I see a lot of confusion about the Type-4 LSA and what it does. This confusion is high in the CCNA world because little is explained about it and one might think it is useless; however, this is valuable when you’re learning or doing route redistributing into OSPF using External type-2 redistributed routes with multple ASBRs redistributing the same external routes. Read more…

When you redistribute a route into OSPF the default metric is External Type-2, the route remains static (by default 20) and the cost never increases. ASBRs advertise these as Type-5 LSAs and flood them throughout the OSPF domain and ABRs will flood them inside normal OSPF areas or will NOT advertise them into stubby areas (relying on a default route inside that stubby area). These article applies to normal OSPF areas.

When that Type-5 LSA is received on a ABR the receiving ABR router records the LSID (which is the router ID of the ASBR) and that ABR calculates the cost to reach the advertising ASBR router and generates a Tpye-4 LSA; thus, the reason for a Type-4 LSA is for an ABR to advertise its cost to reach the ASBR to routers inside a specific normal area.

There is much more to explain but I did say this would be brief.


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