We can generally put dynamic routing protocols
into two categories-- ones that we
use on the inside of our network, and those
that we would use on the outside of our network.
So in this video, we'll look at the difference
between interior gateway protocols and exterior gateway
protocols.
Before we can make a determination
on what is inside or outside of the network,
we need to define an autonomous system, or an AS.
An autonomous system means that this is something that
is independent of anyone else.
Generally, it is a network that you manage.
It is under your control.
And as long as this entire network is under your control,
it is an autonomous system.
If there's part of the network that is controlled by someone
else that would be in a separate autonomous system.
If we look at RFC 1930 under Section 3,
"An AS is a connected group of one or more IP prefixes
run by one or more network operators, which has a single
and clearly defined routing policy."
And this was probably going to be pretty straightforward.
If you start working with routers
and you have your own network, there's
always a very clear delineation between what is your network
and what is someone else's network.
As we start discussing the differences
between an interior gateway protocol and an exterior
gateway protocol, we will absolutely
define these based on what the autonomous system might be.
And when you start working with routing protocols,
the protocols themselves maybe referencing
one AS versus another.
For all of these ISPs to be able to communicate with each other,
they need to be assigned a unique AS, a unique autonomous
system number.
That ASN is assigned by the Internet Assigned Numbers
Authority, or the IANA.
This is the same organization that manages the IPv4 address
blocks.
They manage the root name servers
and other administrative parts of the internet.
This is something that you'll see
used if you're ever taking advantage of a BGP,
or Border Gateway Protocol.
To be able to route, it needs to be
able to know where these autonomous systems are,
and it routes from one AS number to another AS number.
The number of autonomous systems has grown rapidly
from the times of early internet to today.
And you can see, it is a straight line up.
And because of that, BGP has become a very complex set
of protocols that needs some strict administration
to be able to work properly.
And as long as we have our internet service providers that
are working with the IANA, we're able to route
all of our internet traffic successfully.
Here's a very simplified example of how this works.
We have different autonomous systems.
These might be different ISPs.
And those ISPs have been assigned different AS numbers.
We have an AS 100, an AS 200, and an AS 300.
And so the routing tables in each one of these routers
is set up to know where these different numbers happen to be.
So if we need to route information between one
internet service provider and another,
we'll simply send it through that network using those BGP
routes that take advantage of these autonomous system
numbers.
If we need to route between autonomous systems,
we're using EGP, or an Exterior Gateway Protocol.
These are the protocols exterior to our autonomous system
and allows multiple autonomous systems
that may all be configured and managed separately
to all somehow now communicate to each other using an EGP.
A good example of an EGP is Border Gateway Protocol,
or BGP.
This is the most popular protocol
you'll find in use, because this is
what allows all of the devices connected to the internet
to be able to route between each other.
It has effectively become the de facto standard
for exterior gateway protocols, because of its popularity.
An interior gateway protocol is used on our internal network.
It is our own autonomous system.
We don't use IGPs to route between our internal system
and someone else's.
If we need to do that, we have exterior gateway protocols
that handle that particular function.
An example of an IGP might OSPF, Open Shortest Path First.
It might be ISIS, intermediate system to intermediate system
used by many service providers.
There's the routing information protocol, or RIP.
Generally, we see RIPv2 being used today.
And we also see EIGRP, which is a Cisco
protocol for internal gateways.
This is the Enhanced Interior Gateway Routing Protocol.
The interior part is part of the name itself.
If you're managing a rallied network
and you have your own dynamic routing,
you're probably working with one of these every day.
So if we put all of this together,
you would have something that looks like this.
We have these multiple autonomous systems
at these different companies.
Company A is using RIP internally
to perform its dynamic routing.
Company B is using OSPF.
Company C is using EIGRP, and Company D is using RIPv2.
And to be able to communicate across all of them.
We have an exterior gateway protocol, a BGP,
that's allowing all of these companies
to be able to send routing information,
and yet, still be able to run their own routing inside
of their networks.