routing "wireless-guest" traffic through a sub-interface or through the inside interface?

Posted on 2013-02-02
Last Modified: 2013-02-07
Sorry for the long post...

I'm using a Cisco ASA 5510.  So far I'm not using any sub interfaces.  But now all of my physical interfaces are used.

I have a guest-wireless network that I use just for guest laptops.  In the past I've used one of the physical interfaces on the ASA as the gateway for this network, with a route defined to the outside interface.  But now I need to use this physical interface for something else so I need to change the way the ASA handles the guest traffic.

I was thinking about either:

1. defining a sub-interface on the inside interface, setting it to a lower security level, and configuring this sub-interface as the gateway for my guest network.  Since the sub-interface will be a lower security level than the inside interface, traffic from the sub-interface won't be allowed into the inside interface.


2. NOT use a sub-interface, but create access lists on my internal routers so that guest traffic will only be allowed out to the internet.  Then all guest traffic will pass through the inside interface of the ASA, along with all the other internal traffic.

Which one is the best or most secure way to do it?  If I use the first option I don't think I would need to define access lists on my internal routers because the gateway for the guest network would be the sub-interface on the ASA.

Thanks so much for any input!
Question by:luchianoduckman
  • 2
LVL 16

Accepted Solution

max_the_king earned 500 total points
ID: 38846600
i'm not sure about the second point, how can you write an access-list of a subnet you haven't declared on the ASA ...
but first option should work fine.
Please keep in mind that if you configure subinterfaces (VLANs) on a physical interface, then this physical interface must be connected to a Trunk Port on a Layer 2 switch. In addition, if you enable subinterfaces, you typically do not want the main physical interface to also be passing traffic. You can achieve this by omitting the nameif command (no nameif) on the physical interface.
So in your case, you should disable one of your physical interfaces (not necessarily the inside) and create two subinterfaces under that interface:
for example:

ciscoasa(config)# interface gigabitethernet 0/1
ciscoasa(config-if)# no nameif
ciscoasa(config-if)# no security-level
ciscoasa(config-if)# no ip address
ciscoasa(config-if)# exit

ciscoasa(config)# interface gigabitethernet 0/1.1
ciscoasa(config-subif)# vlan 10
ciscoasa(config-subif)# nameif inside1
ciscoasa(config-subif)# security-level 80
ciscoasa(config-subif)# ip address
ciscoasa(config)# interface gigabitethernet 0/1.2
ciscoasa(config-subif)# vlan 20
ciscoasa(config-subif)# nameif inside2
ciscoasa(config-subif)# security-level 90
ciscoasa(config-subif)# ip address

hope this helps

Author Comment

ID: 38848140
Thanks for the input.
What's the problem with having an operational physical interface along with a sub interface, like Ethernet0/1 and Ethernet0/1.1?
I've tried it both ways (1 physical plus 1 sub, and the 2 sub interface method as you described)  and they both worked.
So what's the advantage of one over the other?

Thanks again!
LVL 16

Expert Comment

ID: 38848235
Well, i know it works in both ways, but you may want to eliminate the chance that an unexpected VLAN appears on the trunk.
At times of old PIX at version 6.X you hadn't interfaces on it, you needed to force the firewall to tag packets on the physical trunk interface, like this:
Pix (config)# interface ethernet0 vlan30 physical
the "physical" keyword makes the logical VLAN interface overlay with physical interface so that any packets passing over the interface receive a VLAN ID tag. After a VLAN has been assigned to a physical interface, the firewall drops any untagged packets that are received over the trunk interface's native VLAN.
Well, the good news is that from PIX/ASA 7.X release this step is unnecessary because you can configure the interface with the "no nameif" command, and this forces all traffic to pass through one or more subinterfaces that are configured with a VLAN number, that requires a VLAN tag.
I don't know if you really wanted to get so deep ... or you could just take my advice for granted :)

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