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This article mainly explains "what are the basic knowledge of OSPF dynamic routing protocol". The explanation in this article is simple and clear, easy to learn and understand. Please follow the ideas of Xiaobian and go deep into it slowly to study and learn "what are the basic knowledge of OSPF dynamic routing protocol" together!

OSPF (Internet Engineering Task Force) is a link-state-based interior gateway protocol developed by the IETF. OSPF Version 2 is currently used for IPv4.

Introduction to OSPF

OSPF(Open Shortest Path First) is a link-state-based interior gateway protocol developed by IETF(Internet Engineering Task Force). OSPF Version 2 is currently used for IPv4.

Characteristics of OSPF

OSPF has the following characteristics:

(1)Wide range of adaptation: support networks of various sizes, up to hundreds of routers.

(2)Fast convergence: sending update messages immediately after a change in the topology of the network, synchronizing the change in the autonomous system.

(3)No self-loops: Because OSPF uses the shortest path tree algorithm to calculate routes based on the collected link state, the algorithm itself guarantees that no self-loops will be generated.

(4)Regionalization: Allows autonomous systems to be managed by dividing their networks into regions. The reduction of router link-state database reduces memory consumption and CPU burden; the reduction of routing information transmitted between regions reduces network bandwidth consumption.

(5)Equivalence routing: Support multiple equivalence routes to the same destination address.

(6)Routing hierarchy: four different types of routing are used, in order of priority: intra-area routing, inter-area routing, type 1 external routing, and type 2 external routing.

(7)Support validation: Support message validation based on area and interface to ensure the security of message interaction and routing calculation.

(8)Multicast transmission: Protocol messages are sent with multicast addresses over certain types of links to reduce interference to other devices.

OSPF Message Type

OSPF protocol messages are encapsulated directly into IP messages with protocol number 89.

OSPF has five types of protocol messages:

(1)Hello message: periodically sent to discover and maintain OSPF neighbor relations, and DR(Designated Router)/BDR(Backup Designated Router) elections.

(2)DD(Database Description) message: describes the summary information of each LSA(Link State Advertisement) in the local LSDB(Link State Database), and is used for database synchronization between two routers.

(3) Link State Request (LSR) message: Request the required LSA from the other party. After exchanging DD messages, two routers know which LSAs the opposite router lacks, and then need to send LSR messages to request the LSAs needed by the other router.

(4)LSU(Link State Update) message: Send the required LSA to the other party.

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

LSA type

The description of link state information in OSPF is encapsulated and published in LSA. The commonly used LSA types are as follows:

Router LSA(Type-1): Produced by each router, describes the router's link state and cost, and propagates within the area where it originated.

(2)Network LSA(Type-2): Generated by DR, describes the link state of all routers in the segment and propagates in the originating area.

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

(4)ASBR Summary LSA(Type-4): Produced by ABR, describes the route to ASBR(Autonomous System Boundary Router), and advertises it to the relevant area.

(5)AS External LSA(Type-5): Produced by ASBR, describes routes outside the AS(Autonomous System) and advertises to all areas (except Stub area and NSSA area).

(6)NSSA External LSA(Type-7): Produced by ASBRs in the NSSA(Not-So-Stubby Area) area, describes routes to the outside of the AS and propagates only within the NSSA area.

Opaque LSA: Extended generic mechanism for OSPF, currently available in Type-9, Type-10, and Type-11. Type-9 LSA floods only in the local link range, Grace LSA for GR(Graceful Restart) is a type of Type-9;Type-10 LSA floods only in the area range, LSA for MPLS TE is a type of Type-10;Type-11 LSA can flood in an autonomous system range.

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