Question about OSPF LSA type 4

[annayuzhao]

Does anybody know what's the purpose of LSA type 4, ASBR summary LSA. My understanding is LSA 5 (AS external LSA) will flood to all the routers which are in OSPF autonomous. So why we need LSA 4, unless LSA 4 is used to update the gateway to all intra routes about how to reach the ASBR because I see in LSA 5 packet ,the advertising router is ASBR itself not ABR, which means that internal router won't know how to reach ASBR. To solve this, type 4 is used. Is this right?

Thanks

[rais]

Yes, your understanding is correct. LSA type 4 contain a route to a host i.e. ASBR.

Thanks.

[ipotts]

One further point is that the ASBR will not generate the type 4, it will generate a type 1 LSA with the ASBR flag set. The ABR will receive this local scope LSA, and inject it into area 0 as a type 4, with LSA.

[ruwhite]

Correct. When an ABR receives a router LSA with the E bit set, it generates a type 4 into area 0 providing the path to the ASBR. This is used for two reasons, to provide a path to the ASBR for routers outside the area (in case the route is summarized/etc), and to provide the cost of reaching the ASBe, in case the metric is summarized/etc.

Russ.W