Hi Vipin,
It is not possible to configure two ip addresses on the same interface of the ASA.
If you have a free interface on the ASA, then you could configure that for the second ISP. Once you have configured all NAT rules, default route, etc, to match the ip scheme of the new ISP, all you need to do is go into the interface of the current ISP, and do "no nameif". This will remove all configuration (NAT, routes, crypto maps, etc) that was relevant to that interface, and what is remaining would be that for the new ISP. You might still have to do a "clear xlate" and "clear conn" to clear existing connections, as I am not sure if they get removed when you remove name of primary ISPs interface.(They most probably should clear out on their own.)
However, do note, that you would lose the part of the config related to the current ISP when you do this. Take a backup, in case you require it. Also, if you reload, without saving running-config, you can get the old config back, if for some reason that is required.
I would recommend having a small downtime window in any case. If all goes well, and it isn't required, then you get bonus points from your client.
Hope this helps.
-Shrikant
P.S.: Please mark the question as answered, if it has been resolved. Do rate helpful posts. Thanks.
