1 backbone area (Area 0)
1. There is only one;
2. Other areas must be connected to the backbone area. If they are not connected, they cannot learn routes from other areas;
3. All routes between areas must pass through the backbone area (ABR must be connected to the backbone area);
4. The backbone area cannot be divided. Once divided, it must be connected through a Vlink or GRE tunnel.
The role of these principles :
Prevent LSA3 in one area from flowing between multiple areas, and may flow back through other areas to form a loop. Therefore, non-ABR cannot generate LSA3. Avoid inter-area loops.
2 Ordinary area (Nomarl)
Non-Area 0, general area, bearer Vlink, transmission area routing, inter-area routing and external routing.
3 tip region (Stub)
Special areas do not accept LSA4/LSA5 . External traffic in this area can only pass through ABR, and all traffic and routes enter this area through ABR. Vlink is not supported, and LSA3 (0.0.0.0) is generated by ABR .
4 totally stub area (totally stub)
On the basis of the stub area, LSA3 continues to be filtered. Vlink is not supported, and LSA3 (0.0.0.0) is generated by ABR.
5 Not-So Stubby Area (NSSA)
The difference between the NSSA area and the Stub area is that the Stub area cannot import external routes, while the NSSA area can import external routes. The ABR of the NSSA area produces 0.0.0.0 of LSA7.
6 Totally Not-So Stubby Area (NSSA)
The difference between a totally NSSA area and a totally stub area is that the stub area cannot import external routes, while the totally NSSA area can import external routes. The ABR of the totally NSSA area produces 0.0.0.0 of LSA7.
7 summary
Special area type |
Stub |
Totally Stub |
NSSA |
Totally NSSA |
Types of LSAs in the area |
LSA1/2/3 ABR generates LSA3 (0.0.0.0) |
LSA1/2 ABR generates LSA3 (0.0.0.0) |
LSA1/2/3/7 ABR/ASBR generates LSA7 (0.0.0.0) |
LSA1/2/7 LSA3 (0.0.0.0) ABR/ASBR generates LSA7 (0.0.0.0) |
APR / ASBR |
ASBR is not allowed in the area |
ASBR is not allowed in the area |
ASBR runs in the area, and the area border router is both ABR and ASBR |
ASBR runs in the area, and the area border router is both ABR and ASBR |
Configuration command |
Area stub |
Area stub no-summary |
Area nssa |
Area nssa no-summary |