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The full name of OSPF protocol is opend shortest path first Open shortest path first Protocol. It automatically selects the best route dynamically according to SPF algorithm.

The advantages of OSPF protocol: suitable for large-scale networks, fast convergence of route changes, loop-free routing, support for subnetting LVSM, support for area division, support for multicast to send messages, support for trigger updates of routing entries.

OSPF protocol is a link-state routing protocol with an administrative distance of 110. Its protocol number is 89.

SPF's formula for calculating path cost: 10 to the eighth power divided by BW (bandwidth). Unit: Bit/S.

AS is called autonomous system: we call the network run by a group of routes under common management as autonomous system.

The working process of OSPF: establish adjacency, link-state database, shortest path tree, and form routing table.

OSPF has three tables: neighbor table, link-state database, and routing table.

The process of OSPF forming adjacency state: Down failure state, Init initialization state, 2-Way neighbor state, Exstart start state, Exchange exchange state, Loading wait state, Full adjacency state.

Definition: from Down to 2-Way, routing and routing confirm that they are neighbors, and then the adjacency is completed from Exstart to Full. Adjacency means establishing a connection between neighbors.

OSPF package type:

V Hell packet: send Hell messages to establish adjacency.

V database description package DBD: send a list of descriptions, which are the network segments I have here.

V link-state request packet LSR: send request packet, request route list.

V link-state update package LSU: sends update packets and updates routing lists.

V Link State acknowledgement packet LSACK: sends an acknowledgement packet to confirm the route list.

RouterID: all routers running OSPF need an identity, which is called RouterID.

Election of RouterID:

V manual configuration

V the largest IP address in the loopback interface

V the largest IP address in the physical interface

MA network is called multi-access network, which means that in the same network segment, the router is connected with the switch and the router, and there are many ways for the router to reach the router.

DR and BDR technology: DR represents the designated router and BDR represents the backup router. In the MA network, in order to reduce the flooding of LSA advertisement entries, OSPF selects a DR and a BDR from the routes of the same network segment. When DR learns the route entries of other routes, while other routes learn route entries through DR. BDR is designed to prevent the failure of DR lines, and BDR acts as DR.

DR versus BDR elections: according to priority, according to RouterID. The higher the priority value, the higher the priority, and the larger the RouterID number, the more priority.

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