My company presently has a T-1 connected to the Internet via an ISP.
The ISP provides mail exchange and DNS, the LAN has email server, FTP,
web, etc. Normal stuff. What we'd like to do is have a backup
connection to the Internet using ADSL from a different ISP. When the
T-1 goes down, or there is connection problems at the T-1 ISP, I'd like
to be able to connect the LAN to the ADSL line and have all services
available as usual albeit perhaps at a bit slower speed. The best
scenario would be one where there would no changes necessary at the
desktops, little or none on the LAN servers (email server, ftp, web).
I'm thinking I could use a linux box with 2 NICs as the emergency
router. One NIC is connected to the ADSL and one NIC is connected to
the LAN. The ADSL NIC is given an IP address via DHCP by the ISP,
although if necessary we could request a static IP. The LAN NIC I
give the same IP number as used by the Cisco router on the T-1. Of
course, the LAN-side NIC on the Linux box would be left disconnected
until it was needed. I would disconnect the Cisco router from the LAN
and plug in the Linux box when something went wrong with the T-1.
I'm trying to think through all the various gotchas, like would mail
exchange still work, DNS, etc. Do you see anything wrong with my