Chapter Summary

Routers use a routing table to figure out where to send packets. If a packet arrives at a router, and that router does not have a matching entry in its routing table, the router discards the packet. For routing to work well, the collective routers in an internetwork need to have routes to all the IP networks and IP subnets in the internetwork.

Routers first learn about any directly connected routes. If a router interface is working and has an IP address configured, the router can add a route for the connected subnet to the routing table. Network engineers can also statically configure a route, telling the router the routing table entry that it should add to its routing table.

The most common way that a router learns routes is by running a routing protocol. Routers exchange information about the routes they know about by sending routing update messages. Other routers receive those messages and learn the routes. If all routers advertise their routes, eventually all routers will know about all the IP networks and subnets.

Many IP routing protocols are available. RIP and IGRP are older interior routing protocols, meant for use inside a single company. IGRP and OSPF are the most popular interior routing protocols today, with OSPF being a public standard, and EIGRP being Cisco proprietary. BGP is the main option for an exterior routing protocol.