Frame Relay vs OSPF

bglamb

Hi folks.

So I saw a practice question that contained the following topology and this possible answer:



"If OSPF is running on all routers, then static Frame Relay mappings are required for this topology to work correctly."

Now to me, this looks legit. OSPF will see the networks connected to R2 and R3 and the frame relay network, but it will not route *inside* the Frame Relay network, as it's a single subnet. Now the routers will dynamically learn the IP addresses of the routers that are directly connected, via RARP, but R2 and R3 will never dynamically learn how to reach each other. They need a static mapping telling then to go via R1 if they need to communicate and, without this, the topology will not work.

However this is listed as an incorrect answer on the site I'm using, and I'm not sure why. Can somebody help me see where I am going wrong?

Thanks folks!

Jon_Cisco

Without seeing the choices and looking into it to deeply I think it's a question of does the layer two connectivity effect the layer 3 routing protocol.

Seems like each routers would connect via the frame relay connection. Then ospf would be configured for the networks that are attched.

davenull

While it's true that 'frame-relay map' commands are needed here, they probably mean that you also have to use 'ip ospf network point-to-multipoint' command on all frame-relay interfaces so the routers can become OSPF neighbors.

bglamb

Thanks to both of you.

So, OSPF to one side for a minute, without entering static Frame Relay mappings, are you saying that R2 can ping R3? Because it was my understanding that this would not work!

davenull

No, you need frame relay maps for R2 to ping R3 and vice versa.

But note how that answer is worded: "If OSPF is running on all routers then static Frame Relay mappings are required for this topology to work correctly".

As a matter of fact, Frame Relay mappings are required here regardless of whether you are running OSPF, EIGRP or even static routes, so that answer is not technically correct. It's a bit tricky I guess.

bglamb

Uhhh....yeah that sounds like that means it *is* correct. It's not like they said "Static Frame relay mappings are only necessary if OSPF is running on all routers."

Tbh, as long as you're saying my basic understanding of the concepts seems fine, that's what I really wanted to check. I was worried I was really misunderstanding something here. If it's a case of it just being a crappy question, then we need say no more!

DCD

What out seeing the question and answer it kind of hard know what they are looking for but it could be that you need to change the priority on the spoke routers.

bglamb

The question is just a simple "Which of the following is true" and all the other answers are just checking that you know how DLCIs work. There are 3 obvious wrong answers, and 1 obvious right answer, and then this one. So yeah, I understand why the other answers are what they are, I just thought that the inclusion of this answer as a wrong one meant I was misunderstanding something.

But from the sounds of it, the question is just wrong, basically.

Thank you all for your help.