Original subtitles

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So what is the difference between a routed protocol and a routing protocol?

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Now a routed protocol carries user data, example, would be IPv4 or IPv6.

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When you use a higher layer protocols such asHTTP or FTP

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that protocol is using a lower layer protocols

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such as IPv4 or IPv6 to carry the user data from one device to another.

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So when you connect to a website

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and you're viewing a webpage that would be deemed to be routed data.

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Data from the web server is being routed to your PC

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now the addressing scheme used by routed protocol is based on this specific protocol

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such a IPv4 using a 32 bit address and IPv6 using 128 bit address.

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Now how do routers know where devices are in a network?

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As an example, my PC is based in the UK

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but when I go to Facebook.com traffic is sent from my PC

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to Facebook based in California and back again,

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How does my device actually reach the facebook service

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in a data center in California and how does the data get back to my PC in the UK?

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How is the user data forwarded from one device to another?

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Now it’s important to realize that every router

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along the path between my PC in the UK and facebook.com

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makes an independent routing decision.

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As an example, if I trace to facebook.com

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and in this case, I’m going to set the timeout

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to a low value such as 50 milliseconds.

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Traffic is being forwarded from my PC

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on hop-by-hop bases from 1 router to the next

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until it hopefully reaches facebook.com

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every one of these hops is an independent router

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making independent routing decisions.

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Now Facebook and a lot of other big websites

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will have data centers scattered around the world

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so my traffic may not actually be going all the way to the US

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but maybe going to a local data center in Europe

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that all depends on how the network is configured.

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These routing decisions made by routers

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is known as the hop-by-hop routing paradigm

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with unicast traffic routing is based on destination address only

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not on source address

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so routers decide where traffic goes based on the destination IP address

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in my example it's this address and routers will decide

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where to route traffic based on the destination IP address

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and information stored in routing tables.

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Every router along the path needs to determine

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an outgoing interface to forward traffic to reach the destination IP address.

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To do that routers communicate information about networks using routing protocols.

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They will then determine the best path to the destination IP address

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using criteria specific to that individual routing protocol.

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As an example, RIP uses hop count to determine the best path.

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OSPF uses the bandwidth of interfaces to determine the best path.

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EIGRP uses bandwidth and delays to determine the best path.

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So routing protocols are used to automatically advertise networks

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between routers and that’s how routers learn

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about the available networks in a topology.

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It’s also important to note

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that if a router doesn’t know about the destination IP address.

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In other words, information about that destination IP address

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is not in its routing table it will drop unicast packets.

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Destination IP address is a match against networks

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and subnets in the routers routing table

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so if a router receives traffic going to an IP address of let say 10.1.1.1

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but that IP address doesn’t match a network in the routers routing table.

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The router will drop the packets because it doesn’t know where to forward them

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Essentially if you tell a router send traffic to IP address 10.1.1.1

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and the router doesn’t know how to get to that network or IP address.

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The router will drop that traffic

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if there is no matching route in the routing table traffic gets dropped.

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This applies specifically to unicast packets

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where we are doing routers based on destination IP address.

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So in summary, routing protocols allow routers to learn about destination networks.

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That facilitates the change of routed information between devices.

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Routers can dynamically learn about networks in the topology

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and can then make routing decisions based on different criteria

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such as bandwidth hop count or delay to determine the best path.

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Routers then simply choose an outgoing interface based on the routing table

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and will then forward packets out of that interface to reach a destination.