Hi Ruby,
If you design a network, you will always use one routing protocol (if possible). Here are some scenarios where you might have to use two routing protocols:
- You are migrating your company network with the network from another company. Maybe you are running OSPF and they are running EIGRP. As a temporary solution you could use redistribution and run both protocols.
- You run OSPF on your internal network and are installing a new site. Your SP offers you a MPLS VPN connection but only supports BGP as the routing protocol.
- You use EIGRP on your network but you want a backup static route. In this case you can play with the administrative distance so that the static route is not installed until the EIGRP route dissapears.