Summary

This introductory chapter reviews the concepts underlying IP routing and explains why routing is relevant for information transfer in a connectionless networking environment. You learned that protocols such as IP, which provide connectionless delivery of information, allow data to be transmitted in chunks of information, known as datagrams. IP datagrams also are referred to as packets. Packets consist of a payload and a header. The headers in IP packets contain target addresses that allow them to be independently routed over optimal paths in the network toward their destinations. IP is a network layer protocol; routers, which process and forward packets, run routing protocols that automate the gathering of routing information in internetworks.

Classful and classless notions of IP addressing are covered, leading to a discussion on VLSMs and CIDR. The relevance of CIDR and VLSMs as vehicles for efficient address allocation and use is covered as well.

The subsequent text of the chapter discusses various classifications of dynamic routing protocols, categorizing them into unicast versus multicast, classless versus classful, IGP versus EGP, and, finally, distance vector versus link-state. Key characteristics of distance vector and link-state protocols are discussed and compared.

Brief coverage of Cisco IOS Software protocol-independent commands led to the discus-sion of administrative distances associated with routing protocols. Administrative distance is defined as a mechanism for distinguishing between routing protocol sources and asso-ciating an IOS default trust factor with various routing protocols.

The final section briefly touches on how the routing information gathered by routing protocols actually is used in forwarding. It is pointed out that Cisco routers convert the information in a routing table into optimized data structures for high-speed packet forwarding.