1) Different routing protocols to the same destination use the concept of "administrative distance" to determine who has priority.

The smaller the administrative distance, the higher the confidence...

On a cisco router

The default ospf is 110 and the default RIP is 120

That is, if there are both ospf and RIP routes to the same destination, then ospf takes precedence...

You can also modify the administrative distance via ip route [distance]

One of the important applications is the "floating static route"

Originally, the default static route has an administrative distance of 0 or 1 (set the outgoing interface to 0, set the next-hop ip to 1), which is preferred. ..... If you want to use this static route as a backup route... If you want to use this static route as a backup route, then change its administrative distance to be higher than that of your current route by using the above statement. Then it achieves the effect of a backup...

2) A router belongs to different routing protocols at the same time if you want to interoperate,

The methods are:

a. Redistribute

For example, if you belong to both rip and ospf and you want to redistribute the route from rip to ospf, you have to use the config)#router to redistribute the route from ospf to ospf. ospf [process number]

config-router)#redistribute [rip] metric [metric after redistribution]

b. It can also be used as a default route to point to other routing protocols

config)#router ospf [process number]

config)#router ospf [process number]

config-router)#default-information originate [always]

To always advertise a 0.0.0.0 to ospf to tell it to give the route to that router

-. -0