HCNP Study notes OSPF protocol principle and configuration 5-lsa analysis

Earlier, we have introduced the establishment of adjacency relationship and the synchronization of LSDB. Through the introduction of the synchronization process, we can understand that LSDB synchronization is implemented through the interactive LSA.

The content of LSA emitted by different role routers is different, this article will introduce the structure and kind of LSA message, which can help us analyze the process of routing calculation and generation.

1 LSA header (very important)

The contents of the LSA header form the main content of the LSDB.

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In addition to the Hello message, the other OSPF messages carry LSA information.

LS Age: This field indicates the time at which the LSA has survived , and the smaller the new unit is the second. 3,600 seconds of aging, 1800 seconds to refresh.

LS Type: This field identifies the format and functionality of the LSA. There are five common types of LSA. Determines the generation of the link state ID.

Link State ID: This field is the identity of the part of the link that the LSA describes. such as router ID, and so on. Link identification, different LSA.

Advertising Router: This field is the router ID of the router that generated this LSA.

LS Sequence Number: This field is used to detect old and duplicate LSA. Each change is +1, the bigger the more new.

The combination of LS Type,link state ID and advertising Router identifies an LSA.

The OSPF protocol determines the new and old LSA based on LS age and LS sequence number.

2 Types of LSA (very important to help analyze the route generation process )

A. Intra-domain routing (Router-lsa Network-las), delivered within the area

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Note: each router will publish Router-lsa to the external server. Only the DR is released Network-lsa. Will not be passed across domains.

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B. Interval routing ( NETWORK-SUMMARY-LSA) is used to calculate inter-zone routing information. This type of LSA describes the streamlined routing information, not the detailed link state information. Transfer only between the backbone and non-backbone areas. The default route can also be published through Network-summary-lsa.

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C as External route

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The ASBR uses a class fifth LSA to describe the external routes, which flood the entire as inside the class fifth LSA.

When the ABR advertises the received class fifth LSA to other regions, a class fourth LSA is generated for the zone to describe how to reach asbr.

The fourth LSA can only flood within one region , and the fifth LSA is flooded to a region, and the associated ABR regenerates a new fourth class of LSA for that region.

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3 different link state ID values (important, used when viewing and analyzing Lsdb)

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HCNP Study notes OSPF protocol principle and configuration 5-lsa analysis