Dynamic routing protocols are used by routers to facilitate the exchange of routing information between routers. The purpose of dynamic routing protocols includes: discovery of remote networks, maintaining up-to-date routing information, choosing the best path to destination networks, and ability to find a new best path if the current path is no longer available. While dynamic routing protocols require less administrative overhead than static routing, they do require dedicating part of a router’s resources for protocol operation, including CPU time and network link bandwidth.
Networks typically use a combination of both static and dynamic routing. Dynamic routing is the best choice for large networks and static routing is better for stub networks.
Routing protocols are responsible for discovering remote networks, as well as maintaining accurate network information. When there is a change in the topology, routing protocols propagate that information throughout the routing domain. The process of bringing all routing tables to a state of consistency, where all of the routers in the same routing domain or area have complete and accurate information about the network, is called convergence. Some routing protocols converge faster than others.
Routing protocols can be classified as either classful or classless, as distance vector or link-state, and as an Interior Gateway Protocol or an Exterior Gateway Protocol.
Distance vector protocols use routers as “sign posts” along the path to the final destination. The only information a router knows about a remote network is the distance or metric to reach that network and which path or interface to use to get there. Distance vector routing protocols do not have an actual map of the network topology.
A router configured with a link-state routing protocol can create a complete view or topology of the network by gathering information from all of the other routers.
Metrics are used by routing protocols to determine the best path or shortest path to reach a destination network. Different routing protocols may use different metrics. Typically, a lower metric means a better path. Metrics can be determined by hops, bandwidth, delay, reliability, and load.
Routers sometimes learn about multiple routes to the same network from both static routes and dynamic routing protocols. When a router learns about a destination network from more than one routing source, Cisco routers use the administrative distance value to determine which source to use. Each dynamic routing protocol has a unique administrative value, along with static routes and directly connected networks. The lower the administrative value, the more preferred the route source. A directly connected network is always the preferred source, followed by static routes and then various dynamic routing protocols.
The show ip protocols command displays the IPv4 routing protocol settings currently configured on the router. For IPv6, use show ipv6 protocols.
With link-state routing protocols such as OSPF, a link is an interface on a router. Information about the state of those links is known as link-states. All link-state routing protocols apply Dijkstra’s algorithm to calculate the best path route. The algorithm is commonly referred to as the shortest path first (SPF) algorithm. This algorithm uses accumulated costs along each path, from source to destination, to determine the total cost of a route.