We are currently in the process of switching ISP's from one to another. I was hoping that I could, with the Cisco ASA 5510, run both ISP's in tandem without having to do a "hard cutover" of changing IP's of all public facing devices on a weekend.
Both of the ISP's terminate into the same router (one is NxT1 and the other is Ethernet). Initially, I created a secondary IP subnet (for the new block) on the same interface that the existing public subnet is on. Then I planned on setting up PBR on the external ISP terminating router to make sure that the traffic is routed correctly based on what was presented to it. I haven't got this to work.
In theory will this even work? I would like to look at changing the PAT address to the new ISP (it's faster) and then start migrating the other devices at my pace.
If anyone has any input, please let me know.