In our Introduction to OSPF we had discussed that OSPF builds complete map of topology like a navigation system. In this article we shall learn different LSA types, OSPF uses a LSDB (Link State Database) and this database contains different types of LSAs (Link State Advertisements).
Table of contents
OSPF LSA Overview
There are 8 different LSAs types which are listed below.
- LSA Type 1: Router LSA
- LSA Type 2: Network LSA
- LSA Type 3: Summary LSA
- LSA Type 4: ASBR Summary LSA
- LSA Type 5: Autonomous System External LSA
- LSA Type 6: Multicast LSA
- LSA Type 7: NSSA LSA
- LSA Type 8: External Attribute LSA for BGP
LSA Type 1: Router LSA
In this LSA you will find a list with all the directly connected links of this router. Router LSA is the LSA type used in standard areas. Each router within the area will flood a type 1 router LSA within the area. Keep in mind that the router LSA always stays within the area. Information Included in LSA Type 1 (Router LSA) is Router ID, Router interfaces, neighbors, ip addresses and cost.
LSA Type 2: Network LSA
The second LSA type (network LSA) is created for multi-access networks: Network LSA is the other LSA type used in standard areas. Multi-Access Network included broadcast and non-broadcast which required DR/BDR. This LSA is sent by DR (Designated Router). The main aim of Network LSA 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.
LSA Type 3: Summary LSA
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.
Type 1 router LSAs always stay within the area. OSPF however works with multiple areas and you may want full connectivity within all of the areas. R3 is flooding a router LSA within its area 31 so R1 will store this in its LSDB. R2 and R4 also need to know about the networks in Area 31. R1 is going to create a Type 3 summary LSA and flood it into area 0. This LSA will flood into all the other areas of our OSPF network. In this way all the routers in other areas will know about the prefixes from other areas.
Summary LSA does not summarize route between areas, for summarization we have ospf summarization commands that let us summarize inter-area OSPD routes.
How do i identify a Type 3 Summary LSA in OSPF routing table?
LSA Type 4: ASBR Summary LSA
ASBR Summary LSA 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.
In above topology, we have R4 router acting as ASBR because it is redistributing EIGRP routes into OSPF domain. R3 is an ABR as it have interfaces in more than one areas, when R3 will receive LSA from R4 it will convert it into Type 4 Summary LSA and floor it into area 0.
LSA 4 is required to tell the routers in other areas how to reach the ASBR to get to external network eventually.
What is the difference between an LSA 3 and an LSA 4?
LSA Type 5: Autonomous System External LSA
Now with Type 4 LSA routers in OSPF domain are aware how to reach ASBR, lets advertise a route from EIGRP domain and redistribute it to OSPF. ASBR will take care of this and create Type 5 external LSA. External prefixes are identified in OSPF routing table as “O E1” & “O E2”.
How to identify Type 5 route on OSPF routing table?
LSA Type 6: Multicast LSA
LSA Type 6 is often used and not supported on Cisco, as for Multicast routing another specific protocol is available which is called PIM (Protocol Independent Multicast).
LSA Type 7: NSSA LSA
In our example topology we have area 24 router connected to EIGRP router. Area 24 is standard OSPF area which allows Type 5 LSA, but if it would be an NSSA area which does not allow Type 5 LSA we can not redistribute EIGRP routes to OSPF NSSA area. Since type 5 is not allowed in NSSA Area, that’s why we have type 7 external LSA that carries the exact same information but is not blocked within the NSSA area. R3 will translate this type 7 into a type 5 and flood it into the other areas.
Normally Stub Areas don’t allow redistribution, but as a Not So Stubby Area (NSSA), redistribution is allowed. But, redistributed prefixes are not seen as Type 5 LSA, they are seen as Type 7 LSA.
LSA Type 8: External Attribute LSA for BGP
Normally BGP prefixes are redistributed into OSPF or any other routing protocol, and BGP attributes are lost. But, you may need to carry BGP attributes with your Autonomous System between the Routers.
Let’s say, for the given destination IP prefix, you have two exit points from your network, and for the outbound direction, you want to prefer one of those exit points as Primary. You can use this BGP Local Preference attribute. Two Routers exchange the prefixes with each other, and when they check the BGP Local Preference attribute, which every Router has the higher Local Preference, that router is used as an exit point by both of the routers.
But BGP local preference e attributes cannot be carried in OSPF normally. Because of reachability, you need to redistribute from BGP to OSPF, and if you redistribute, attributes are lost.
Type 8 LSA in OSPF LSA Types, was considered for this purpose. BGP Attributes would be carried even if we would redistribute. But yet another LSA that we don’t use in computer networking. Instead of this LSA, IBGP – Internal BGP is used in the networks. Hope Type 8 LSA as one of the OSPF LSA Types is understood better now.
That’s it, In this lesson we learn different OSPF LSA Types and discussed one by one in detail. Hope you have like it. See you in next lesson.