- Bronze, 100 points or more
In my OSPF lab, all routers have a Loopback 0 interface with the address 222.1.x.1; where x equals the number of the router's letter (RouterA=1, B=2, etc).
I have a backbone area of Routers E, F, G, and H; all connected via tokenring and all on subnet 10.4.4.0/24.
Router E is the ABR to Area 1; via a serial link (18.104.22.168/30; A=.1 E=.2).
Within Area 1, Router A has an Ethernet link to Router B; 10.3.3.0/24.
I have full connectivity everywhere; the routing tables all look like I think they should.
The thing that's confusing me is that the 22.214.171.124 link isn't showing up in the "sh ip ospf database" output anywhere in Area 1.
All of the backbone routers list a Link ID of 126.96.36.199 under "Summary Net Link States (Area 0)" with 188.8.131.52 as the adv router. That's all well and good but how come the routers in Area 1 don't list it under "Net Link States (Area 1)?"
If so, why ain't they...
If not, why's that?
The reason behind this is tied to the function on each LSA as to what it advertises.
- A Router LSA's function is to advertise a network node and its connected links. How do we identify a node on the SPF-tree? By its router-id.
This is why the Router LSA's link-id is the router-id.
- A Network LSA's function is to advertise a multiaccess network and its DR. So practically the best choice to identify this LSA is with the IP address of the DR on this multiaccess network.
- A Summary LSA's function is to advertise subnets in other areas. So the best choice to identify this LSA with the subnet address that it advertises.
And so on.
In case of Summary and External LSA's there is no sense to advertise network nodes and DR's of OTHER areas, because the OSPF routers in THIS area do not need that topology information. THIS area's OSPF routers calculate the SPF-tree for THIS area only.
OSPF routers use the topology information of only ares that they are members of.
I hope I explained understandably.