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[AUDIO LOGO]

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So let's begin our
journey here by taking

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a look at AS-Path Prepending.

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This is probably the most
common and also the easiest way

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to influence the return
traffic into your AS.

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You're simply adding ASes to
the path to artificially make

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that AS path longer.

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And therefore, other devices
that are sending traffic to you

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will not prefer that path.

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So this is actually a
fairly easy implementation.

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Let's go ahead and take
a look at our topology,

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get an idea of what
we're going to do.

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And then we'll jump
into the command line

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and see AS-Path
Prepending in action.

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So for the demonstrations
in this skill,

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we will be using BGP
topology 1 if you want

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to have that diagram handy.

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And for this particular
demonstration,

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we're going to be
taking a look at AS 300

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and its return path
coming to AS 100.

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So what should be
happening right now

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is router 9 should
actually be sending

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all of the traffic to router 1.

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What we're going to
do is we're going

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to have router 1 prepend its AS
number a few times when sending

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its updates to router 9.

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And what we'll see
is this will cause

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router 9 to flip over and
start sending the traffic

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to router 2.

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Now, what makes this inbound
path selection is the fact

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that we're going to make this
configuration change on router

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1.

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So it's AS 100 that's going
to be influencing as 300.

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So let's jump into the
command line and get started.

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We'll begin actually on router
9 to examine its current routes.

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So again, let's start
by verifying that route.

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We'll just do a show ip bgp.

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And what we're
seeing here is all

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of the routes coming
from AS 100 are currently

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going through router 1.

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So we see that there's
numerous routes here.

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But just to highlight
two of them,

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we see these first two entries
with the next hop of 50.1.19.1

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being selected as the best.

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And we see that's the same
for all of the entries.

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So let's go ahead and implement
our policy on router 1.

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This is a fairly
straightforward policy.

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We're just going to
make a route policy.

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We'll call it RP_TO_AS300.

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Now, we could of course
here use a prefixed

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set, as we've done
numerous times before,

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just to match on
certain prefixes.

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But in this case,
our design goal

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was to have all of the traffic
returning from AS 300 to AS 100

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to come in through router 2.

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So in this case,
we're going to do this

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without specifying the prefixes.

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So this will just
apply to all of them.

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Just keep that in mind as
we're writing our policy here.

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So without any kind
of policy statement,

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all we're going to
have to say is that we

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want to pretend the AS path.

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And we see here, we can actually
put in any numbers we want.

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Technically, you don't have
to use your own AS number.

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But since own AS is the
most common thing to do,

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we actually have a
special setting for that.

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We can just say own-as.

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And then we can tell
it how many times.

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In this case, we'll
just go ahead and do 2,

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like I had put on our diagram.

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Once it's done
prepending the path,

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then we simply want it to
pass the routes through.

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We'll end our
policy, go into BGP,

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go to the neighbor
configuration for router 9,

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into the address family,
and apply our route policy.

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This will be an outbound
policy because this

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is what we're going to be
sending to our neighbors.

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So this is ready to go.

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But before we do
it, we want to see

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what the update looks like
in Wireshark when we actually

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implement this.

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So let's jump into
Wireshark, set up a capture

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before we actually commit
this change and router 1

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sends its update.

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So here, we have our capture
running on router 1's gigabit 2

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interface looking for any BGP
traffic other than keepalives.

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So now, let's jump back
over to our command line

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and commit our changes.

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So we'll simply say commit.

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We'll exit configuration mode.

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And let's go ahead and do a
show bgp neighbor for router 9.

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And let's do an
advertised routes.

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We can see here on the
AS path where we're

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prepending our AS two times.

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So that's a total of three. .

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Remember one of these is the AS
that gets put on automatically

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on EBGP updates and then
the two ASes that we're

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adding through our policy.

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So now, of course, we
want to take a look

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at what this looks like in
Wireshark and then hop over

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and make sure that
it made the change we

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wanted it to make on router 9.

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So back here in
Wireshark, I did have

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to modify our filter here
just a little bit because it

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sent a keepalive at the same
time as the update messages.

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And it ended up
matching our filter.

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So I did have to take that
part of our filter off

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to be able to see this.

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But here, we can see this
message coming from 50.1.19.1

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going to router
9, the 50.1.19.9.

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And if we look down here and
expand the UPDATE Message

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and the Path Attributes, we
can see here under the Path

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Attribute that the AS
path is 100 100 100 1,000.

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So once again, 3
copies, one that's

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normal for EBGP and then
the two that, of course,

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we're adding through our policy.

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Now, let's jump over
to router 9 and see

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if this actually made the change
that we wanted it to make.

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So on router 9, we'll simply
do our show ip bgp again.

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And there's a few
things here we can see.

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And in this case, we'll just
focus on the first entry,

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the 100..8.8.0/24.

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And in this case, we
can see that now it's

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choosing router 2 as the
next hop, the 50.1.29.2,

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with the shorter AS
path that's only 100.

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We can also see that the
path through router 1

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has in fact been prepended
with those three ASes

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in the AS path, which is why
router 9 is no longer choosing

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these as best.

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And therefore, we can
see that local preference

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is doing exactly what
it's designed to do.

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In this video, we
took a detailed look

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at how AS-Path
Prepending can actually

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be used to influence our
return, or our inbound traffic,

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depending on how you
want to look at it.

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The way we did this was by
adding ASes to the AS path

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going out of router
1 towards router 9,

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therefore making that path not
look as desirable as the path

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that we wanted it to use
going through router 2.

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So again, the basic
idea here is we're

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tainting the path that we don't
want the other devices to use.

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Now, keep in mind just like
any other inbound influencing,

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this could easily be
overridden on router 9

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if they chose to by
implementing either weight

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or local preference
because those

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are higher on the
BGP path selection

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criteria than shortest AS path.

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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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