Transparent Firewall

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Aug 20th, 2007
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I have read the following definition a couple times:

Transparent mode, the FWSM acts like a "bump in the wire," or a "stealth firewall," and is not a router hop.

What I understand with the previous sentence is that: The FWSM connects the same network on its inside and outside interfaces, but each interface must be on a different VLAN.

However, are both vlans going to share same subnet???

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rochopra Mon, 08/20/2007 - 16:06
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Transparent firewall will bridge between vlan and not route, so if you are using different subnets you will need layer 3 routing device to route packets between subnets.


Jon Marshall Mon, 08/20/2007 - 22:57
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In answer to your question yes you have 2 vlans but only one IP subnet. This setup is also used on other devices such as load balancers.

The reason you need to do this is to avoid a spanning-tree loop. Assuming you are running PVST+ then having 2 vlans but only one subnet allows you to bridge the subnet with the FWSM.




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