Detailed description of ospf lsa type

 

All routers connected to the same OSPF region will learn the same topology data. Each vro stores data in the link status database consisting of The Link Status announcement (LSA. Then, the router runs the SPF (Shortest Path First) algorithm on LSDB to determine the optimal route to each subnet.

 

The following describes all types of LSA in OSPF.

 

In general, there are the following types:

1. Type 1: Router LSA

2. Type 2: Network LSA

3. Type 3: Network Summary LSA

4. Type 4: ASBR Summary LSA

5. Type 5: AS External LSA

6. Type 6: Group Membership LSA

7. Type 7: NSSA External LSA

8. Type 8: External Attributes LSA

9. Type 9: Opaque LSA (link-local scope)

10. Type 10: Opaque LSA (area-local scope)

11. Type 11: Opaque LSA (AS scope)

 

The following is a detailed explanation.

1. Router LSA

Each vro creates an LSA class to describe itself to each region connected to it. In each vro, The LSDB in each region contains a class-1 LSA, which indicates the RID Of the current vro and the IP addresses of all interfaces. Class-1 LSA is also used to describe the peripheral network.

 

Type 1 LSA uses the OSPF router ID to mark the OSPF router. Each vro creates a class-1 LSA and floods it to the entire region. To flood LSA, the originating router sends a Class 1 LSA to the neighbor in the current region, and then sends it to another neighbor in the current region, and so on, it is known that all routers in the region have copies of the LSA.

 

Class 1 LSA contains information: indicates the subnet number/mask and OSPF overhead of each interface without DR election.

For each DR interface selected, the IP address of the DR and the link connecting to the transit network are pointed out.

If a DR is not elected but it can be used to reach a neighbor's interface, it indicates the neighbor's RID.

 

Each internal router creates a class-1 LSA, but the API creates multiple class-1 LSA, each region has one.

You can use show ip ospf database router to view the router LSA.

 

2: Network LSA

In each multi-channel access Network, the DR in the subnet creates a Network LSA, which describes the subnet and the excuse for connecting to the router in this subnet. It only describes all routers connected to the Network LSA (including the DR itself) in the region where the Network LSA is generated ).

Show ip ospf database network: The Network LSA is displayed.

 

3: Network Summary LSA

Created by the API, it describes the subnets contained in Class 1 and Class 2 LSA in one region and is advertised to another region. It indicates the links (subnets) and overhead of the origin region, but there is no topology data.

If the ABR knows that there are multiple paths to reach the target address, but it still sends only a single Network Summary LSA, and it is the one with the lowest overhead. Similarly, if the ABR receives multiple Network Summary LSA records from other BRS, it selects the least overhead and declares the Network Summary LSA to other regions.

When other routers receive the NetworkSummary LSA from the API, they do not run the SPF algorithm. It simply adds the overhead to the API and the overhead contained in the Network Summary LSA, the route entry and overhead to the target address are added to the routing table. This dependency on the intermediate router is used to determine the full route to the target address (full route ).

 

You can use show ip ospf database summary to view Network Summary LSA.

 

4: ASBR Summary LSA

Similar to the three types of LSA, only a host route for ASBR is advertised, rather than a network.

Use show ip ospf database asbr-summary to view ASBR Summary LSA

 

5: AS External LSA

AS external LSA, created by ASBR, is used to describe the external route injected into OSPF. This LSA will flood throughout the.

You can use show ip ospf database external to view AS External LSA.

 

 

6: Group Membership LSA

Group member LSA, which is defined for MOSPF and not supported by Cisco IOS.

 

7: NSSA External LSA

NSSA external LSA, from a non-full Stub region (not-so-stubby area), similar to 5 LSA, but created by ASBR In The NSSA region, flood occurs only in the NSSA region.

Run the Show ip ospf database nssa-external command to check the NSSA External LSA.

 

8: External Attributes LSA

The external attribute LSA cannot be implemented by the Cisco router.

 

9--11: Opaque LSA

The non-transparent LSA is used as a general LSA to facilitate OSPF extension. (For example, the LSA of type 10 is modified to support MPLS Traffic Engineering .)

 

 

Finally, let's talk about the LSA generated in various regions of OSPF:

 

Backbone: 12345

 

STUB: 1234

 

Nsssa: 1237