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I have always read that a router is a layer three device. Then how could it use RIP which is an application layer protocol ?

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    – Ron Maupin
    Feb 19 '18 at 4:53
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The layers of the OSI Model are exactly that, a model. It exists today as a framework to help understand how networks function. A network connected device, generally speaking, doesn't "live" as simply a single element in a single layer. In order to function, it participates at many OSI layers simultaneously. Some network elements (routers, switches, etc) may have primary functions, however they will utilize elements from other layers.


For example, to your specific question, a router's primary function is to route packets. And you are correct, IP Packets exist at Layer 3 of the OSI model. However, in order to gather information about WHERE to route these packets, a router often uses a routing protocol (RIP, OSPF, EIGRP, BGP, IS-IS) to determine reachable networks.

These are applications, however they are application that are exchanging information about route destinations so that the router can perform it's primary Layer 3 function: Route Packets.

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Don't confuse OSI layer abstractions. RIP is a protocol used to share addresses which are understood to be part of the L3. Think of applications as for end user.

RIP is to Layer 3 as Quake online gaming is to Layer 7.

Does a layer understands application layer protools?

Layers can be thought as a structural design. Another way to put it, layers are a common ground for all to understand.

Layer 3 whether it is Layer 3: Network Layer from the OSI model or Internet Layer from the TCP/IP stack, they are just abstractions/models.

RIP as other existing protocols when implemented by vendors can run as daemons (services) or tasks, you may think of this as an application, however this is at the implementation side of things, if you think about OSI this is Layer3 because it's used to communicate end points.

Taken from wiki Osi model

Protocols enable an entity in one host to interact with a corresponding entity at the same layer in another host.

So if we try to boil it down hardcore, you can think of L3 being source IP and destination IP. An application has to do with the end user. As an Internet user, you don't need RIP to know all addresses from the internet on your computer because what you are needing is just the application, i.e online Game, as long as your PC has a way out i.e default gateway (your modem).

The application layer is the OSI layer closest to the end user, which means both the OSI application layer and the user interact directly with the software application. Such application programs fall outside the scope of the OSI model.

ISP's on the other hand need a way of communicating and to do so they need a set of rules that everyone understands in order to share the necessary information (IP addresses), which can be RIP, OSPF, BGP or any other depending on the extent.

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