This article only explains the type of LSA. For the specific message format of LSA, see:
https://blog.51cto.com/xxy12345/2518384
1 Router-LSA（Type 1）
LSA-1, which is generated by every device, describes the link status and overhead of the device, and only floods in the area it belongs to.
2 Network-LSA（Type 2）
LSA-2, generated by DR, describes the link status of the MA network, and only floods in the area it belongs to. Describe the routers and network segments connected to the MA network.
3 Network-Summary-LSA（Type 3）
LSA-3, described in a routing area segment, is generated and advertised by an ABR to other areas, the LSA-3 flooding scope only one region . For example, ABR connects areas 0 and 1. ABR generates LSA-3 and advertises the route of area 1 to area 0 in area 0. The LSA-3 is only flooded in area 0. When area 0 needs to advertise the route to other areas , Need to regenerate LSA-3 notification through ABR.
l ABR generates an LSA3 for each OSPF route in the area and advertises it to other areas;
l When there are multiple ABRs on the boundary, each ABR will generate the same LSA-3, which is distinguished by the Adveritsing Router field;
l Inter-area transfers are routes. LSA-3 is generated by the ABR of each area and flooded only in that area . After the route enters other areas, the ABR in the area generates LSA-3 and continues to flood;
l OSPF has vector characteristics at the area boundary , does not carry any topology information, only describes network information to other areas, and only routes appearing in the ABR routing table will be advertised to neighboring areas;
l Routing cost calculation : the cost from the current router to the target = the cost from itself to the ABR in the area + the inter-area cost transmitted by LSA3;
l ABR route withdrawal : If an OSPF route in the routing table on the ABR router is unreachable, an LSA-3 with an Age of 3600s will be immediately flooded into the area to withdraw the network in the area;
l LSA3 route calculation: When OSPF calculates routes, it considers LSA3 routes to be calculated through leaf nodes on the router.
4 ASBR-Summary-LSA（Type 4）
LSA-4, generated by the ABR, describes the distance to the ASBR, and advertises it to other relevant areas except the area where the ASBR is located. The flooding range of this LSA-4 is only in one area, and the ABR will generate LSA- for other areas at the boundary of the area. 4 and continue to flood.
5 AS-External-LSA（Type 5）
LSA 5 only advertises routes of other protocols outside the OSPF routing domain, such as RIP, BGP, etc. The imported external routes are flooded to the OSPF routing domain through LSA 5, unlike LSA 3/4, which can only be flooded in a certain area and advertised to others When the area is regenerated by ABR and then flooded again, LSA5 has a flooding range that other LSAs do not have, and LSA 5 can flood all areas (except Stub and NSSA) . During the flooding period, only age increases, and nothing else changes. In addition to external routes, LSA5 will also inform the router how to access external routes, that is, Forwarding Address .
6 NSSA AS-External-LSA（Type 7）
LSA7 has the same format as LSA5, and is different from LSA5:
l LSA7 floods only in the NSSA area;
l The FA of LSA7 is non-zero. If it is 0, it will not be converted to LSA5;
l External routes are transmitted in NSSA area using LSA7, and in non-NSSA area using LSA5, ABR is responsible for 7/5 translation;
l The option of LSA7 is P-bit (Propagate bit) to inform the translation router whether this LSA7 should be translated;
l By default, the translation router is the area border router with the largest Router ID in the NSSA area;
l Only LSA7 whose P-bit is set and FA (Forwarding Address) is not 0 can be converted to LSA5;
l If an external route is imported to the ABR, the generated LSA7 will not set the P-bit, so it will not be advertised to Area 0;