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TOPIC: One VLANs spread over two sites

One VLANs spread over two sites 7 years 5 months ago #30879

  • SinghJ
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Hi

Is it possible to have one VLAN spread across two geographically seperate sites?

Can we have one IP subnet over two sites.

Thanks

J
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Re: One VLANs spread over two sites 7 years 5 months ago #30882

  • S0lo
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Yes, it's possible as long as the two sites are interconnected via switches, not routers. In other words the two sites should be connected like this:

[Site 1 Switch]
[Site 2 Switch]

Not like this:

[Site 1 Router]
[Site 2 Router]

And not like this:

[Site 1 Switch]
[Site 2 Router]

The link between the two switches is typically a trunk.

The thing is that a router is a split point between vlans/subnets. A Vlan cannot exist on one router interface and another router interface at the same time, unless those two interfaces share the same physical link.
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Ammar Muqaddas
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Re: One VLANs spread over two sites 7 years 5 months ago #30884

  • next_virus
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Could you explain brefly. Means we have to use layer 3 switch or what. What will be the connecting media. Can we use PIX / ASA to connect sites with VPN and then to switch & still we can have the same VLAN.

Regards
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Re: One VLANs spread over two sites 7 years 5 months ago #30885

  • S0lo
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Could you explain brefly. Means we have to use layer 3 switch or what. What will be the connecting media. Can we use PIX / ASA to connect sites with VPN and then to switch & still we can have the same VLAN.

Regards

You can use either a Layer 2 or Layer 3 switch. But Layer 3 switching is essentially routing. You can use a layer 3 switch but you have to configure it properly to do only layer 2 switching between the two sites.

Yes, it's good that you reminded me about VPN. You can have one VLAN extending through the two sites using a Site-to-Site VPN. This way you CAN have 2 routers and/or PIX/ASA in between. Or you can configure VPN on one router/PIX/ASA on one site and then connect the other site using a switch, in this case the VPN client will be the user PC it self.

A VPN is as you know a tunnel done over IP, so it doesn't matter how many routers or switches are in between as long as the two ends of VPN (client/server) are configured properly and the IP traffic is routable.
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