OSPF LSA Types

In OSPF, Link-State information, in other words, Routing information is exchanged between routers. This exchange is done with LSAs (Link State Advertisements). There are eleven different OSPF LSA types and each of them has a special purpose. Some of the OSPF LSA types are the main LSA types that are used in OSPF operations often. Let’s see all the LSA types:

• Router LSA (Type 1)
• Network LSA (Type 2)
• ABR Summary LSA (Type 3)
• ASBR Summary LSA (Type 4)
• ASBR Externel LSA (Type 5)
• Group Summary (Type 6)
• NSSA External LSA (Type 7)
• External Attributes LSA (Type 8)
• Opaque LSAs (Type 9, 10, 11)

 

Now, let’s talk about these OSPF LSA types detailly. Here, it is better to explain these LSA types with different area types and their LSA Adversisement requirements. So, the below diagram will be very useful for you.

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OSPF Router LSA (Type 1)

Router LSA (Type 1) is the LSA type used in standard areas. The aim of this LSA type is giving information about the router. This LSA includes information like Router ID, Router interfaces, neighbors, ip addresses and cost. Router LSA can not pass ABR, so it can not reach to the other areas.

 

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You can also reach other Cisco CCNP ENCOR lessons

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OSPF Network LSA (Type 2)

Network LSA (Type 2) is the other LSA type used in standard areas. This LSA is sent by DR. The main aim of this LSA type is listing the connected routers in the segment and informing the other routers. This LSA includes information like DR, BDR IP addresses, subnet masks. Network LSA can not pass ABR, so it can not reach to the other areas.

 

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OSPF ABR Summary LSA (Type 3)

ABR Summary LSA (Type 3) is generated by ABR (Area Border Router) to advertise one Area’s networks to other Areas. ABR Summary LSA (Type 3) includes all prefixes available in the Area.

 

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OSPF ASBR Summary LSA (Type 4)

ASBR Summary LSA (Type 4) is generated by ABR (Area Border Router) to inform its areas about how to reach the ASBR (Autonomous System Border Router). ASBR Summary LSA (Type 4) includes ASBR’s Router ID.

 

OSPF ASBR Externel LSA (Type 5)

ASBR Externel LSA (Type 5) is generated by ASBR (Autonomous System Border Router) for advertising External Routes for Open Shortest Path First.

 

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OSPF NSSA External LSA (Type 7)

NSSA External LSA (Type 7) is generated by ASBR (Autonomous System Border Router), for advertising external routes to NSSA. Then the ABR (Area Border Router) in this area convert Type 7 LSA to Type 5 LSA, and then advertise it to other Areas.

 

 

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OSPF Extended LSA Types

There are also some other LSA types used in Open Shortest Path First. These specific LSA types are also used for specific purposes. These LSA types are:

• Group Summary (Type 6)
• External Attributes LSA (Type 8)
• Opaque LSAs (Type 9, Type 10, Type 11)

 

Group Summary (Type 6) is used in Multicast. This type is not supported in some vendors.

 

External Attributes LSA (Type 8) is rarely used when BGP info is carried across OSPF AS.

 

Opaque LSAs (Type 9, Type 10, Type 11) are designated for future application specific use. For example Type 10 Opaque LSA is used for OSPF Traffic Engineering.

 

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OSPF Routes in the Routing Table

When you configure Open Shortest Path First as your IGP router, you will see the OSPF routes in the Routing Table. In the Routing Table there are some specific codes for these Routing informations. Let’s talk about these specific signs.

 

O means that, this route is propagated within the area. LSA 1 generated by all the routers in the area and LSA2 by DR.

 

O IA means that, this route is propagated from different area.

 

O E1 means that, this route is advertised externally, this is an external route to OSPF that is send from ASBR. Here, there is a special formula:

E1 = Cost to ASBR + Advertised External CostO E2 means that, this is an external route to OSPF that is send from ASBR. This is Default External route sign. Here, there is a special formula:

E2 = Advertised External CostN1/N2 means that, these routes are NSSA routes if they are used instead of E1 or E2.

 

OSPF is a very important and commonly used Routing Protocol. Our OSPF Adventure is going on, so kepp on with the other lessons;)