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Transparent Firewall on Trunk Links

Can we implement the ASA firewall in transparent mode in between the trunk links? Example I have a trunk link between L2 and L3 switch. Can I put the ASA firewall in between them? Is there any special configuration needed?


Re: Transparent Firewall on Trunk Links


Yes, you can insert the ASA between the switches and act as a Layer 2 firewall.

You'll need to issue the 'firewall transparent' global config command. Note this command will auto clear all existing config.

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Re: Transparent Firewall on Trunk Links

Hello avilt,

As John said you can place it in transparent mode.

As a quick reminder there is no need to use a Ether-Type ACL in order to allow BPDU packets as they are allowed by default (CDP and IPv6 packets are not).

So Spanning-tree will still be used between the trunk links through the ASA.

Any other question you have let us know!

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Julio Carvajal Segura

Julio Carvajal
Senior Network Security and Core Specialist
New Member

Transparent Firewall on Trunk Links

I have tried this in a lab environment and it is not at all working. The trunk link carries multiple vlan traffic and each packet is tagged with vlan number. How can the firewall in transparent mode inspect the vlan tagged traffic?

The same setup with normal lan link works fine.

VIP Green

Transparent Firewall on Trunk Links

to my understanding you will need to create multiple subinterfaces on each link on the transparent firewall, and associate each of those subinterfaces with the respective VLAN and (i believe) you need to associate a seperate BVI interface for each subinterface...though this I am a little unsure of.

Once this is done, traffic should pass as expected.

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