OSPF question

White Wizard

I have the following question in Bosons ICND 2 software and I am slightly confused:



If you look at the commands entered into Router A, you will see the default-information originate command which distributes Router A's gateway of last resort to all routers in OSPF. Why is Boson saying Router D's gateway of last resort is the s0/2 int on Router C then?

If you look at the OSPF configuration, you can see all 4 routers are in OSPF. There is an IP route cmd as well at the bottom saying all traffic not in router A's table to go out int fa0/1 but I do not see how that ties into this problem at all as Router D is in the OSPF configuration so its gateway of last resort should be Router A's gateway of last resort.

Thank You.

EDIT: I think Boson is assuming you already know Router A is the gateway of last resort but they want to know how the router will go about sending traffic to that int. Sound right?

Jon_Cisco

I believe if a default route already exists it will not replace it. I'm not sure if that's whats going on here but something to look at.

AwesomeGarrett

I'm not following you on your confusion. Are you confused as to why Router D sees the gateway of last resort as Router C's interface se0/2?

White Wizard

AwesomeGarrett wrote: »

I'm not following you on your confusion. Are you confused as to why Router D sees the gateway of last resort as Router C's interface se0/2?

Yes. Its default gateway should be Router A's default gateway due to the default info cmd on Router A no?

Also, Kai Greene fan I see?

networker050184

Look into how the LSA is generated for the default route. Who generates the LSA? What is the forwarding address? How would router D get there? Might clear it up a bit for you.

White Wizard

networker050184 wrote: »

Look into how the LSA is generated for the default route. Who generates the LSA? What is the forwarding address? How would router D get there? Might clear it up a bit for you.

Unfortunately this is not a simulation question, I cannot access the cmd line for any of the routers in this question.

AwesomeGarrett

We all need motivation

It appears you may have gotten confused on LSA recursion vs the routing table. OSPF is going to find the LSA for this route/link in the LSDB. It is then going to compute the shortest path to this link via type 1 LSA, type 2 LSA, etc... Once the SPF algorithm is complete it will have the next-hop information for the destination and input that in the routing table.

If you were to lab this out, you would see the LSDB have an LSA for the default route via the router advertising the route and the next hop to reach that router is the se0/2 interface of Router C.

networker050184

You don't need a router. Plenty of Cisco docs!

http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/47868-ospfdb9.html

White Wizard

networker050184 wrote: »

You don't need a router. Plenty of Cisco docs!

How OSPF Injects a Default Route into a Normal Area - Cisco

I was actually just going over that page. I think I'm over analyzing the question as I know what the default info originate cmd does and understand the route the router will take to get there. I don't agree with the wording used in the question which is what confused me.

Thanks for the input everyone!

networker050184

I think your confusion is because the LSA is not going to contain router A's gateway of last resort. The LSA just has 0.0.0.0 via 1.1.1.1 (for example that is router A's router-id). So there is nothing to put in router D's routing table about router A's gateway of last resort. It just knows to go to router A. It already has a SPT built of the entire area so it knows how to get there. That is via it's interface to router C.