Configuring Ospf; Overview; Ospf Packets; Lsa Types - HPE FlexNetwork HSR6800 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 (defined in RFC 2328) throughout this document.
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.
•
Equal-cost multi-path (ECMP) routing—Supports multiple equal-cost routes to a destination.
•
Routing hierarchy—Supports a four-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 the
security of 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 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 the neighbor. After exchanging the
DD 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.

LSA types

OSPF advertises routing information in Link State Advertisements (LSAs). The following describes
some commonly used LSAs:
•
Router LSA—Type-1 LSA, originated by all routers and flooded throughout a single area only.
This LSA describes the collected states of the router's interfaces to an area.
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