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. These include:
- Distance vector routing protocols, inlcuding RIP and IGRP
- Link-state routing protocols, inlcuding OSPF
- Hybrid routing protocols, like EIGRP
Excerpted from CCNA Flash Cards and Exam Practice Pack by Eric Rivard and Jim Doherty (ISBN: 1587200791).
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