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10. OSPF Link State Advertisement (LSA)

1. Introduction to Link State Advertisement

(1) Overview of LAS

Link State Advertisement (LSA)It is the carrier of link status information between routers.LSAIt is the smallest unit of LSDB. LSDB is composed of LSAs.。yesOSPFThe important basis for calculating routing。

LSAUsed to pass traffic to other adjacent OSPF routersTopology informationandRouting information。

LSID：In OSPF, each LSA has its own unique identifier.LSID，Used to distinguish different LSAs。

Each LSA has its own sequence number, which will change according to the changes in the LSA, in an incremental manner. And the information is propagated throughout the network, which ensures that each router has the latest topology status and can calculate the best and latest shortest path.

In OSPFIt mainly learns routes by flooding LSA and synchronizing LSDB to achieve full network connectivity.。

Flooding LSA:The process of copying and advertising LSAs to each link in the area.Flooding every 30 minutes (aging after 60 minutes)

Flooding is a reliable process with a confirmation mechanism.

1) Display confirmation,Use LSAck to confirm the LSA in the LSU message (send a data packet containing a copy of the LSA to confirm, which is an information-level confirmation) to confirm that the content has been received

2) Implicit confirmation,DD message sequence number 1 is confirmed (only message-level confirmation) to confirm that the message has been received.

(2) LSA composition

Link state advertisements mainly consist of:LSAHeader information (LSA summary) and link status (Some LSAs only have LSA header information but no link state information).

Each router using OSPF generates a Link State Advertisement (LSA).，Contains the following:Link status information of directly connected networks and link status information of neighboring routers.

The LSA header is the same for each type of LSA, including type (type)/link state ID (LS ID)/advertiser router (ADV rtr), which are the three elements of LSA.Used to uniquely represent an LSA；Ls age, seq, and checksum are used to compare the new and old LSAs.(The following figure)