Why does a router create an ASBRSum (Type 4) LSA?
A.
An ABR creates an ASBRSum (Type 4) LSA with the information contained in NSSA (Type 7) LSAs it receives from NSSAs.
B.
An ABR creates an ASBRSum (Type 4) LSA to describe the router ID of ASBRs located in other areas.
C.
An ASBR creates an ASBRSum (Type 4) LSA to describe its router ID so that routers in other areas can reach the external networks it advertises.
D.
An ABR creates an ASBRSum (Type 4) LSA to summarize routes received from other areas.
Explanation:
Autonomous system boundary routerAn autonomous system boundary router (ASBR) is a router that is connected to more than one Routing protocol and that exchanges routing information with routers in other protocols. ASBRs typically also run an exterior routing protocol (e.g., BGP), or use static routes, or both. An ASBR is used to distribute routes received from other, external ASs throughout its own autonomous system. (An interactive ASBR simulation shows how an ASBR creates External LSA for external addresses and floods them to all areas via ABR.) Routers in other areas use ABR as next hop to access external addresses. Then ABR forwards packets to the ASBR that announces the external addresses
What is the type 4 ASBR LSA for? The type 4 LSA is an LSA that instructs the rest of the OSPF domain how to get to the ASBR so that other routes in the OSPF domain can route to external prefixes redistributed into OSPF by the ASBR. If we have no way to reach the actual ASBR that redistributed the route, we obviously cant reach the external route. Makes sense. However, there is a lot of bad information out there about what type of router actually generates the type 4 LSA. Sometimes you hear it said that the ASBR injects a type 4 LSA into OSPF. This is not true. Actually, area border routers (ABRs) are responsible for injecting type 4 LSAs.
http://blog.markom.info/2012/02/ospf-database-on-junos-external-type-5-and-asbr-summary-type-4-lsa/
http://astorinonetworks.com/2011/06/06/understanding-ospf-type-4-lsas/