Dynamic routing protocol OSPF basics, OSPF message, LSA, one minute to understand

1. Introduction to OSPF

OSPF (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 IPv4 protocol.

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2. Characteristics of OSPF

OSPF has the following features:

• Wide adaptability: supports networks of various sizes and up to several hundred routers.
• Fast convergence: Send update messages immediately after the network topology changes to synchronize the changes in the autonomous system.
• No self-loop: Since OSPF uses the shortest path tree algorithm to calculate routes based on the collected link status, the algorithm itself ensures that no self-loop routes will be generated.
• Regional division: Allows the network of autonomous systems to be divided into regions for management. The reduction of router link state database reduces memory consumption and CPU burden; the reduction of routing information transmitted between regions reduces network bandwidth usage.
• Equal-cost routing: supports multiple equal-cost routes to the same destination address.
• Routing classification: Use 4 different types of routing, in order of priority: intra-area routing, inter-area routing, first-class external routing, and second-class external routing.
• Support verification: Supports area and interface-based message verification to ensure the security of message interaction and routing calculation.
• Multicast transmission: Send protocol messages using multicast addresses on certain types of links to reduce interference with other devices.

3. OSPF Message Types

OSPF protocol packets are directly encapsulated as IP packets, 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 elect DR (Designated Router)/BDR (Backup Designated Router).
• 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.
• LSR (Link State Request) message: Requests the required LSA from the other party. After two routers exchange DD messages, they know which LSAs the other router has that are missing from the local LSDB. At this time, they need to send LSR messages to request the required LSAs from the other party.
• LSU (Link State Update) message: sends the LSA required by the other party.
• LSAck (Link State Acknowledgment) message: used to confirm the received LSA.

IV. LSA Type

The description of link state information in OSPF is encapsulated in LSA and published. Commonly used LSAs are of the following types:

• Router LSA (Type-1): generated by each router, describes the link status and cost of the router, and is propagated in the area where it originates.
• Network LSA (Type-2): generated by DR, describes the link status of all routers in the network segment and is propagated within the area where it originates.
• Network Summary LSA (Type-3): Generated by ABR (Area Border Router), describes the route of a network segment in the area and announces it to other areas.
• ASBR Summary LSA (Type-4): generated by ABR, describes the route to ASBR (Autonomous System Boundary Router) and advertises it to related areas.
• AS External LSA (Type-5): generated by ASBR, describes the routes outside the AS (Autonomous System) and is advertised to all areas (except the Stub area and NSSA area).
• NSSA External LSA (Type-7): generated by ASBR in the NSSA (Not-So-Stubby Area), describes the routes outside the AS and is propagated only in the NSSA area.
• Opaque LSA: An extended general mechanism for OSPF, currently including Type-9, Type-10 and Type-11. Type-9 LSA is flooded only in the local link scope, and Grace LSA used to support GR (Graceful Restart) is a type of Type-9; Type-10 LSA is flooded only in the area scope, and LSA used to support MPLS TE is a type of Type-10; Type-11 LSA can be flooded in an autonomous system scope.