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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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Chapter12 Configuring Private VLANs Configuring Private VLANs

Private VLANs and Unicast, Broadcast, and Multicast Traffic

In regular VLANs, devices in the same VLAN can communicate with each other at the Laye r 2 le vel, b ut
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 i n tha t V LAN . Private-VLAN b roadc as t
forwarding depends on the port sending the broadcast:
•An isolated port sends a broadcast only to the promiscuous po rts o r trunk p orts.
•A community port sends a broadcast to all promiscuous ports, trun k por ts, and po rts in t he s am e
community VLAN.
•A promiscuous port (only NNI) 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 running the metro IP access image), a switch virtual interface (SVI)
represents the Layer 3 interface of a VLAN. Layer 3 devices communicate with a private VLAN only
through the primary VLAN and not through secondary VLA Ns . Con figure La yer 3 V L AN int erfaces
(SVIs) only for primary VLANs. You cannot configure Layer 3 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 VLA N, the co nfigurat ion is no t
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 re turn ed. 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.
Configuring Private VLANs
These sections contain this configuration information:
•Tasks for Configuring Private VLANs, page 12-6
•Default Private-VLAN Configuration, page 12-6
•Private-VLAN Configuration Guidelines, page 12-6
•Configuring and Associating VLANs in a Private VLAN, page 12-10
•Configuring a Layer 2 Interface as a Private-VLAN Host Port, page 12-12