FTOS Configuration Guide for the C-Series - Force10 Networks

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Chapter 26 Open Shortest Path First
Open Shortest Path First is supported only on platforms: c e s
This chapter is intended to provide a general description of OSPF as it is supported in the Force 10
Operating System (FTOS). It is not intended to provide a complete understanding of OSPF; refer to the
RFC 2328, OSPF Version 2 for a complete discussion of this protocol.
The Force 10 Operating System (FTOS) supports the following OSPF RFCs:
• RFC 1587 The OSPF NSSA Option
• RFC 1850 OSPF Version 2 Management Information Base
• RFC 2328 OSPF Version 2
• RFC 2370 The OSPF Opaque LSA Option
• RFC 3623 Graceful OSPF Restart
Protocol Overview
Open Shortest Path First (OSPF) routing is a link-state routing protocol that calls for the sending of
Link-State Advertisements (LSAs) to all other routers within the same Autonomous System (AS) Areas.
Information on attached interfaces, metrics used, and other variables is included in OSPF LSAs. As OSPF
routers accumulate link-state information, they use the SPF algorithm (Shortest Path First algorithm) to
calculate the shortest path to each node.
OSPF routers initially exchange HELLO messages to set up adjacencies with neighbor routers. The
HELLO process is used to establish adjacencies between routers of the AS. It is not required that every
router within the Autonomous System areas establish adjacencies. If two routers on the same subnet agree
to become neighbors through the HELLO process, they begin to exchange network topology information
in the form of Link State Advertisements (LSAs).
Autonomous System (AS) Areas
OSPF operate in a type of hierarchy. The largest entity within the hierarchy is the autonomous
system (AS), which is a collection of networks under a common administration that share a
common routing strategy. OSPF is an intra-AS (interior gateway) routing protocol, although it is
capable of receiving routes from and sending routes to other ASs.
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