Connecting ASA 5510s to a DSL modem with a static IP range

Question

I have DSL service with AT&T and I have a Motorola 3360 modem.  We also have a /28 network of static IPs from AT&T.  When I login using PPPoE on the modem it gets x.x.x.190 as it's address.  Our range is 177-190.  I have two ASA 5510s in an active/passive failover configuration with the Ethernet port of the modem and one interface of each of the ASAs on a dumb layer 2 switch. 
I want to setup this DSL connection as a backup to our main Internet connection.  I cannot figure out what setting on the DSL modem to use to make this happen.  I know I cannot use PPPoE in a failover setting so I can't have the modem in bridged mode.  There is some mode where it passes the 190 address to the connected device and when I plug in a PC directly to the modem and set it for DHCP it does get 190 as it's address.  So do I configure the ASA interface as 190 with one of the other addresses as it's standby?  What do I set my route on the ASA to for use of this connection?  Can I then make use of these other static addresses when plugging other devices into the layer 2 switch?

Answer 1

Thanks for your prompt response.  From your information, your network near the firewalls looks like this:
Your cable modem connects to your provider without any intervention from your equipment, and you are free to assign IP addresses from your assigned block.  The cable ISP knows to route traffic to your block down to the layer 2 segment attached to the cable mode. 
As you described, the Motorola 3360 DSL modem is an odd fish.  I do not have personal experience with that device,  but from internet searches that appears to be a model AT&T bundles with small business DSL service.  The 3360 appears to have three modes:
--router mode where it uses a single public IP on the WAN side and issues IP addresses in the 192.168.1.x range on the LAN side.  The modem performs the PPPoE function in this mode.
--hybrid mode where it gets a single public IP on the WAN side and then passes that through to one device connected on the LAN side.  The modem performs the PPPoE function in this mode.
--bridge mode.  A device on the LAN side must perform the PPPoE function.
Various links I found indicate folks with static IP address assignments from their ISP (usually AT&T) have difficulty getting those static IP addresses to work with the Motorol 3360 except in bridge mode.
To your original question, I'm guessing you match the configuration you performed on the cable modem side and use two of your static IPs for the ASA's.  Howver, it's unclear if the additional IP addresses will work with 3360's odd behavior.  If you have internet-exposed hosts (as shown in my simple drawing), try assigning some of the DSL static IPs to those hosts and test communications both ways -- host-->internet, internet-->host.  If possible, test two hosts at the same time to verify the 3360 can handle multiple public IPs at the same time (one posting I found claimed it could only handle one public IP address at a time).