Private VLAN basic question

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Aug 25th, 2008
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Hi friends,

I have a basic question on Communication between two different community vlans.

If i have a switch called S with two community vlans viz. A and B and two promiscious ports connecting to a non-Cisco switch (that does not understand private vlans). Can this non-Cisco switch achieve communication between two distint community vlans? Or do i need only a Layer 3 device to achieve inter-community vlan communication?

Thanks a lot


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misramanish Mon, 08/25/2008 - 11:05
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Hi Gautam,

While all the switches (Cisco or not) can pass traffic to each other for their respective VLAN's, to my knowledge you will require a Layer 3 device to pass data between VLANs, whether Cisco or otherwise.

Hope this helps!

Giuseppe Larosa Mon, 08/25/2008 - 11:16
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Hello Gautam,

if the non Cisco switch is a L2 only switch being the community vlans different vlan numbers it will not create a backdoor between them: they are separate broadcast domains.

If it has L3 capabilities it could provide a way to an attacker to get access from one community vlan to the other.

However, a L2 switch has at least a management IP avoid to have it in the private vlans' IP subnet.

If it allows inter-vlan bridging like it is possible on a C3550 or other switch it can be configured to make a bridge so defeating the private Vlans deployment.

If someone connects with a crossover cable one port in vlan x and one port in vlan y on the L2 standard switch the same result is achieved of defeating the private vlans.

Being vlan x and y the two community vlans.

So disable all the unused ports and put them on a non routed vlan.

Private Vlans try to segment a single IP subnet in multiple broadcast domains under the control of the switch.

A l3 device cannot have overlapping ip addresses on different L3 interfaces.

Hope to help


sushant.u Mon, 09/06/2010 - 02:52
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Hi Giuseppe,

I have one mroe question, if we configure separte private vlan as community VLAN for example 346 and 355 and connect two differnt devices to a primary vlan 309 which is Layer vlan, will vlan 346 and vlan 355 will be able to communicate to each other.

As i was goning throug the CISCO doc and came across the following line which says community vlan is doing isolation at layer 2 but allow communication at layer 2. This statement is very ambigious to me. . As without layer 2 information how there will be communication at layer 3.

Although private VLANs provide host isolation at Layer 2, hosts can communicate with each other at Layer 3.

Please suggest




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