Dynamic Routing Protocol Overview

Routers add IP routes to their routing tables using three methods: connected routes, static routes, and routes learned by using dynamic routing protocols.

* Routing protocol: A set of messages, rules, and algorithms used by routers for the overall purpose of learning routes.  This process includes the exchange and analysis of routing information.  Each router chooses the best route to each subnet and finally places those best routes in its IP routing table.  Examples: RIP, EIGRP, OSPF, and BGP.

* Routed protocol and routable protocol: Both terms refer to a protocol that defines a packet structure and logical addressing, allowing routers to forward or route the packets.  Routers forward, or route, packets defined by routed and routable protocols.  Examples: IP and IPX.

Path selection sometimes refers to part of the job of a routing protocol, in which the routing protocol chooses the best route.

The routing process forwards IP packets, but if a router does not have any routes in its IP routing table that match a packet’s destination address, the router discards the packet.  Routers need routing protocols so that the routers can learn all the possible routes and add them to the routing table so that the routing process can forward routable protocols such as IP.

Routing Protocol Functions

Cisco IOS software supports several IP routing protocols, performing the same general functions:

1. Learn routing information about IP subnets from other neighboring routers.

2. Advertise routing information about IP subnets to other neighboring routers.

3. If more than one possible route exists to reach one subnet, pick the best route based on a metric.

4. If the network topology changes, react by advertising that some routes have failed, and pick a new currently best route (convergence.)

Convergence refers to a process that occurs when the topology changes.  Convergence refers to the process by which all the routers collectively realize something has changed, advertise the information about the changes to all the other routers, and all the routers then choose the currently best routes for each subnet.