Determining IP Routes
Routers perform two main functions: switching and routing. The switching function is the process of moving packets from an inbound interface to an outbound interface. The switching function is also responsible for stripping the data link layer information from a packet it receives and encapsulating it with the data link layer information (MAC address) of the router's exiting interface. Routing is a relay function in which packets are forwarded from one location to another. The routing mechanism is responsible for learning and maintaining awareness of the network topology. A router functions as a hop-by-hop paradigm and performs best-effort packet delivery. The packet is delivered to the downstream router that the sending router feels is closest to the final destination. For a router to be an effective relay device, it must perform both routing and switching functions.
There are two ways a router learns about routes a packet must take: statically and dynamically. Static routes are entered manually by a network administrator. Dynamic routes are learned by a routing protocol. Routing protocols can be classified into two categories: interior and exterior. Interior routing protocols learn about routes and route packets within an autonomous system (AS). Exterior routing protocols learn about routes and route packets between autonomous systems. Routing protocols use metrics to determine what path is best for a packet to travel. A metric is a standard measurement, such as distance, used by routing protocols to determine the optimum path to a destination. Each routing protocol uses a different algorithm to determine its metric. This chapter focuses on the interior routing protocols covered on the ICND and CCNA exams.
Distance Vector Routing Protocols
RIP and IGRP are distance vector routing protocols. The name is derived from the fact that routes are advertised as vectors of distance and direction. Routers running distance vector protocols learn routes from neighboring routers' perspectives and then advertise the routes from their own perspective. Because each router depends on its neighbors for information, distance vector routing is sometimes called "routing by rumor." Routers running distance vector protocols store the route that has the best administrative distance and metric in its routing table. RIP and IGRP are also classful routing protocols. Classful routing protocols exchange routes to subnetworks in the same major network (Class A, B, or C). In other words, all networks in the internetwork must have the same subnet mask. This is because RIP and IGRP do not send the subnet mask in their routing updates, thus assuming that all networks use the same subnet mask assigned to the exiting interface.