Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

Question about OSPF LSA type 4

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?



Re: Question about OSPF LSA type 4

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


New Member

Re: Question about OSPF LSA type 4

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.


Re: Question about OSPF LSA type 4

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.