RFC 1247 (rfc1247) - Page 2 of 189

OSPF Version 2

RFC 1247                     OSPF Version 2                    July 1991


from the Bellman-Ford base used by traditional internet routing
protocols.

The OSPF protocol was developed by the OSPF working group of the
Internet Engineering Task Force.  It has been designed expressly for the
internet environment, including explicit support for IP subnetting,
TOS-based routing and the tagging of externally-derived routing
information.  OSPF also provides for the authentication of routing
updates, and utilizes IP multicast when sending/receiving the updates.
In addition, much work has been done to produce a protocol that responds
quickly to topology changes, yet involves small amounts of routing
protocol traffic.

The author would like to thank Rob Coltun, Milo Medin, Mike Petry and
the rest of the OSPF working group for the ideas and support they have
given to this project.


1.1 Protocol overview

OSPF routes IP packets based solely on the destination IP address and IP
Type of Service found in the IP packet header.  IP packets are routed
"as is" -- they are not encapsulated in any further protocol headers as
they transit the Autonomous System.  OSPF is a dynamic routing protocol.
It quickly detects topological changes in the AS (such as router
interface failures) and calculates new loop-free routes after a period
of convergence.  This period of convergence is short and involves a
minimum of routing traffic.

In an SPF-based routing protocol, each router maintains a database
describing the Autonomous System's topology.  Each participating router
has an identical database.  Each individual piece of this database is a
particular router's local state (e.g., the router's usable interfaces
and reachable neighbors).  The router distributes its local state
throughout the Autonomous System by flooding.

All routers run the exact same algorithm, in parallel.  From the
topological database, each router constructs a tree of shortest paths
with itself as root.  This shortest-path tree gives the route to each
destination in the Autonomous System.  Externally derived routing
information appears on the tree as leaves.

OSPF calculates separate routes for each Type of Service (TOS).  When
several equal-cost routes to a destination exist, traffic is distributed
equally among them.  The cost of a route is described by a single
dimensionless metric.

OSPF allows sets of networks to be grouped together.  Such a grouping is



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