Dynamic Routing is when protocols are used to find networks and update routing tables instead of manually entering the routing information. There are two major types of routing protocols, IGP Interior Gateway Protocols and EGP Exterior Gateway Protocols.
IGP is used to exchange routing information between routers on a same network or collection of inter connected networks known as Autonomous System (AS). Most of dynamic protocols in use nowadays are IGP. Interior gateway protocols (IGP) is categorized into two categories: distance-vector routing protocols and link-state routing protocols. Examples of IGP protocols include Open Shortest Path First (OSPF), Routing Information Protocol (RIP) and Intermediate System to Intermediate System (IS-IS).
EGP is a protocol used to exchanging routing information between routers in a network of autonomous systems. In the past, EGP is commonly used between hosts on the Internet to exchange routing table information. A good example of EGP protocol is BGP (Border Gateway Protocol) which is now obsolete.
The Administrative Distance is used to rate the trustworthiness of the routing information received on a router from a neighboring router. The best route is determined terms of lowest route value of the advertising router.
Below is a list of default Administrative Distance value for each IGP protocol.
0 – Directly connected router
1 – Manually entered route (static route)
90 – EIGRP ( Enhanced Interior Gateway Routing Protocol)
110 – OSPF (Open Shortest Path First)
120 – RIP
170 – External EIGRP
255 – Unknown