Upgrading and Repairing Networks (5th Edition)

SOME OF THE MAIN TOPICS IN THIS CHAPTER ARE

Basic Types of Routing Protocols

Multi-Protocol Label Switching

Routers are devices that examine Network layer protocol addresses and make decisions based on those addresses on how best to send a network packet on its way to its destination. Routers can be used in a corporate network to interconnect various LAN segments, to connect to a wide area network ”for connecting branch offices to the headquarters, for example ”or, more commonly, to connect the local network to the Internet.

Routers also play an important part in firewalls, which are covered in Chapter 49, "Firewalls."

However, to make decisions on the best path a packet needs to take as it travels through the network, a router must keep a table in memory that it can use to locate the destination network for each packet that passes through it. Because routers generally are used to connect many different networks, and because networks usually undergo changes frequently, there must be a method for keeping the routing table up-to-date. Network transport protocols (such as TCP/IP) are used to transfer data across a network. Routing protocols are used by routers to communicate with other routers to exchange information, such as routes or routes that no longer exist.

Chapter 25, "Overview of the TCP/IP Protocol Suite," not only gives you a detailed overview of TCP/IP, but also contains a great deal of information about address classes, subnet masks, subnetting, and the Classless Interdomain Routing (CIDR), among other basic TCP/IP topics. I strongly recommend that you read Chapter 25 before this one to make it easier for you to comprehend the topics covered here.

For example, suppose that an important router suddenly fails. All other routers that have this router in their routing table need to know this so that they can discover another route, if there is one, that can be used to bypass this failed device. Routing protocols come in all sizes and shapes , but all generally perform the same function: keeping routing tables up-to-date.