There are times when you need to go beyond just turning on a routing protocol in your network. You may need to use multiple protocols, control exactly which routes are advertised or redistributed, or which paths are chosen. Most networks use DHCP; your router may need to be a DHCP server, or relay DHCP broadcasts.
There are several reasons you may need to run multiple routing protocols in your network. Some include:
Migrating from one routing protocol to another, where both protocols will run in the network temporarily
Applications that run under certain routing protocols but not others
Areas of the network under different administrative control (“layer 8” issues)
A multi-vendor environment ...