[HCIA continuous update] working principle of OSPF Protocol

OSPF How the protocol works

OSPF Adjacency relationship establishment

OSPF The relationship between routers

1、 About OSPF The relationship between routers has two important concepts , Neighborhood and neighborhood

2、 Consider a simple topology , Two routers are connected directly . Activate... On the interconnection interface of both parties OSPF, The router starts sending and listening Hello message . Re pass Hello When the messages find each other , The two routers form a neighbor relationship

3、 The establishment of neighborhood relations is only the beginning , A series of messages will be exchanged later , For example, as mentioned above DD、LSR、LSU and LS ACK etc. . When two routers LSDB Synchronization complete , And start to calculate the route independently , The two routers form an adjacency relationship

First time to know OSPF The process of establishing adjacency

OSPF Establish process adjacency

OSPF Introduction to network types

1、OSPF Network type is a very important interface variable , This variable will affect OSPF Operation on the interface , For example, how to send OSPF Protocol message , And whether elections are needed DR、BDR etc.

2、 The interface defaults to OSPF The network type depends on the data link layer encapsulation used by the interface

3、 As shown in the figure ,OSPF There are four types of networks ：Broadcast,NBMA,P2MP,P2P

OSPF Network type

1、 In general , On both ends of the link OSPF The interface network type must be consistent , Otherwise, the two sides cannot establish a neighbor relationship

2、OSPF The network type can be manually modified through commands under the interface to adapt to different network scenarios

DR and BDR

1、 To optimize NA In the network OSPF Adjacency relations ,OSPF Three are specified OSPF Router identity ,DR（ Specify router ）、BDR（ Standby designated router ） and DRother Router

2、 Only DR、BDR And others OSPF Routers establish adjacency .DRother Will not establish an all on the brink OSPF

Adjacency relations , Both sides stagnated in 2——way state

3、BDR Will monitor DR The state of , And when the DR Replace the role in case of failure

OSPF Single region and multi region

OSPF Domain and single region

1、OSPF Domain ： A series of connections using the same strategy OSPF Network equipment

2、OSPF Routers flood the network in the same area LSA. To ensure that each router has a consistent understanding of the network topology ,LSDB Need to synchronize in the region

3、 If OSPF There is only one area , With the increasing scale of the network ,OSPF The number of routers is also increasing , This will lead to many problems

• LSDB It's getting bigger and bigger , At the same time OSPF The size of the routing table increases . The scale of routing resources increases . Router resource consumption is high , Equipment performance degradation , Affect data forwarding

• Based on the huge LSDB Routing calculation becomes difficult

• When the network topology changes ,LSA Global flooding and the whole network SPF Recalculation brings a huge burden

OSPF Multi region

1、OSPF Introduce the concept of area , Will a OSPF The domain is divided into multiple regions , You can make OSPF Support larger scale networking

2、OSPF Multi area design reduces LSA The extent of the flooding , Effectively control the influence of topology change in the region , Achieve the goal of network optimization

3、 Route summary can be done at the regional boundary , Reduce the size of the routing table

4、 Multi region improves network expansibility , Conducive to the formation of a larger network

OSPF Router type

• Regional routers
• Area border router ABR
• Backbone router
• Autonomous system border router ASBR

OSPF Single area & Multi area typical networking