BROCADE IP PRIMER

Link State Advertisement (LSA)A NSSA will typically have an ASBR in its area (like our example). The idea isthat it probably won't be the only ASBR in the autonomous system (again, likeour example). This is a way that you can say, “I'll accept routes from ASBRs thatare in my area, but not routes from any ASBRs that are not in my area.”Totally Stubby AreaThe names keep getting sillier, don't they? We don't have an example for thisone in our image. A Totally Stubby Area is an area that won't accept routesfrom another autonomous system (like a stub area), but it also won't evenaccept routes from other areas. You would typically only configure a TotallyStubby Area if there was only one link connecting this area to the backbone,and it was a router that was not very powerful. In this situation, the only routethat would be advertised would be a default gateway to Area 0.This is similar to a Stub Area, but let me rehash the difference again. A StubArea will accept routes from within the autonomous system (e.g., routes toother areas advertised from Area 0). A Stub Area will not accept routes fromoutside the autonomous system. A Totally Stubby Area will not accept routesfrom within the autonomous system or without the autonomous system. Nothingother than a simple default gateway to its ABR in Area 0 is accepted.Link State Advertisement (LSA)A Link State Advertisement (LSA) is a packet that is sent between OSPF routers.They come in six types (that we'll talk about; there are actually 11,currently). LSAs are the packets that contain routing information that is passedfrom router to router. Different types travel to and from different routers.• Type 1 (Router LSA). These are sent by every router participating inOSPF; they list all of the router's links (and, thus, the connected routes forall interfaces participating in OSPF); these are only sent to routers withintheir respective area• Type 2 (Network LSA). These are sent by the DR only; these packets listall of the routers on the same broadcast multi-access segment; if youthink of the DR as the king, this advertisement is listing his loyal subjects• Type 3 (Summary LSA). These are generated by ABRs only; they summarizeall of the routes within their respective areas, and advertise them toArea 0; Area 0, in turn, advertises these summarized routes to otherareas; Totally Stubby Areas would receive only a default gateway• Type 4 (ASBR-Summary LSA). These are generated by ASBRs only; forthose areas that need nothing more than a summarized gateway toroutes outside the autonomous system, a Type 4 LSA is sent; a Stub Areaor NSSA would receive these; a Totally Stubby Area would not• Type 5 (External LSA). These are the non-summarized routes generatedby ASBRs; these are advertised to Area 0, and Area 0 will, in turn, advertisethem to the rest of the areas (except for Stub, NSSA, and TotallyStubby areas)Brocade IP Primer 249