Ok this one is straight forward. But you must use a Layer 2 switch for HSRP messages to work (sending a ARP request to the switch when change over occurs to update the switch’s mac address table).
The config is really simple just add the commands in the interface configuration mode on the interface that router will have internally. Please note the router itself must have a default route in order to route traffic.
Routing rable of IOU-2:
Here is the log messages on IOU-1 after it’s interfaces is shutdown:
Here are the log messages on IOU-2 after it takes over the router role:
Here is the uninterrupted pings demonstrating uninterrupted network access:
Download pre-configured lab here.