Hi,
I am afraid that you will have to convert an existing interface to be used as a trunk interface for this purpose.
The Cisco ASA doesnt really give you a smooth way to do this change. The most important thing related to the ASA configurations that you have to notice that as soon as you start changing a single physical interface to be used as Trunk, you will be removing almost all of its configurations. (Namely the "nameif")
What this will do is that all the configurations referring to the "nameif" on the original physical interface will be removed and need to be entered manually for things to work again. There doesnt seem to be any way around this fact.
To prepare for such a change I usually take the output of
show run | inc
So that I get a list of all configurations referring to the interface that already existed.
The configuration might be something like this Before and After the changes
Before
interface GigabitEthernet0/0
description LAN
nameif inside
security-level 100
ip add 10.10.10.1 255.255.255.0
After
interface GigabitEthernet0/0
description LAN and DMZ Trunk
no nameif
no security-level
no ip add
interface GigabitEthernet0/0.10
vlan 10
description LAN
nameif inside
security-level 100
ip add 10.10.10.1 255.255.255.0
interface GigabitEthernet0/0.20
vlan 20
description DMZ
nameif dmz
security-level 50
ip add 10.10.20.1 255.255.255.0
Hope this helps
- Jouni
