We currently have a Cisco ASA on a range with only two usable IP addresses. The Cisco has and the BT NTE box (for the leased line) has (on /32)

Now we want to split this line across two firewalls, so the existing ASA and another ASA 5510, so both firewalls have access to the line . This will be done via a switch - i.e. NTE Box --> Switch --> Link to each firewall. The ISP has sent has two additional IPs but in a totally separate subnet for the new firewall.

Now the ISP have apparently routed the above range (/31) to which is the interface on the current firewall.

But on the new firewall, we assign one of the above IPs to it's WAN interface, what do we assign as the default gateway for this interface? The IP of the NTE box? How does this work?

  • 1
    This is very unclear - your 'firewalls' are Cisco ASAs which you want to configure as failover? Or are you putting two firewalls behind your ASA? – MikeyB Aug 6 '13 at 16:45
  • We have 2 Cisco's - we have a BT NTE Box with one cable connected to a switch, from that switch, a cable to each Cisco on it's own interface on each Cisco. – PnP Aug 6 '13 at 17:19
  • Please see edit – PnP Aug 6 '13 at 19:38

You can only put the address on one of your ASAs.

You'll need to set up failover using HSRP/VRRP so that only one of them picks up the IP address at a time - that ASA will become the gateway for your internal network (part of which is and

Sounds like what you want is similar to what we have set up at Discourse:

Assigned subnet: 
upstream gateway: 
HA IP:  

Any further subnet assignments are routed by the ISP to


The interface you have is a /30, period. You can't have an extra interface on that circuit. You CAN have (which they provided you) extra IPs that can be used across that circuit (not sub-IFs, but routable IPs).

If you want a simple setup with 2 different ASA's in place, your ISP will need to expand the actual circuit to a /29 or larger to allow for multiple interfaces on your side.

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