Key points of lldp protocol preparation

LLDP A protocol is a protocol that identifies a switch , The name of the switch comes to mind first , Description, etc. , In fact, there are many other contents , such as , Description of the port ,ttl, management ip,mac Address, etc .

Grab one lldp message , Its composition is shown in the figure below

There is a destination address （01:80:c2:00:00:0e） And source address （00:26:f8:99:00:77）, There are also protocol types 0x88cc

Down there tlv 了 , such as Chassis subtype、port subtype、Time to Live(ttl)、port description wait

These data are put into the array in the form of characters

Analyze one by one

One 、 The destination address is 01:80:c2:00:00:0X form , This is the management frame , You can refer to stp Destination address of message , It means that you can pass block or discard Status port message , This kind of message will enter cpu Processed . After opening some ring network protocols, the processing methods of these messages are different from those of ordinary messages .

Two 、 The source address is the address of the switch , This address can be changed as needed

3、 ... and 、TLV As shown in the figure below

The first few of them are necessary data , But generally speaking, all of the above tlv Write it into the code . Of course, only when sending shutdown The above several... Will be used directly during the frame . And will ttl=0, In that way, the device can be directly aged to remove it from the switch lldp In the table to delete .

PS: Connect at the switch PC Computer time , The computer will also send lldp message , But it has only the first few contents . It is advisable to write code to identify these messages , Then cancel such messages . The messages sent in some old exchanges will only contain the first few TLV Suggestions and computer lldp Let the switch discard the message .

TLV Generally, it is composed of three or four parts. Of course, for specific composition, please refer to the instruction documents

One 、 Four parts

  The picture above is in 88cc Followed by tlv

  Its type is 02, however wireshark It is shown that 1, In fact, this value has been shifted to the left , Therefore, you need to shift one bit to the right when restoring 2>>1=1, The length is 07 Refer to 04-00-26-f8-99-00-77 this 7 Array values , The subtype is 04 Please refer to lldp In the manual chassis Type values of subtypes , among 4 The corresponding is mac Address , What follows is the content ,mac The address is 6 individual 16 Composed of hexadecimal numbers . It is the source mac Address . When we send and receive this type, we can use the corresponding function to process one by one in the array , Note that the value here is char The number of types 0 The corresponding is ascii The code 0 value , So when it is transferred or printed into character or string form , Note the conversion type , For example, it's the same 0,0 yes ascii The first of the codes BLANK NULL Nonprintable characters for , And characters ‘0’ The corresponding value is 48.

Two 、 In the third part of

As shown in the above figure, the meaning of its value is somewhat different from the above , It is the system name , There are no subtypes

 

The first value is 0x0a>>1=5 That is, its type , The second value represents a length of 5, The system name has no subtypes . That is, the following value is its name , These values are 16 It's binary , Find their corresponding ascii code ,0x54,0x39,0x30,x034 The decimal system is 84,57,48,48,52 Look up the table below and you will find that T9004

The composition of the message has been finished , It is relatively simple. You only need to write it in the corresponding position

Next is the point , The state machine , But I put it simply , Only the most basic form , For example, there is a status of only receiving but not sending , Only send but not receive status 、 The state of receiving and sending 、 Neither receive nor send status （ The closed position ） And so on

I look for a picture on the Internet , What initialization is there on it , What is the sending status , Receiving status , When the state changes , Idle state processing , Beginners may be confused , What is this? , How do you write it . In fact, it's not hard to . Library functions written by others are only used to #define It's just written in the advanced usage of .

But it's better to read other people's code and realize that the biggest thing is .

Create a new thread . Those who have learned SCM can be understood as a while(1) Circulatory body of , But in linux Real time and other operations can be used in the operating system while(1) The program is running . This while(1) There is one 1s Time delay of , That is, run the program every second .

The core of the program 1 yes ttl That is, survival time , In fact, it is the aging time in the switch . Once it's time , The device will be deleted by the table .

The core 2 If the state of the switch changes, the state machine will enter another state , In fact, simply put, it's just a switch sentence , If something changes, go to another case In the . For example idle state （ Idle state ） Turn it off when , Switchboard lldp The status immediately enters shutdown state , Send a ttl=0 Message of , After receiving this message, the peer switch will send it ttl Set up 0 Then it will age immediately , The peer switch switches it from lldp Delete... From the equipment list

The core 3 Is the interval transmission time , In fact, it is to keep it in the peer switch table , Prevent aging , Will be sent automatically at regular intervals lldp message . The message transmission time should be less than the interval time 1/4. There is an interval , Survival time multiplier , Reinitialization delay , Transmission delay and other parameters . If you are interested, please visit Baidu , There is no discussion here .

The core 4 Combine this information with mib Libraries , There is no discussion here