” section on page16-7 under the
“Private-VLAN Configuration Guidelines” section.
Private VLANs and Unicast, Broadcast, and Multicast Traffic
In regular VLANs, devices in the same VLAN can communicate with each other at the Layer 2 level, but
devices connected to interfaces in different VLANs must communicate at the Layer 3 level. In private
VLANs, the promiscuous ports are members of the primary VLAN, while the host ports belong to
secondary VLANs. Because the secondary VLAN is associated to the primary VLAN, members of the
these VLANs can communicate with each other at the Layer 2 level.
In a regular VLAN, broadcasts are forwarded to all ports in that VLAN. Private VLAN broadcast
forwarding depends on the port sending the broadcast:
•An isolated port sends a broadcast only to the promiscuous ports or trunk ports.
•A community port sends a broadcast to all promiscuous ports, trunk ports, and ports in the same
community VLAN.
•A promiscuous port sends a broadcast to all ports in the private VLAN (other promiscuous ports,
trunk ports, isolated ports, and community ports).
Multicast traffic is routed or bridged across private-VLAN boundaries and within a single community
VLAN. Multicast traffic is not forwarded between ports in the same isolated VLAN or between ports in
different secondary VLANs.
Private VLANs and SVIs
In a Layer 3 switch, a switch virtual interface (SVI) represents the Layer 3 interface of a VLAN. Layer3
devices communicate with a private VLAN only through the primary VLAN and not through secondary
VLANs. Configure Layer 3 VLAN interfaces only for primary VLANs. You cannot configure Layer3
VLAN interfaces for secondary VLANs. SVIs for secondary VLANs are inactive while the VLAN is
configured as a secondary VLAN.
•If you try to configure a VLAN with an active SVI as a secondary VLAN, the configuration is not
allowed until you disable the SVI.
•If you try to create an SVI on a VLAN that is configured as a secondary VLAN and the secondary
VLAN is already mapped at Layer 3, the SVI is not created, and an error is returned. If the SVI is
not mapped at Layer 3, the SVI is created, but it is automatically shut down.
When the primary VLAN is associated with and mapped to the secondary VLAN, any configuration on
the primary VLAN is propagated to the secondary VLAN SVIs. For example, if you assign an IP subnet
to the primary VLAN SVI, this subnet is the IP subnet address of the entire private VLAN.