Configuring Ospf; Overview; Ospf Packets; Lsa Types - HP FlexFabric 12900E Series Configuration Manual

Configuring OSPF

Overview

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.
OSPF has the following features:
•
Wide scope—Supports multiple network sizes and 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).

LSA types

OSPF advertises routing information in Link State Advertisements (LSAs). The following LSAs are
commonly used:
•
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.
64