The simplest way to achieve a failover is by using floating static routes. You are going to need a default route (
0.0.0.0/0) to your ISPs. Cisco has AD (Administrative Distance) that basically tells you which of identical routes is preferred. Static routes normally have an AD of
1 (lower is better, like golf), but you can give a static route a specific AD. When an interface goes down, the route is withdrawn from the routing table, and an identical route with a higher AD will then take over.
For example (where
220.127.116.11 is ISP 1 and
10.11.12.13 is ISP 2):
ip route 0.0.0.0 0.0.0.0 18.104.22.168
ip route 0.0.0.0 0.0.0.0 10.11.12.13 10
Both commands create an identical static, default route, but the first command leaves the AD as
1 to the primary ISP, while the second command sets the AD to
10 to the secondary ISP. If the interface to the primary ISP fails, then the default route automatically fails over to the secondary ISP.
This works if the interface on the router loses connection to the ISP (
Up/Down). It may be possible, depending on the physical connection to the ISP, that your interface still shows
Up/Up, but for some reason the ISP doesn't respond, and this this configuration will not work correctly. That is where IP SLA would work.