[Series of OSPF] Episode 3 how the routers exchange the detailed LSAs to each other Highlighted

Latest reply: Nov 28, 2018 19:14:16 306 6 1 0

During the last episode(please refer to [Series of OSPF] Episode 2 How the OSPF routers establish adjacency relationship and exchange LSAs.), we talked about how the OSPF routers establish adjacency relationship and begin to exchange LSAs.
As the episode 2 said, there is only LSA header in the DBD packets, OSPF cann't caculated out the route path based on the LSA header. In this episode, we will introduce how the routers exchange the detailed LSAs to each other and calculate the route path out.
When router find some LSAs that doesn't exist in its own LSDB, it will set neighbor router to loading state and begin to send LSR packet to request relevant detailed LSAs.

LSR packet
as the capture result above shows, we can see that LSR packet is spreaded to the destination through unicast. In the LSR payload, there is three fields

  • link-state advertisement type: type of the LSA that router requested. 
  • link state ID: the value of this field depends on type of the LSA. 
  • advertising router: RID of the router which generate this LSA, not the router which generate this LSR packet. 
when router receive LSR packet, it will answer it with LSU packet. In the LSU payload, it encapsulates the detailed LSAs to answer the request router. 

LSU packet
When all the LSR requests have been answered, the router set neighbor router to full state. Up to now, two routers establish full adjacency relationship.

As we said in episode 2, 2-way is the first steady state in OSPF, here, the full adjacency state is the second steady state in OSPF. Unlike 2-way state, full adjacency state is established between DR and DRother or BDR and DRother. By the way, in a multi-access network, DR and BDR will establish full adjacency relationship.
By now, we introduced the whole process of the OSPF establish neighbor and adjacency relationship. The difference between neighbor relationship and adjacency relationship is that two DRother routers only can establish neighbor relationship and DR and DRother, BDR and DRother or DR and BDR can establish adjacency relationship.
In the next, we use images to briefly review the process of OSPF neighbor or adjacency relationship establishment in the multicast network.

OSPF establish adjacency relationship part_1OSPF establish adjacency relationship part_2OSPF establish adjacency relationship part_3


In the last three episodes, we introduced how the router establish adjacency relationship in multi-access network, and how they build the whole LSDB of the local area.

More about OSPF, please refer to

[Series of OSPF] Episode 2 How the OSPF routers establish adjacency relationship and exchange LSAs. 

[Series of OSPF] Episode 1 OSPF summary and concept

In the next episode, we will introduce the difference between the different types of LSA, and how the router rebuild the route topology based on the LSDB.

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MVE Created Nov 26, 2018 15:36:16 Helpful(0) Helpful(0)

very good
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Passion%20to%20learn
Created Nov 27, 2018 21:45:08 Helpful(0) Helpful(0)

Each router in AS generates one or more LSAs according to the classification of routers. The set of LSA forms LSDE (Link-state Database). The description of routing information in OSPF is encapsulated and published in LSA. There are five commonly used LSAs: Router LSA, Network LSA, Summary LSA, AS-Extermal LSA and NSSA External LSA.
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Created Nov 28, 2018 19:11:56 Helpful(0) Helpful(0)

good tutorial
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Created Nov 28, 2018 19:12:41 Helpful(0) Helpful(0)

more of such series should be created
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Created Nov 28, 2018 19:12:53 Helpful(0) Helpful(0)

thanks for the efforts
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Created Nov 28, 2018 19:14:16 Helpful(0) Helpful(0)

good to know about it
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