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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-21521-01
Chapter 18 Configuring Private VLANs Configuring Private VLANs
With VTP version 1 or 2, after you have configured private VLANs, use the copy running-config
startup config privileged EXEC command to save the VTP transparent mode configuration and
private-VLAN configuration in the switch startup configuration file. Otherwise, if the switch resets,
it defaults to VTP server mode, which does not support private VLANs. VTP version 3 does support
private VLANs.
VTP version 1 and 2 do not propagate private-VLAN configuration. You must configure private
VLANs on each device where you want private-VLAN ports unless the devices are running VTP
version 3.
You cannot configure VLAN 1 or VLANs 1002 to 1005 as primary or secondary VLANs. Extended
VLANs (VLAN IDs 1006 to 4094) can belong to private VLANs
A primary VLAN can have one isolated VLAN and multiple community VLANs associated with it.
An isolated or community VLAN can have only one primary VLAN associated with it.
Although a private VLAN contains more than one VLAN, only one Spanning Tree Protocol (STP)
instance runs for the entire private VLAN. When a secondary VLAN is associated with the primary
VLAN, the STP parameters of the primary VLAN are propagated to the secondary VLAN.
You can enable DHCP snooping on private VLANs. When you enable DHCP snooping on the
primary VLAN, it is propagated to the secondary VLANs. If you configure DHCP on a secondary
VLAN, the configuration does not take effect if the primary VLAN is already configured.
When you enable IP source guard on private-VLAN ports, you must enable DHCP snooping on the
primary VLAN.
We recommend that you prune the private VLANs from the trunks on devices that carry no traffic
in the private VLANs.
You can apply different quality of service (QoS) configurations to primary, isolated, and community
VLANs.
Sticky ARP
Sticky ARP entries are those learned on SVIs and Layer 3 interfaces. They entries do not age
out.
The ip sticky-arp global configuration command is supported only on SVIs belonging to
private VLANs.
The ip sticky-arp interface configuration command is only supported on
Layer 3 interfaces
SVIs belonging to normal VLANs
SVIs belonging to private VLANs
For more information about using the ip sticky-arp global configuration and the ip sticky-arp
interface configuration commands, see the command reference for this release.
You can configure VLAN maps on primary and secondary VLANs (see the “Configuring VLAN
Maps” section on page 37-31). However, we recommend that you configure the same VLAN maps
on private-VLAN primary and secondary VLANs.
When a frame is Layer-2 forwarded within a private VLAN, the same VLAN map is applied at the
ingress side and at the egress side. When a frame is routed from inside a private VLAN to an external
port, the private-VLAN map is applied at the ingress side.
For frames going upstream from a host port to a promiscuous port, the VLAN map configured
on the secondary VLAN is applied.
For frames going downstream from a promiscuous port to a host port, the VLAN map
configured on the primary VLAN is applied.