Routing Tables

Routers are devices that function at the network layer; they use network numbers to make routing decisions: how to get a packet to its destination. Routers build a routing table, which contains path information. This information includes the network number, which interface the router should use to reach the network number, the metric of the path (what it costs to reach the destination), and how the router learned about this network number. Metrics are used to weight the different paths to a destination. If there is more than one way to
reach the destination, the metric is used as a tie-breaker. The router will put the best metric paths in its routing table.

There are many different types of metrics, such as bandwidth, delay, and hop count. Each routing protocol uses its own metric structure. For instance, IP RIP uses hop count, while Cisco’s EIGRP uses bandwidth, delay, reliability, load, and frame size (MTU). Routing and routing metrics are discussed in Chapters 9, 10, and 11.
When a router receives an inbound packet, it examines the destination layer-3 address in the packet header. The router then determines what the network number is in the address and then compares this network number to its routing table entries. If the router finds a match, it forwards the packet out of the destination interface.

However, if the router does not find a match, the router drops the packet. This is unlike bridges and switches at layer 2: with these devices, unknown unicast destinations are flooded, not dropped.