1
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Won't you please,
won't you be my router?

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You know, a router's really only
as good as its, first of all,

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knowledge of how
to forward a packet

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and know where networks are and,
secondly, its ability to do so.

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So for this discussion, I'd
like to use this topology right

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here.

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This represents right here,
for example, a VLAN, a network.

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And associated
with that network,

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we've got a network address.

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And let's use 10.1.0.

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And we'll use, in this
example, the first three

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numbers in the IP address to
represent the network numbers.

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So down here, we have
the 10.2.0 network.

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Between routers 1 and 2,
we have the 10.3.0 network.

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Up here, we have the 10.4.0
network, and down here,

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the 10.5.0 network.

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Also, I've assigned IP addresses
to each of the interfaces

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on the routers.

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And each of the
interfaces on router 1

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ends in .1, just to make it
really easy from a learning

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perspective to remember
who owns those addresses.

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And then on router 2,
each of its interfaces

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has a last number in
its IP address of .2.

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So as an example, on
this network right

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here, the 10.1.0 network,
the router's interface

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has an IP address of 10.1.0.1,
and this PC is at 10.1.0.11.

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So let's talk about training a
router, router 1 and router 2,

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on how they can reach
a given network.

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And there are three basic ways
of training a router on how

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to reach a specific network.

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The first of those is simply
to configure an IP address

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on an interface of a router,
because once we configure an IP

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address on an interface,
the router says, hey,

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I know how to get
to this network.

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It's directly connected.

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Just like this
interface right here--

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1/0 on this router.

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If we configure the
interface with the IP

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address of 10.1.0.1,
it knows, badaboom,

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I'm directly connected.

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I know how to reach
the 10.1.0 network.

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It's off of this interface.

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So that's option number one.

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The problem, however, is
that the router is not going

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to learn about remote networks--
for example, the 10.4 network--

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because router 1 is not
directly connected to it.

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So for networks that are
not directly connected

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to the router, option two would
be to configure a static route.

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A static route is simply
giving an instruction.

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Like, if somebody came over
to your house and said, hey,

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how do I get to the
nearest convenience store?

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You could probably tell them
based on where you live.

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Think about it.

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You go down the
street, make a left,

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and you give them
the directions.

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Well, the same
holds true if we're

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telling R1 and
configuring R1 to tell it

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how to reach the 10.4.0 network,
as an example, or the 10.5.0

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network.

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And so what we
can do is we could

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go into the configuration
of R1 and say,

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dear Mr. R1, if you ever need to
forward a packet to the network

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10.5.0, here's what
we want you to do.

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We want you to forward the
packet out this interface

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and send it over here
to your good buddy R2.

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And that's your job.

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And then because R2 is directly
connected to that network,

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R2 already knows how to
get to that 10.5 network.

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So those are two
options to begin with--

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configure a router
interface to be directly

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connected to a network by giving
that interface an IP address,

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and, for remote networks that
aren't directly connected,

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we can configure a static
route, specific instructions

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on how to forward a
packet in the direction

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of that remote network.

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Now, the big bummer
in most networks

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is that there are lots
and lots of routes,

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and having these static routes
for every single network

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is a pain in the bazooski.

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So instead of configuring
static routes,

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the more popular
and practical option

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is to configure and use a
dynamic routing protocol.

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This is where the routers
talk to each other.

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Now, they're not
trying to take over

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the world by talking to each
other, but what they will do

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is they will share with each
other information about what

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they know regarding
how to reach a network.

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So if we configured a
dynamic routing protocol

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between R1 and R2, here are
some things that they might say.

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R1 might say, hey, I can
reach the 10.1.0 network

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and the 10.2.0 network.

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And, R1 says to R2,
if you ever need

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to reach either
of those networks,

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go ahead and send those
packets to me, because I

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can reach them.

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And, conversely, R2 may say
to R1, hey, thanks a lot.

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Hey, you know what, Mr. R1?

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I know how to reach, says
R2, the 10.4.0 and the 10.5.0

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networks.

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So if you ever need to
reach those networks,

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you can forward
those packets to me,

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and I will forward them to
those final destinations.

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And that's an example of
a dynamic routing protocol

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configured on both
routers so that they

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can share routing
information and reachability

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information with each other.

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And the cool thing
about it being dynamic--

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if we lose a network or
we change the topology,

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the dynamic routing
protocols can dynamically

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update each other, so we
don't have to manually go back

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in and statically configure
different routes based

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on our topologies changing.

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In this Nugget,
we've taken a look

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at the three most common methods
for training a router on how

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to reach a specific network.

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I hope this has been
informative for you,

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and I'd like to thank
you for viewing.

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