Using the same IP Address in two different VLANs

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Nov 17th, 2007
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Haven't tried this, but is it possible in an STP? any link to documents that supports this?



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Jon Marshall Sat, 11/17/2007 - 05:44
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Hi Dandy

Do you mean using one IP subnet over 2 vlans. If so, yes it is entirely possible and in fact is the way you setup FWSM, CSM-S modules etc. in bridge mode.

You have to use 2 separate vlans with the same subnet otherwise STP sees the same IP address on two ports in the same vlan so for example with the CSM-S you would

1) Create 2 vlans eg. 10 + 11

2) Create a Layer 3 SVI on the 6500 switch for vlan 10 only.

3) Allocate your servers into vlan 11.

4) The servers and the L3 SVI use the same IP subnet.

5) The CSM-S then bridges between the vlans.

Hope i have understood your question


Danilo Dy Sat, 11/17/2007 - 15:45
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Hi Jon,

Thank you for your reply.

In my planned setup I make it simple by using a L2 switch, the L3 is taken care by two different device (a router for vlan10 and a firewall for vlan11) - a third vlan12 is where the router and the firewall see each other. I will be configuring two-way NAT in the router so that the two networks will see each other as a different network. Servers in vlan10 and vlan11 have the same IP and gateway. I don't have equipment to try the setup :)




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