I'm in the middle of equipment migration and the setup is somewhat similar to Mike's (Configure Cisco ASA in Transparent mode: Layer2 DMZ w/ Vlan translation) but with a difference that I need the inside and outside VLANs to be different.
We have a X 'outside' VLANS (with IDs from 600-699) and X 'inside' VLANS (with IDs from 700 to 799). I was going with the same idea as in Mike's post: two subinterfaces (one outside, one inside) and one bridge group. Setup looked fine... on the paper.
First thing I found out is that you need to have a BVI interface for bridge group to work, so burning one (public in our case) IP for no apparent reason. Second thing, that some of you probably know but I didn't even think i should check, is that ASA allows maximum of 8 BVI interfaces (or 8 per context if in multimode).
I would mostly like to know the why ASA has such requirements.
- Why is there a need for BVI interface with a valid IP address if we want just to bridge two interfaces? Documentation vaguely states that BVI is used for management and packets sourced from ASA (ARP, etc.) but why is any of that necessary for a transparent L2 firewall? I understand the need when we're talking about IRB on routers but I really don't on a device that basically should just drop or pass frames.
- What is a reason for a limit of 8 bridge groups? Is there any technical reason or is this just so you would need to buy more context licenses if you want to do something like this.
- And of course do you have any different solutions for this problem?
I know that their idea is probably to have a different context for each VLAN (or a mix of contexts and bridge groups). My concern is management, for example can you share objects between contexts? We have a lot of network devices and servers on the 'outside' and making object for them for each context would not be manageable.