OSPF LSA types

Hi, friend

This is a small summary

LSAs describe the state of a network or a router. This description covers the status of all the routers' interfaces and their adjacencies.

In OSPF we use 4 types of LSAs(More use):

• Type 1: They are called router link, these LSAs describe the status and cost of the links between area routers. These LSAs only spread within the same area, not throughout the Autonomous System.

• Type 2: They are called network links, these LSAs describe all the routers in a particular network. These LSAs spread within the area that contains the network.

• Type 3/4: Those are the summary links. These LSAs are generated by the ABRs, and describe the links between the ABRs and the Irs in the local area. The summary links are propagated through area 0 or backbone to other areas through the ABRs of the AS. Type 3 and Type 4 LSAs have differences. Type 3 LSAs describe the routes to the networks through the AS and are sent over area 0. However, Type 4 LSAs describe the location of the ASBRs.

• Type 5: They are also known as external links, which are created in the ASBRs. These LSAs describe the routes to destinations outside the AS. These LSAs go through the standard areas and through the backbone. There are two types of external links:

  • External link type 1: This is calculated by adding to the external cost the internal cost to reach the destination.

  • External link type 2: It is the external cost without taking into account the internal cost.

The different types of LSAs are reflected in the LSDB or Topological Table that we visualize using the show ip ospf database command. In this table the LSAs are sorted first by type and then by ID.

When it comes to an ABR, the LSAs are grouped by each area the device is connected to.

I don't know if with this I can clarify your doubts regarding the LSAs, but I recommend that you use Wireshark so that you can see all the states by filtering the OSPF packets.

Best regards

Hi  @hamza11

OSPF relies on several types of Link State Advertisements (LSAs) to communicate link state information between neighbors. A brief review of the most applicable LSA types:

LSA Type1, Router:
Each router creates its own Type 1 LSA to represent itself for each area to which it connects. The LSDB for one area contains one Type 1 LSA per router per area, listing the RID and all interface IP addresses on that router that are in that area. Represents stub networks as well

LSA Type2, Network:
One per transit network. Created by the DR on the subnet, and represents the subnet and the router interfaces connected to the subnet

LSA Type3, Net Summary:
Created by ABRs to represent subnets listed in one area’s type 1 and 2 LSAs when being advertised into another area. Defines the links (subnets) in the origin area, and cost, but no topology data.

Type 4 LSA, ASBR-Summary
this is needed because Type 5 External LSAs are flooded to all areas and the detailed next-hop information may not be available in those other areas. This is solved by an Area Border Router flooding the information for the router (i.e. the Autonomous System Boundary Router) where the type 5 originated. The link-state ID is the router ID of the described ASBR for type 4 LSAs.

Type 5 LSA, External LSA
These LSAs contain information imported into OSPF from other routing processes. They are flooded to all areas unchanged (except stub and NSSA areas). For “External Metric Type 1” LSAs routing decisions are made using the Type 1 metric cost sent, as the total cost to get to the external destination and includes the cost to the ASBR; while for “External Type 2” LSAs the metric sent is the cost from the ASBR to the External destination network and must be added to the OSPF cost to the ASBR advertising the Type 5. The link-state ID of the type 5 LSA is the external network number.

LSA Type7, NSSA External:
Created by ASBRs inside an NSSA area, instead of a type 5 LSA



Also refere to the below video from Huawei Documentation Insights:

https://www.youtube.com/watch?v=FiRFCtisvHk

Thanks 

Hi Hamza,

You can find the LSA Type details in the below,also i share with you some command outputs as well.

                                                                LSA TYPES
Router-LSA  is a Type 1 LSA. It describes a router's link state and cost.Every router on the OSPF network generates Router-LSAs and advertises them within its areas.

Network-LSA  is a Type 2 LSA. It describes the link states of all routers on the local network segment. A DR generates Network-LSAs and advertises them within its area.

Network-summary-LSA is a Type 3 LSA. It describes inter-area routing information. An ABR generates Network-summary-LSAs and advertises them within its area to notify destination addresses of routes from this area to other areas.

ASBR-summary-LSA is a Type 4 LSA. It describes routes to an ASBR. An ABR generates ASBR-summary-LSAs and advertises them to areas except the area to which the ASBR is located.

AS-external-LSA is a Type 5 LSA. It describes routes to destinations outside an AS. An ASBR generates AS-external-LSAs and advertises them to all areas except stub areas and NSSAs.

<sysname> display ospf lsdb

          OSPF Process 1 with Router ID 1.1.1.1                                 

                  Link State Database                                           

                                                                                

                          Area: 0.0.0.0                                         

 Type      LinkState ID    AdvRouter          Age  Len   Sequence   Metric      

 Router    2.2.2.2         2.2.2.2             98  36    8000000B       1       

 Router    1.1.1.1         1.1.1.1             92  36    80000005       1       

 Network   10.1.1.2        2.2.2.2             98  32    80000004       0       

 Sum-Net   10.2.1.0        2.2.2.2            286  28    80000001       1       

 Sum-Net   10.2.1.0        1.1.1.1            282  28    80000001       1       

 Sum-Asbr  2.2.2.2         1.1.1.1             61  28    80000001       1


                                                                                

                          Area: 0.0.0.1                                         

 Type      LinkState ID    AdvRouter          Age  Len   Sequence   Metric      

 Router    2.2.2.2         2.2.2.2             99  36    8000000D       1       

 Router    1.1.1.1         1.1.1.1             93  36    80000005       1       

 Network   10.2.1.2        2.2.2.2             99  32    80000004       0       

 Sum-Net   10.1.1.0        2.2.2.2            534  28    80000001       1       

 Sum-Net   10.1.1.0        1.1.1.1            288  28    80000002       1       

 NSSA      10.10.1.0       2.2.2.2            293  36    80000009       1       

                                                                                

                                                                                

                  AS External Database                                          

 Type      LinkState ID    AdvRouter          Age  Len   Sequence   Metric      

 External  0.0.0.0         2.2.2.2           1128  36    80000001       1       

 External  10.10.1.0       2.2.2.2            538  36    80000002       1       

                                                                                

          Type 9 Opaque (Link-Local Scope) Database. Area: 0.0.0.0              

 Type      LinkState ID    AdvRouter          Age  Len   Sequence   Interfaces  

 Opq-Link  10.3.0.0        2.2.2.2             12  44    80000001    10.1.1.1   

                                                                                

                  Type 10 Opaque (Area-Local Scope) Database                    

 Type      LinkState ID    AdvRouter          Age  Len   Sequence   Area        

 Opq-Area  1.0.0.1         1.1.1.1              4  200   80000003    0.0.0.0    

 Opq-Area  1.0.0.0         1.1.1.1           1641  28    80000001    0.0.0.0