One 、 Matching conditions

1. Concept

stay MA In the network , If all equipment is adjacent , There will be a large number of repeated updates , therefore , Need to carry out DR/BDR The election of , All non DR/BDR(DROther) Only maintain the neighbor relationship between them .

2. Router type

（1） Specify router （DR）： and MA Establish adjacency relationship with all other devices in the network .
（2） Back up the specified router （BDR）： and MA Establish adjacency relationship with all other devices in the network .
Tip：
① stay DR and BDR In the presence of both , One MA At least four devices are required in the network to See adjacency .

②DR and BDR Although it is called designated router （ Back up the specified router ）, Its scope of work is in a MA In the network , Its essence is the concept of interface .

3．DR/BDR The election of

background ： stay MA In the network , If all equipment is adjacent , There will be a large number of repeated updates , therefore , Need to carry out DR/BDR The election of .
Method ：
（1） Compare interface priorities , The highest priority is DR, The next largest is BDR.
① The initial default value of priority is 1;
② If the priority of an interface is changed to 0, It means that the interface abandons DR/BDR The election of ;
③ Priority range （0-255）.

（2） When priorities are the same , Compare RID.RID The corresponding interface of a large router is DR, The next largest is BDR.
RID： Indicates the router that sent this packet RID.
Area ID： The area of the interface that sends the packet ID.
Tip：DR/BDR The election of — Non preemptive election — Election time equals death time

Two 、 OSPF Data packets of

1、OSPF The serial number of ：89

2、 OSPF Message header

（1） edition ：OSPF Version of protocol （OSPFv2 The version field is 2）.
（2） type ：OSPF The type of packet
Hello — 1
DBD ---- 2
LSR — 3
LSU — 4
LSACK – 5
（3） packet length ： Whole OSPF Length of data message , Unit byte .
（4） RID： Indicates the router that sent this packet RID
（5） Area ID： Of the area where the interface that sent the packet is located ID
（6） Certification type and certification data ：OSPF At the time of certification , You need to compare the certification type first , Only when the authentication type is the same can the authentication data be compared .
Authentication type ：
null — Null authentication — 0
simple – Plaintext Authentication – 1
MD5 — Compare summary value authentication — 2

3、OSPF Five data packets

（1）hello package ：

effect ：① Cycle discovery , Establish and maintain neighborhood relations .
②DR/BDR The election used hello package

Hello Parameters in the package that restrict the establishment of neighbor relationships :
1、 Subnet mask ： issue hello Package interface ip Mask information corresponding to the address
（ Huawei equipment sets that the subnet mask must be the same for both sides of neighbor establishment , Otherwise, it will not be possible to establish Neighborhood （ Yes p2p The network does not work ））

2、Hello Time （ Both neighbors hello The time and time of death must be the same , Otherwise, the establishment of neighbor relationship will fail ）

3、Dead Time

4、Ospf Marking of special areas
optional ：8 position （8 Tag bits ）, Each marking position 1, Represents that it conforms to OSPF A certain characteristic of .
（ There is OSPF Mark bit of special area , If the marker bits of the special areas of the neighbors are different , You will not be able to establish a normal neighbor relationship ）
5、 authentication
（2）DBD package ： Database description message
1、 Use a that does not carry data DBD Package for master-slave relationship election ;
2、 Use... That carries data DBD Package for directory information sharing .

I— init — This location 1, It stands for this DBD A packet is a data packet that elects a master-slave relationship , Such a packet does not contain LSA Head content .
— Before the master-slave relationship is selected , All devices consider themselves Masters .
M — more – This location 1, There are others behind the representative DBD Package to send .
MS — Master – This location 1, It means that the device sending this packet is the main .
Serial number ： stay DBD In the process of message sending and receiving , Will gradually increase 1, Used to ensure that DBD Order and reliability of message transmission .
（3）LSR package ： Link status request message （ be based on DBD Package request local unknown LSA Information ）

“LSA A triple ”：① Link state type ;② Link state ID;③ Notify router .
The above three parameters can uniquely identify a LSA.
（4）LSU package ： Link state update message （ Really carry LSA Packets of information ）

（5）LSACK package ： Link status acknowledgement message .

3、 ... and 、 OSPF The interface network type of

Definition ： finger ospf Interfaces work in different default modes under different network types .
type ：

1.P2P（PPP,HDLC）

P2P, Only one neighborhood can be established , There is no need for DR and BDR The election , hello 10s, dead time 40s.

2.MA

① BMA（ Ethernet ）：Broadcast, Multiple neighbors can be established . Need to carry out DR and BDR The election ,hello 10s ,dead time 40s
② NBMA（ Frame relay ）：NBMA, Multiple neighbors can be established , Need to carry out DR and BDR The election of ,hello 30s, dead time 120s, Unable to automatically establish neighbors .

3. Loopback interface （ Virtual interface ）

① P2P, Huawei equipment is defined as P2P type , In fact, there is no data sending and receiving . Loopback interface default learning 32 Bit host content .
② P2MP, Multiple neighborhood relationships can be established , There is no need for DR and BDR The election of , hello 30s,
Dead time 120s. Learn the host routing of neighbor interface .