RFC 1245 (rfc1245) - Page 2 of 12
OSPF Protocol Analysis
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RFC 1245 OSPF protocol analysis July 1991 of magnitude larger than the current environment. o What are the limits of the protocol for these metrics? (I.e., when will the routing protocol break?) o For what environments is the protocol well suited, and for what is it not suitable? 1.1 Acknowledgments The OSPF protocol has been developed by the OSPF Working Group of the Internet Engineering Task Force. 2.0 Key features of the OSPF protocol This section summarizes the key features of the OSPF protocol. OSPF is an Internal gateway protocol; it is designed to be used internal to a single Autonomous System. OSPF uses link-state or SPF-based technology (as compared to the distance-vector or Bellman-Ford technology found in routing protocols such as RIP). Individual link state advertisements (LSAs) describe pieces of the OSPF routing domain (Autonomous System). These LSAs are flooded throughout the routing domain, forming the link state database. Each router has an identical link state database; synchronization of link state databases is maintained via a reliable flooding algorithm. From this link state database, each router builds a routing table by calculating a shortest-path tree, with the root of the tree being the calculating router itself. This calculation is commonly referred to as the Dijkstra procedure. Link state advertisements are small. Each advertisement describes a small pieces of the OSPF routing domain, namely either: the neighborhood of a single router, the neighborhood of a single transit network, a single inter-area route (see below) or a single external route. The other key features of the OSPF protocol are: o Adjacency bringup. Certain pairs of OSPF routers become "adjacent". As an adjacency is formed, the two routers synchronize their link state databases by exchanging database summaries in the form of OSPF Database Exchange packets. Adjacent routers then maintain syn- chronization of their link state databases through the reliable flooding algorithm. Routers connected by serial lines always become adjacent. On multi-access networks (e.g., ethernets or X.25 PDNs), all routers attached to the network become adjacent to both the Designated Router and the Backup Designated router. o Designated router. A Designated Router is elected on all multi-access networks (e.g., ethernets or X.25 PDNs). The network's Designated [Moy]
