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OSPF routing protocol:

Open shortest path first (Open Shortest Path First,OSPF) is a widely used dynamic routing protocol, which belongs to link-state routing protocol. It has the advantages of fast convergence of routing changes, no routing loop, support for variable length subnet mask (VLSM) and aggregation, hierarchical area division and so on. After using the OSPF protocol in the network, most of the routes will be calculated and generated by the OSPF protocol without manual configuration by the network administrator. When the network topology changes, the protocol can automatically calculate and correct the route, which greatly facilitates the network management.

Four routing types: DR BDR ABR ASBR

Five regions: backbone area, standard area, stub area, stub area, non-stub area

Five big data package types: hello, DBD, LSR, LSU (LSA), LSACK

Six types of lLSA:

Seven states:

Basic concepts:

OSPF region

In order to adapt to large networks, OSPF divides multiple areas within the AS

Each OSPF router maintains only complete link-state information in its area

Regional ID

The region ID can be expressed as-a decimal number

It can also be expressed as an IP

Backbone area Area 0

Responsible for inter-area routing information dissemination

Non-backbone area

Router ID

The IP address that uniquely identifies the router in the OSPF area

Router ID selection rules

Select the router loopback interface. The IP address with the highest value on

If there is no loopback interface, select the one with the highest IP address in the physical port

You can also use the router-id command to specify Router ID

DR and BDR (designated router)

Other routers (DRothers) only form adjacencies with DR and BDR

The election method of DR and BDR

Automatically elect DR and BDR

The largest router of Router ID on the network segment will be elected as DR, and the second largest will be elected as BDR.

Manually select DR and BDR

The priority range is 0255. the higher the value, the higher the priority. The default is 1.

If the priority is the same, you need to compare Router ID

If the priority of the router is set to 0, it will not participate in the election of DR and DBR

The election process of DR and BDR

The priority of the router can affect the election process, but it cannot force the replacement of an existing DR or BDR router

Multicast address of Ospf

224.0.0.5

224.0.0.6

The measure of OSPF is COST_

COST= 10 to the power of 8 / BW

The shortest path is calculated based on the cost specified by the interface (cost)

OSPF packet

Carried in an IP packet, using protocol number 89

Package type of OSPF

Package type of OSPF

Description

Hello packets are used to discover and maintain neighbor relationships, and DR and BDR database description packages (DBD) are elected to send summary information to neighbors to synchronize link-state database link-state request packets (LSR). After the router receives a DBD containing new information, it is sent to request more detailed information link-state update packets (LSU)

Send a link-state advertisement (LSA) upon receipt of a LSR. A LSU packet may contain several LSA

Link-state acknowledgement packet (LSAck) acknowledges that LSU has been received, and each LSA needs to be acknowledged separately

Establishment of OSPF adjacency (that is, seven states)

The first stage of OSPF startup is the process of establishing two-way communication using Hello messages.

The first stage of OSPF startup is to establish a full adjacency.

OSPF divides networks into four types

Point-to-point network (Point-to-Point)

Broadcast multiple access Network (Broadcast MultiAccess, BMA)

Non-broadcast multiaccess network (None Broadcast MultiAccess,NBMA)

Point-to-multipoint network (Point-to-Multipoint)

Application Environment of OSPF

Consider the use of OSPF from the following aspects

Network scale

Network topology

Other special requirements

Router's own requirements

Characteristics of OSPF

Adaptable to large-scale network

Fast convergence speed of route change

No routing ring

Support variable length subnet mask VLSM

Support for zoning

Support for sending protocol packets with a multicast address

Comparison between OSPF and RIP

Configuration commands for OSPF

Start the OSPF routing process

Router (config) # router ospf process-id

Specify the interface and area on which the OSPF protocol runs

Router (config-router) # network address inverse-mask area area-id

Modify the priority of an interface

Router (config-if) # ip ospf priority priority

Modify the Cost value of the interface

Router (config-if) # ip ospf cost cost

View routing tabl

Router#show ip route

View the list of neighbors and their status

Router#show ip ospf neighbor

View the configuration of OSPF

Router#show ip ospf

View the data structure of the OSPF interface

Router#show ip ospf interface type number

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