RFC 1245 (rfc1245) - Page 2 of 12

OSPF Protocol Analysis

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



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