Configuring Ospf; Overview; Ospf Packets - HP MSR Series Configuration Manual

Configuring OSPF

This chapter describes how to configure OSPF.

Overview

Open Shortest Path First (OSPF) is a link state interior gateway protocol developed by the OSPF working
group of the IETF. OSPF version 2 is used for IPv4. Unless otherwise stated, OSPF refers to OSPFv2
throughout this document.
OSPF has the following features:
Wide scope—Supports various network sizes and up to several hundred routers in an OSPF routing
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domain.
Fast convergence—Advertises routing updates instantly upon network topology changes.
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• 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
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feature reduces the LSDB size on routers to save memory and CPU resources, and reduces route
updates transmitted between areas to save bandwidth.
Equal-cost multi-path (ECMP) routing—Supports multiple equal-cost routes to a destination.
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Routing hierarchy—Supports a 4-level routing hierarchy that prioritizes routes into intra-area,
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inter-area, external Type- 1 , and external Type-2 routes.
Authentication—Supports area- and interface-based packet authentication to ensure the security of
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packet exchange.
• Support for multicasting—Multicasts protocol packets on some types of links to avoid impacting
other devices.

OSPF packets

OSPF packets are encapsulated into IP packets and use the protocol number 89.
OSPF uses the following packet types:
Hello—Periodically sent to find and maintain neighbors, containing the values of some timers, and
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information about the DR, BDR, and known neighbors.
Database description (DD)—Describes the digest of each LSA in the LSDB, exchanged between two
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routers for data synchronization.
Link state request (LSR)—Requests needed LSAs from the neighbor. After exchanging the DD
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packets, the two routers know which LSAs of the neighbor are missing from their LSDBs. They then
send an LSR packet to each other, requesting the missing LSAs.
• Link state update (LSU)—Transmits the requested LSAs to the neighbor.
Link state acknowledgment (LSAck)—Acknowledges received LSU packets.
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