Cisco Cookbook

Chapter 8. OSPF

8.0. Introduction

Open Shortest Path First (OSPF) is a popular routing protocol for IP networks for several key reasons. It is classless, offering full CIDR and VLSM support, it scales well, converges quickly, and guarantees loop free routing. It also supports address summarization and the tagging of external routes, similar to EIGRP. For networks that require additional security, you can configure OSPF routers to authenticate with one another to ensure that unauthorized devices can’t affect routing tables.

Perhaps the most important reasons for OSPF’s popularity are that it is both an open standard and a mature protocol. Virtually every vendor of routing hardware and software supports it. This makes it the routing protocol of choice in multivendor enterprise networks. It is also frequently found in ISP networks for the same reasons.

But, for all of these benefits, OSPF is also considerably more complicated to set up than EIGRP or RIP. Unlike EIGRP, which can be readily retrofitted into almost any existing network, your network has to be designed with OSPF in mind if you want it to scale well. For more information on OSPF network design, refer to Designing Large-Scale LANs (O’Reilly). You can find more information about the protocol itself in IP Routing (O’Reilly). The remainder of this section is intended only to serve as a reminder to readers who are already familiar with OSPF.

OSPF is currently in its second version, which is documented in RFC 2328. It uses a large, ...