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ThinkMo EDU Share – network 57.Detailed explanation of OSPF neighbor establishment process

蒂娜 1 Comment 01/04/2023

ThinkMo EDU Share – network 57.Detailed explanation of OSPF neighbor establishment process

Five packages of OSPF

1. Hello packet – only sent in state init, state exstart .

Function: used to establish and maintain neighbor relationship (send Hello packets to each other every 10s, if not sent within 40s, the neighbor relationship will be deleted ).

Important fields:

The router-id of the router.

The area-id of the router interface (must be the same).

The IP address and mask of the router interface.

Authentication of router interface, hello interval, and dead interval (must be the same).

neighbor field.

STUB logo (stub logo) (must be consistent).

2. DBD package (database description) – only sent in the state exchange .

Only send in the following state: exchange

Function: used to describe the network topology of OSPF.

3. LSR packet (Link State Request) – only sent in state loading.

Function: used to request the routing information of the other party.

4. LSU packet (Link State Update) – only sent in state loading.

Function: used to store and transmit routing information.

5. LSACK packet (Link State Acknowledgment) – only sent in the state loading.

Function: Used for reliable confirmation of LSU and other packages.

7 State Machines of OSPF Neighbor Establishment Process

down—>init—>two-way—>exstart—>exchange—>loading—>full

1.down: The two parties have not had any interaction yet.

2.init: In this state, only hello packets are exchanged, and no routing information is synchronized.

3. two-way: reach the state two-way, and establish a neighbor relationship (DR/BDR election is required after the neighbor relationship is formed).

4. exstart: Election of master-slave relationship, in order to determine who is in the exchange state to send the DBD packet first.

5.exchange: Create their own topology tables by interacting with DBD packages.

6.loading: requesting the routing information of the other party, this state exchanges LSR packets, LSU packets, and LSACK packets.

7.full: Establish an adjacency relationship to achieve LSDB synchronization.

Analysis of OSPF Neighborhood Establishment Process

The process of establishing OSPF can be simply divided into the following three processes:

①Establish neighbor relationship ②Establish respective topology table ③Establish adjacency relationship

①Establish neighbor relationship: down –>init –>two-way —–>

Analysis: In state init, routers will send hello packets to each other, and the important fields of hello packets are as follows:

The router-id of the router.

The area-id of the router interface (must be consistent).

The IP address and mask of the router interface.

Authentication of the router interface, hello time interval, and death time interval (must be consistent).

neighbor field.

STUB logo (end section logo) (must be consistent).

Only the interface aera-id fields of the two routers, interface authentication, and hello time interval.

Only when the dead time interval and STUB identifier are consistent, the router will enter the two-way state and establish a neighbor relationship.

After the neighbor relationship is established, the DR/BDR election is performed. Function: reduce the number of adjacencies and reduce link consumption.

Election rules: 1) Comparing the priority, the bigger the better (the default is 1, the maximum is 255, 0 means not to participate in the election). 2) Compare the Router-ID, the bigger the better.

②Create their own topology table: exstart –>exchange —–>

Analysis: In the state exstart, the master-slave relationship election (master/slave) is carried out by sending hello packets. In order to determine who sends the DBD packet first in the exchange state, the master-slave relationship election is by comparing the Router ID size of the two routers, and the Router ID is larger. It is the master router, and the master router first exchanges DBD packets.

After the election of the master-slave relationship, enter the state exchange, in this state, establish their respective topology tables by exchanging DBD packages.

③Establish an adjacency relationship: loading ——>full

Analysis: After establishing their respective topology tables, enter the loading state and request the routing information of the other party. As shown in the figure, the router on the left sends an LSR packet to request the routing information of 172.16.6.0/24. After receiving the LSR packet, the router on the right will reply with the LSU packet. Inform 172.16.6.0/24 of the routing information. After receiving the LSU packet, the router on the left will reply with an LSACK packet for reliable confirmation of the LSU packet.

After exchanging routing information, the full state is reached, and an adjacency relationship is established between routers to achieve LSDB synchronization.

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1 Comment

  • Nick Hawken
    02/12/2023

    Dear ciscodumps.net webmaster, Your posts are always well-timed and relevant.

    Reply

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