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Understanding Private VLANs
Private VLANs across Multiple Switches
As with regular VLANs, private VLANs can span multiple switches. A trunk port carries the primary
VLAN and secondary VLANs to a neighboring switch. The trunk port treats the private VLAN as any
other VLAN. A feature of private VLANs across multiple switches is that traffic from an isolated port
in switch A does not reach an isolated port on Switch B. See
Figure 14-2
VLAN 100
VLAN 100 = Primary VLAN
VLAN 201 = Secondary isolated VLAN
VLAN 202 = Secondary community VLAN
Because VTP does not support private VLANs, you must manually configure private VLANs on all
switches in the Layer 2 network. If you do not configure the primary and secondary VLAN association
in some switches in the network, the Layer 2 databases in these switches are not merged. This can result
in unnecessary flooding of private-VLAN traffic on those switches.
Note
When configuring private VLANs on the switch, always use the default Switch Database Management
(SDM) template to balance system resources between unicast routes and Layer 2 entries. If another SDM
template is configured, use the sdm prefer default global configuration command to set the default
template. See
Private-VLAN Interaction with Other Features
Private VLANs have specific interaction with some other features, described in these sections:
•
•
You should also see the
"Private-VLAN Configuration Guidelines"
Cisco Catalyst Blade Switch 3020 for HP Software Configuration Guide
14-4
Private VLANs across Switches
Switch A
VLAN 201
VLAN 202
Chapter 6, "Configuring SDM Templates."
Private VLANs and Unicast, Broadcast, and Multicast Traffic, page 14-5
Private VLANs and SVIs, page 14-5
"Secondary and Primary VLAN Configuration" section on page 14-6
Figure
Trunk ports
VLAN 201
Carries VLAN 100,
201, and 202 traffic
section.
Chapter 14
Configuring Private VLANs
14-2.
VLAN 100
Switch B
VLAN 202
under the
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Troubleshooting
