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② No algorithm involved , because BGP Just will IGP The calculated routing information is sent to other AS In , amount to Only existing routes are delivered without calculation

IGP --- Distance vector protocol

① Distance vector --- Distance is the embodiment of cost , Take the hops as the evaluation standard of the cost value . Think of a router as yes One unit calculates the distance .

② The concept of algorithm , because IGP The protocol itself needs to calculate the routing information of the unknown network segment by algorithm .

BGP Agreement concerns

1. Controllability ： Controllable means convenient interference and route selection .BGP Discard the cost value , Give each routing information a routing attribute

BGP The protocol needs to transmit a lot of routing information , There is no periodic update , There is a trigger update

2. reliability ： Transport layer protocol uses TCP,TCP Of 179 Port number works (TCP Only unicast )

3.AS-BY-AS： stay BGP among , Will a AS As a whole .BGP Load balancing is not supported , stay BGP among , If there are multiple paths to the same target network segment ,BGP Select an optimal path to load into the routing table according to the path attribute , Without load balancing .

BGP Data packets

BGP Packets are based on TCP The established session channel is sent

stay BGP in , The process of discovering neighbors becomes manually specified by people . Mainly because TCP The protocol needs to establish a session channel , Then the data packets will be sent based on the channel .

open package

effect ： establish BGP Peer relationship . That is, the process of parameter negotiation .

BGP Building neighborhoods requires OPEN Package to carry parameters , Compare and negotiate .

	Negotiate parameters
AS Number	When creating a neighbor relationship, you need to specify the location of the neighbor AS Number , This parameter is carried in OPEN Send to the other party in the message , The other side will compare this AS Number and local AS Whether the numbers are consistent , If you are consistent, you can establish a normal neighborhood relationship
authentication	BGP Jianlin can also do authentication , After authentication, you will carry the authentication password , recognize Both sides need to compare the password , If you are consistent, you can establish a normal neighborhood relationship .
RID	RID Will be in OPEN Carry... In your bag , After sending to the opposite end , The opposite end will detect this RID, If with local RID Different , Will be able to establish normal neighborhood relations
Holdtime Time to live	The default time is 180S, During the life guarantee period , If you don't receive the message sent by the other party keeplive Packages or update package , Will disconnect BGP Neighborhood . This parameter is in open The message will be carried , however , Inconsistency between the two sides will not affect the establishment of neighbor relations , But in execution , This time must be consistent , The smaller of the two parties will be used .
Whether the router supports the refresh function

Keeplive package

effect ：① Life cycle , The cycle sending time is equal to the keeping alive time 1/3

② Confirmation package , stay open The message temporarily acts as a confirmation packet when negotiating parameters .

Confirm that the other party sent open Parameters in the message .

update package

effect ： Carry the routing information that needs to be passed .

Represents a route entry information , The parameters carried are mainly the target network number Subnet mask Path properties .

In update package , There is one. Undo route entry field , The routing entries under this field will need to be deleted at the opposite end , It no longer needs to be expressed by means of toxic transmission .

notification package

effect ： Alarm mechanism

Route-refresh package

effect ： It is used to change the routing policy and request the peer to resend the routing information , Prerequisite ： Both sides support the route refresh function .

BGP State machine

BGP The protocol separates the neighbor establishment process from the routing transceiver process .

BGP State machine description of ：BGP Peer establishment process The change of state in the process .

IDLE --- Idle state

The router starts BGP After the process , First in idle state . After manually specifying the neighborhood ,BGP Will enter into a check link , Check the specified IP Whether the address is reachable in the local routing table . If it can reach , Will enter the next state

Connect--- Connection status

This status is complete TCP The establishment of conversation

TCP Session successfully established , Enter into opensent state , send out open message .

TCP Session establishment failed , Get into ACTIVE state , Try to rebuild TCP conversation .

In establishment TCP During conversation , Because both parties will take the initiative to establish a session

cheng , And the final establishment is a two-way conversation . therefore , In the end, you just need to keep

One session channel is enough . The selection method is through follow-up open In the message RID Conduct

Compare , Choose to keep RID Initiated by large devices TCP conversation

Opensent--- Send local open message

Received from the opposite end open message , Check the parameters , after , If you confirm that the parameters are correct . Will reply keeplive Message operation

For confirmation .

Openconfirm---open Message confirmation status

The opposite end also receives a message sent locally open message , Then confirm according to the parameters inside . If confirmed, it will send keeplive message . This paragraph received from the other party keeplive The message will then enter the next state .