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08: Network layer - Routing vs routed protocols & decapsulation part 2

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Routing : 

Routing is allocated the greatest  part in this course because it is one of the most important topics that a candidate has to master in CCNA R&S. This lesson gives just a general idea about routing, for having little knowledge about routing will help in building a firm grasp of what the Network layer is as well as how data is dealt with when reaching this layer. Besides, this lesson lays the ground to the routing section.  

As a definition to the term ‘routing’, it comes from routers ; it means connecting routers together by a given protocol. The following figure contains a set of routers along with their routing tables. You might be asking what routing tables are you talking about ! do not freak ! the pictured figure encompasses routers connected together by serial cables because in order for all of these routers to see each other, each router must have a routing table that contains some information about the other routers. Each routing table comprises the routing protocol and ip address of each router as well as metric, interfaces which will be covered in routing section.

 Without routing protocols, routers cannot see and communicate with each other, or rather forward packets to one another. It is up to the admin to configure the router with any routing protocols like RIP, RIPv2, EIGRP, OSPF, ISIS, among others ( these are dynamic routing protocols), or default route, static route (which are static protocols). One a router runs one of the dynamic protocols as well as other router, they are ready to forward packet to one another.     Simply put, the router whose ip address is (interface) 10.0.0.1 cannot see the router with 11.0.0.1 ip address (interface) unless a routing protocol is running.

Once routing protocols start functioning, each router constructs its own routing table containing information about other routers. They will have, let us say, a comprehensive map of the network. It critically important to point out that the connection in this layer is connectionless because the protocol used is IP ( or IPv6, IPx, Appletalk). IP protocol is regarded connectionless on the ground that IP datagram can travel from the sender to receiver without the recipient having to send an Acknowledgment.


N.B. A router can see only the next interface if there is no routing protocol running. That is, only one interface of the router that is directly connected. In this case, we can either configure the router by a dynamic or static protocols (you will learn more about that in the next lessons). 


Network layer protocols :


As any other layer, a plethora of protocols reside within the Network layer. IPv4 (Internet Protocol version 4) is one of the most known protocols which are hugely used in data delivery. It is the actual protocol that each device connected to the internet uses to transmit data. 


IPv6 is the next generation protocol. It comes because IPv4 addresses are about to run out. It is currently used in some part of the world, but it will totally replace the preceding protocol on the horizon ( nearly next 30 years). 


IPx (Internetwork Packet Exchange) is another protocol that works at the this layer. This protocol is no longer used ; it had been used before IPv4 came. 


Appletalk is a protocol which also operates within this layer. It is merely used among devices which are manufactured by Apple company. 


Note : all of the above four protocols are referred to as routed protocols.


Routing protocols reside also within this layer. RIP, RIPv2, EIGRP, OSPF, ISIS are all routing protocols that allow router to know and communicate with each other, which means that two computer from different networks cannot reach each other if their routers are not routed by a routing protocol.


Routed vs Routing  protocols :


Now you might be asking what it is the difference between a routing and routed protocols. Routing protocols are only used between routers. It makes possible for routers to build and maintain routing tables. Routed protocols, on the other hand, are what routing is used for.  The following figure may make it plain for you.



Let us suppose that people in the buses are data to be transmitted, and buses are routed protocols, and road signs are routing protocols. As you see, the routed protocols (IP addresses) which are buses carry people which are data, and routing protocols (like OSFP or EIGRP) direct or give them the way to go.

Decapsulation :

Decapsulation at the Network layer is no different from decapsulation at the other layers. When packets get at the Network layer of the receiver, the header which was already generated (IP header) at the Network layer of the sender is removed so that data move up to the above layer which is the Transport layer. So, decapsulation reverses the process of encapsulation which takes place at the source machine.

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08: Network layer - Routing vs routed protocols & decapsulation part 2 Reviewed by BOUFTIRA on 8:18:00 AM Rating: 5

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