Cisco ASA VLANs and Sub-Interfaces

Each interface on a Cisco ASA firewall is a security zone so normally this means that the number of security zones is limited to the number of physical interfaces that we have. For example, the ASA 5510 has 4 physical interfaces and often you will only see the following three security zones:

  • Inside
  • Outside
  • DMZ

For a simple scenario this is more than enough but sometimes it’s useful to create additional security zones. For example, maybe you don’t want one “big” DMZ with all your servers but more separation. You could create a security zone with all your mail servers, another one with all the DNS servers and one more with all web servers. This is a good security practice but we’ll need more interfaces to accomplish this.

Luckily the ASA supports trunking and logical interfaces which means we can create multiple logical sub-interfaces on a single physical interface. Each sub-interface can be assigned to a different security zone and they are separated by VLANs.

This means you can create way more than 4 security zones, depending on your ASA model you can create up to 1024 VLANs.

The physical interface on the ASA will become a trunk interface which is not assigned to any security zone. Each sub-interface will be configured for a VLAN, security zone and security level.

Here’s a picture to visualize this:

ASA Trunk Physical Sub Interfaces

In the example above we have a Ethernet 0/0 physical interface and two sub-interfaces:

  • Ethernet 0/0.10 will be used for security zone “INSIDE1” and uses VLAN 10.
  • Ethernet 0/0.20 will be used for security zone “INSIDE2” and uses VLAN 20.
  • The physical interface is not configured for any security zone.

Basically this is the same thing as the router on a stick configuration on Cisco IOS routers but on the ASA we also have security zones.


Let’s take a look at a configuration example for this. I’ll use the following topology:

Cisco ASA Trunk Switch Routers

On the left side we have our ASA, it’s Ethernet 0/0 interface will be used for trunking. The switch in the middle is connected to two routers, R1 and R2. Each router represents a host in a different security zone:

  • INSIDE1 which uses VLAN 10 and has a security level of 70.
  • INSIDE2 which uses VLAN 20 and has a security level of 80.

Let’s start with the ASA configuration…

ASA Configuration

ASA1(config)# interface Ethernet 0/0
ASA1(config-if)# no nameif
ASA1(config-if)# no security-level 
ASA1(config-if)# no ip address 
ASA1(config-if)# no shutdown

The configuration above is the default configuration for an interface on the ASA, there should be no security zone, no security-level and no IP address. Make sure the interface is not in shutdown and we can continue with the sub-interfaces:

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Forum Replies

  1. Question:

    Why I can not create sub-interfaces if I have the security plus license?
    Maximum Physical Interfaces : 8 perpetual
    VLANs : 20 DMZ Unrestricted
    Dual ISPs : Enabled perpetual
    VLAN Trunk Ports : 8 perpetual
    Inside Hosts : Unlimited perpetual
    Failover : Active/Standby perpetual
    VPN-DES : Enabled perpetual
    VPN-3DES-AES : Enabled

    ... Continue reading in our forum

  2. Hi Aaron,

    It really depends what you are trying to achieve. If you use your switches for Inter-VLAN routing then there’s no need to create a trunk to your ASAs, a static route will do the job.

    If you want the ASAs for inter-VLAN routing then yes, you will need a trunk…your hosts will then use the ASA as their default gateway and you’ll have a bunch of sub (inside) interfaces.


  3. Hi Mark,

    By default, all traffic from a higher security level (OUTSIDE) to a lower security level (INSIDE) will be dropped. The only thing you have to do is to create an access-list and permit the traffic you want. Take a look at this example:

    Look for the “Permit Traffic to DMZ” section. Instead of the DMZ, it will be INSIDE for you.

    The ASA will use regular routing to select the egress interface. Let’s say you have an INSIDE gi0/1.10 sub-interface with IP address on it. If you

    ... Continue reading in our forum

  4. Hi there again

    I have my inside VLANS working a treat on sub interfaces - thanks for all the advice.

    What I cannot get working are multiple sub-interfaces on the outside interface. This is my (basic) config that works:

    interface GigabitEthernet1/1
    description WAN
    nameif OUTSIDE
    security-level 0
    ip address
    no shut
    interface GigabitEthernet1/2
    no ip address
    no nameif
    no security-level
    no shut
    interface GigabitEthernet1/2.60
    description LAN60
    vlan 60
    nameif INSIDE60
    security-level 100
    ip address 255.255.255

    ... Continue reading in our forum

  5. Hi Rene,

    I’m a newbie in need of config help involving a catalyst 4500x and ASA5512X.
    I will be managing 4500x that is connected to my DataCenter provider’s ASA 5512x.

    Here are the details:
    All internal routing is done on the 4500x.
    Inter VLAN on the switch (VLAN 500, VLAN 69, VLAN xxx, VLAN xxy)

    VLAN 500 - 4500x TE1/1/17 <==> ASA Ge0/1 (Primary) and 4500x TE2/1/17 <==> ASA Ge0/1 (Secondary).
    -- / 29

    4500x config:

    interface TenGigabitEthernet1/1/17
     switchport mode trunk
     switchport trunk allowed vlan 500
    interface TenGigabitEthernet2/1/17
    ... Continue reading in our forum

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