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An internal gateway protocol based on link state developed by OSPF IETF (Internet Engineering Task Force, Internet Engineering Task Force). Currently OSPF Version 2 is used for the IPv4 protocol. Introduction to OSPF

OSPF (Open Shortest Path First, Open shortest path first) is a link-state-based internal gateway protocol developed by IETF (Internet Engineering Task Force). Currently OSPF Version 2 is used for the IPv4 protocol.

Characteristics of OSPF

OSPF has the following characteristics:

Wide range of adaptations: supports networks of all sizes and supports up to hundreds of routers.

Fast convergence: send an update message immediately after a change in the topology of the network to synchronize the change in the autonomous system.

No self-loop: because OSPF uses the shortest path tree algorithm to calculate the route according to the collected link state, it ensures that the self-ring route will not be generated from the algorithm itself.

Zoning: allows the network of autonomous systems to be divided into areas to manage. The reduction of the link-state database of the router reduces the consumption of memory and the burden of CPU, and the reduction of routing information between regions reduces the occupation of network bandwidth.

Equal cost routing: supports multiple equal cost routes to the same destination address.

Routing classification: use four different types of routes, which are: intra-area routing, inter-area routing, first-class external routing, and second-class external routing.

Support authentication: support region and interface-based message authentication to ensure the security of message interaction and routing computation.

Multicast transmission: protocol messages are sent at multicast addresses on certain types of links to reduce interference to other devices.

OSPF message type

OSPF protocol message is directly encapsulated into IP message, and the protocol number is 89.

OSPF has five types of protocol messages:

Hello message: sent periodically to discover and maintain OSPF neighbor relationships, and to conduct DR (DesignatedRouter, designated router) / BDR (Backup DesignatedRouter, backup designated router) elections.

DD (Database Description, database description) message: describes the summary information of each LSA (Link State Advertisement, link-state advertisement) in the local LSDB (Link State DataBase, link-state database), which is used for database synchronization between two routers.

LSR (Link State Request, link-state request) message: the required LSA is requested from the other party. After exchanging DD messages with each other, the two routers know which LSA of the opposite router is missing by the local LSDB, so they need to send LSR messages to request the required LSA from each other.

LSU (Link State Update) message: sends the LSA it needs to the other party.

LSAck (Link State Acknowledgment) message: used to acknowledge the received LSA.

LSA Typ

The description of link-state information in OSPF is encapsulated and published in LSA. There are several common types of LSA:

Router LSA (Type-1): generated by each router, describes the link state and cost of the router, and propagates in the area from which it originated.

Network LSA (Type-2): generated by DR, it describes the link state of all routers in this segment and propagates in the area from which it originated.

Network Summary LSA (Type-3): generated by ABR (Area Border Router, area Border Router), it describes the route of one network segment in an area and advertises it to other areas.

ASBR Summary LSA (Type-4): generated by ABR, describes the route to ASBR (Autonomous SystemBoundary Router, autonomous system boundary router) and advertises it to the relevant area.

AS External LSA (Type-5): generated by ASBR, describes routes outside the AS (Autonomous System, autonomous system), advertised to all areas (except Stub areas and NSSA areas).

NSSA External LSA (Type-7): generated by ASBR within the NSSA (Not-So-Stubby Area) area, describes routes outside the AS and propagates only within the NSSA area.

Opaque LSA: an extended general mechanism for OSPF. Currently, there are three kinds of mechanisms: Type-9, Type-10 and Type-11. Among them, Type-9 LSA only floods in the local link scope, Grace LSA used to support GR (Graceful Restart, smooth restart) is a type of Type-9; Type-10 LSA only floods in area scope, and LSA used to support MPLS TE is a type of Type-10; Type-11 LSA can flood in an autonomous system scope.

Original address: https://www.linuxprobe.com/ospf-lsa.html

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