Link-state

Link-state routing protocols are also known as shortest path first protocols and built around Edsger Dijkstra's shortest path first (SPF) algorithm. The SPF algorithm will be discussed in more detail in a later section.

The IP link-state routing protocols are shown in the figure:
- Open Shortest Path First (OSPF)
- Intermediate System-to-Intermediate System (IS-IS)

Link-state routing protocols have the reputation of being much more complex than their distance vector counterparts. However, the basic functionality and configuration of link-state routing protocols is not complex at all. Even the algorithm itself can be easily understood, as you will see in the next topic. Basic OSPF operations can be configured with a router ospfprocess-id command and a network statement, similar to other routing protocols like RIP and EIGRP.

There are two link-state routing protocols used for routing IP today:
- Open Shortest Path First (OSPF)
OSPF was designed by the IETF (Internet Engineering Task Force) OSPF Working Group, which still exists today. Most of the work on OSPF was done by John Moy, author of most of the RFCs regarding OSPF. His book, OSPF, Anatomy of an Internet Routing Protocol, provides interesting insight to the development of OSPF.

- Intermediate System-to-Intermediate System (IS-IS)
IS-IS was designed by ISO (International Organization for Standardization) and is described in ISO 10589.IS-IS was originally designed for the OSI protocol suite and not the TCP/IP protocol suite. Later, Integrated IS-IS, or Dual IS-IS, included support for IP networks. Although IS-IS has been known as the routing protocol used mainly by ISPs and carriers, more enterprise networks are beginning to use IS-IS.

OSPF and IS-IS share many similarities and also have many differences. There are many pro-OSPF and pro-IS-IS factions who discuss and debate the advantages of one routing protocol over the other. Both routing protocols provide the necessary routing functionality needed. You can learn more about IS-IS and OSPF in CCNP and begin to make your own determination if one protocol is more advantageous than the other.

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