In this video we're going to discuss routing.
We'll look at the basics of IP routing and look at different routing protocols including distance vector
and link-state routing protocols. We'll compare the differences between distance vector routing protocols
such as RIP and link-state routing protocols such as OSPF.
We'll also discuss some of the methods that distance vector routing protocols use to stop loops. Routing
protocols are very important because they advertise networks and use various mechanisms to prevent loops.
Before discussing routing protocols such as OSPF
or EIGRP you need to know the difference between a routed versus a routing protocol.
You should also be able to differentiate and explain the advantages and disadvantages of using static
routes versus dynamic routing protocols.
So why would you want to use a static route versus a dynamic writing protocol such as OSPF
and as mentioned we want to talk about distance vector and link-state routing protocols.
So what is the difference between a routed protocol and a routing protocol?
Now a routed protocol carries user data.
Examples would be IP version 4 or IP version 6.
When you use a higher layer protocol such as HTTP or FTP that protocol is using a lower layer protocol such
as IP version 4 or IP version 6 to carry the user data from one device to another.
So when you connect to a website and you're viewing a web page that would be deemed to be routed data,
data from the web server is being routed to your PC.
Now the addressing scheme used to buy routed protocols is based on the specific protocol such as IP
version 4 using a 32 bit address and IP version 6 using a 128
bit address. Now, how do routers know where devices are in a network?
As an example, my PC is based in the UK but when I go to facebook.com traffic is sent from my PC to Facebook
based in California and back again.
How does my device actually reach the Facebook servers in a data center in California
and how does the data get back to my PC in the UK?
How is the user data forwarded from one device to another?
Now, it's important to realize that every router along the path between my PC in the UK and Facebook
.com makes an independent routing decision. As an example, if I trace to Facebook.com
and in this case, I'm going to set the time out to a low value such as 50 milliseconds.
Traffic is being forwarded from my PC on a hop by hop basis from one router to the next until it hopefully
reaches Facebook.com.
Every one of these hops is an independent router making independent routing decisions.
Now Facebook and a lot of other big websites will have data centers scattered around the world.
So my traffic may not actually be going all the way to the U.S. but maybe going to a local data center
in Europe.
That all depends on how the network is configured.
These routing decisions made by routers is known as the hop by hop routing paradigm with unicast traffic
routing is based on destination address only not on source address.
So routers decide which traffic goes based on the destination IP address and by example its address and
routers will decide where to route traffic
based on the destination IP address and information stored in routing tables. Every router along the
path needs to determine an outgoing interface to forward traffic to reach the destination IP address.
To do that routers communicate information about networks using routing protocols.
They will then determine the best path to the destination IP address using criteria specific to that
individual routing protocol.
As an example, RIP uses hop count to determine the best path, OSPF uses the bandwidth of interfaces to
determine the best path.
EIGRP uses bandwidth and delay to determine the best path.
So routing protocols are used to automatically advertise networks between routers and that's how routers
learn about the available networks in a topology.
It's also important to note that if a router doesn't know about a destination IP address.
In other words information about that destination, IP address is not in its routing table
it will drop unicast packets. Destination IP addresses are matched against networks and subnets in the
routers routing table.
So if a router receives traffic going to an IP address of let's say 10.1.1.1 but that
IP address doesn't match a network in the routers routing table
the router will drop the packets because it doesn't know where to forward them.
Essentially if you tell a router, send traffic to IP address 10.1.1.1 and the
router doesn't know how to get to that network or IP address the router will drop the traffic, if there
is no matching route in the routing table
traffic gets dropped.
This applies specifically to unicast packets where we are doing our routing based on destination IP
address.
So in summary routing protocols allow routers to learn about destination networks that facilitates the
exchange of a routed information between devices.
Routers can dynamically learn about networks in the topology and can then make routing decisions based
on different criteria such as bandwidth, hop count or delay to determine the best path route is then
simply choose an outgoing interface based on the routing table and will then forward packets out of
that interface to reach a destination.