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Configuring OSPF
Open Shortest Path First (OSPF) is a link-state IGP developed by the OSPF working group of the IETF.
OSPF version 2 is used for IPv4. OSPF refers to OSPFv2 throughout this chapter.
Overview
OSPF has the following features:
•
Wide scope
—Supports various network sizes and up to several hundred routers in an OSPF routing
domain.
•
Fast convergence
—Advertises routing updates instantly upon network topology changes.
•
Loop free
—Computes routes with the SPF algorithm to avoid routing loops.
•
Area-based network partition
—Splits an AS into multiple areas to facilitate management. This
feature reduces the LSDB size on routers to save memory and CPU resources, and reduces route
updates transmitted between areas to save bandwidth.
•
ECMP routing
—Supports multiple equal-cost routes to a destination.
•
Routing hierarchy
—Supports a 4-level routing hierarchy that prioritizes routes into intra-area,
inter-area, external Type-1, and external Type-2 routes.
•
Authentication
—Supports area- and interface-based packet authentication to ensure secure packet
exchange.
•
Support for multicasting
—Multicasts protocol packets on some types of links to avoid impacting
other devices.
OSPF packets
OSPF messages are carried directly over IP. The protocol number is 89.
OSPF uses the following packet types:
•
Hello
—Periodically sent to find and maintain neighbors, containing timer values, information about
the DR, BDR, and known neighbors.
•
Database description (DD)
—Describes the digest of each LSA in the LSDB, exchanged between two
routers for data synchronization.
•
Link state request (LSR)
—Requests needed LSAs from a neighbor. After exchanging the DD packets,
the two routers know which LSAs of the neighbor are missing from their LSDBs. They then exchange
LSR packets requesting the missing LSAs. LSR packets contain the digest of the missing LSAs.
•
Link state update (LSU)
—Transmits the requested LSAs to the neighbor.
•
Link state acknowledgment (LSAck)
—Acknowledges received LSU packets. It contains the headers
of received LSAs (an LSAck packet can acknowledge multiple LSAs).
