Differnece between ospf and frame relay

Hi, I was wondering if someone can just clarify to me the differnece between ospf and frame relay. Why does one operate at the data link layer and one at the network layer? Is it because frame relay DLCI's operate at the data link later. Also, is ospf just limited to the interior network of a company? E.g if there are multiple sites across the country then can both frame relay and ospf operate and communicate with the sites or can only frame relay do that? I'm not quite sure on the differnece.

Thanks for the clarity

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networker050184

If you don't understand I suppose start at the beginning and learn the OSI model. They operate on different levels with completely different uses. Learn how devices communicate at each layer.

Robbo777

I know how the osi model operates, my mind has just gone a blank with this ONE particular query that's all. I think if someone was to answer my questions i posted it would probably all connect again

OctalDump

OSPF is a routing protocol. It runs at layer 3 and so it doesn't care about the layer 2. For example, you could have OSPF running over ethernet on the inside ports, and over a serial link on the WAN ports. Once the layer 2 link is established, OSPF can communicate its routing information (which interfaces and routers connect with which IPs) over the link.

Frame relay is one option for layer 2 WAN links. It needs to be able to set up at layer 2. It doesn't care really what layer 3 or higher protocols are running over it.

One very bad analogy is telephone. If you are making a telephone call to someone, you don't care if they are using mobile or fixed line. You just dial the number and talk. In this instance, you can think of the dialing a number and connecting as layer 3, the conversation as layer 4 (and higher), and the actual radio or wired link as layer 2. The layer 2 link doesn't care if it is voice or data on the phone connection, the layer 4 link doesn't care if it is going over wires or radio.

As networker050184 says, they have completely different uses.

This is why the OSI model is so nice, each layer can be taken by itself.