Routing Protocol

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There are three classes of routing protocols:

• Distance Vector -- Distance is the cost of reaching a destination, usually based on the number of hosts the path passes through (hops). Distance vector protocols use a distance calculation plus an outgoing network interface (a vector) to choose the best path to a destination network. All the routers in the network send out periodic updates even if there are no changes in the network, every update contains the entire routing table. Upon receiving the routing table of the neighbor, a router can verify all the known routes and make changes to its local routing table based on updated information received from the neighbor router. This process is also known as “routing by rumor.” Examples of this protocol include Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP).
• Link State -- In link-state routing protocols, every router tries to build its own internal map of the network topology. Each router sends messages into the network when it goes online, listing the routers to which it is directly connected and providing information about whether the link to each router is active. The other routers use this information to build a map of the network topology and then use the map to choose the best path to the destination network. Link State protocols track the status and connection type of each link and produces a calculated metric based on these factors. Link state protocols know whether a link is up or down and how fast it is and calculates a cost to 'get there'. Link State protocols will take a path which has more hops, but that uses a faster medium over a path using a slower medium with fewer hops. Compared with Distance Vector routing protocols, Link-state routing protocols are quieter -- only send periodic updates (link-state refreshes) at long time intervals, approximately once every 30 minutes. When there's change in the network, the routers only send information about the change itself, instead of the whole routing table, therefore Link-state routing protocols respond quickly to network changes. Examples of Link state protocols are Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS).
• Hybrid -- combination of distance vector protocol and link state protocol, example is Enhanced Interior Gateway Routing Protocol (EIGRP).

Administrative Distance

The routing table holds only one entry per network. If there are more than one routing protocols provides route information to a particular destination network, the router needs to decide which source of information should be used in the routing table. Different routing protocols use different metrics to measure the quality of path to a destination network, in order to directly compare information provided by different routing protocols, the Cisco routing process assigns a weight, known as the administrative distance, to each source of information.

Administrative distance: Routers use the administrative distance to choose between routes learned through two (or more) routing protocols. Lower administrative distance path will be chose.

Default Administrative distance

• Unknown = 255
• RIP = 120
• IS-IS = 115
• OSPF = 110
• IGRP = 100
• EIGRP = 90
• Static route = 1
• Connected route = 0

note: Static routes using an exit interface have an administrative distance of 0. For example: "ip Route 192.168.3.0 255.255.255.0 Serial0/1"

To display which routing protocols are active on the router, use command "show ip protocols".