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[AUDIO LOGO]

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So now that we've talked
about the details of using

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AS path prepending, let's
jump in with a diagram

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and look at an
operational example of how

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we would actually use AS
path prepending in a multi AS

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environment.

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So in this scenario, we'll
be talking about three ASes.

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So we have AS 100, 200, and 300.

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And this is going to be
about return traffic coming

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to AS 100.

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And the default behavior
would be for router 3

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to send the traffic to
router 1, and for router 4

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to send the traffic
directly to router 2.

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The part that we
don't want here,

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we don't want router 4 to be
sending this traffic to router

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2 across this one gig link.

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So this is what we want to stop.

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We want to get router 4 to
follow the path over the 10 gig

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link.

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Here's how we do that
with AS path prepending.

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What we'll do is on
router 2, we will simply

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prepend our AS number say,
for example, three times.

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So now when router
4 gets the update,

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it would have four
copies of our AS number,

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the three were prepending, plus
the one that would normally

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be put on that update.

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On the update however
coming this direction,

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the AS path would
simply be 200, 100.

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So now we have a comparison
where we have a path with four

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AS numbers versus a path
with two AS numbers.

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And of course, we will
choose the one with two.

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That means that the
traffic from router 4

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will now go up to router 3.

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And of course,
router 3 is already

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forwarding it to router 1.

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So this means that
the traffic is

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going to follow the
path that we want it to.

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So as we mentioned, it would
be possible for router 4

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to not accept this
AS path prepending.

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And therefore, the end
result would be it would not

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accept this path at all.

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It would not even be considered.

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Now interestingly enough,
the effect would be the same.

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Meaning, if I'm not getting
the path from router 2

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at all on router 4,
well then router 4

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would follow the only
path it had left,

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which would be up through
router 3 and over to router 1

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as we've been trying to get our
traffic to go that way anyway.

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The downside, of course,
to that plan is then I

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wouldn't have the
link between router 2

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and router 4 as a backup.

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If I don't learn
the prefix at all,

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I can't go that way
to get to the prefix.

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So yeah, at the
end of the day, it

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would affect our path selection.

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But we don't want to affect
the path selection by making

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the backup path unusable.

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So that would be a problem.

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We still need to make sure that
the AS, in this case AS 300,

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is still actually
accepting the path

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and then making a decision
based on the shortest AS path.

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Just to change our
diagram up here

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for just a moment, let's talk
about another interesting issue

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we have with AS path prepending.

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What happens if our
AS 200 and our AS 300

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are not directly connected?

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What if they're going
through a public network

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such as the internet?

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The problem with this is
we have no idea what's

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going on in the internet.

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We don't know how
many ASes it's going

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to go through as
the update is sent

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from router 3 down to router 4.

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We don't know how long this
path is going to be now.

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So when it comes to us
doing our prepending,

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as we send the update
from router 2 to router 4,

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it becomes an issue
of how many times

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do we have to prepend in
order to make this happen.

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One possibility here
may be to go to AS 300

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if they offer a public looking
glass router that we can

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log into that device
and actually see

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what our prefixes look like
after they've gone through AS

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200 going around
through the internet

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and finally ended up at AS 300.

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We could look at
that AS path and get

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an idea of how many times
the prepend are AS number.

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The primary issue with
that is that things

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change on the internet.

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So you would have
to keep a close eye

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to make sure that
your traffic is still

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going the way you want it to.

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And the other thing
here is we want

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to make sure that we're
not cutting it too close.

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We might go to AS
300, see that the AS

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is let's say six ASes long.

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Well, we wouldn't want to
prepend just six or seven then

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to our path.

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Because again, it would
only take one AS or two ASes

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to change on the internet
to have this go right

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back to default path selection.

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You would probably want to
go to say 10 or something

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along those lines to make
sure the traffic is going

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to go the way we want it to.

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If you don't have any of
these options, in other words,

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you can't see what
AS 300 is learning,

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well the old school way so to
speak is prepend say five ASes

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and see if your
traffic starts coming

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in the way you want it to.

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Keep adding ASes
until no traffic

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is coming in that link
between router 2 and 4.

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That's basically
what you're going

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to have to do if you can't
see what your prefixes look

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like in AS 300 here.

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So, again, let's say we
actually get that information

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by looking at AS
300, and we find out

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that the path being learned
through the internet

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maybe has six ASes
on it, then like I

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said on our
advertisement, when we're

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sending the update
towards router 4 then,

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we should probably put something
like 10 on here to make sure

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that this is going to be treated
just like a backup and not

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as a primary circuit.

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So in this video,
we examined how

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to use AS path prepending to
influence other ASes to send

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the traffic in on
the circuits that we

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would prefer them to use.

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Now, again, there are
some caveats with this.

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One of them is
that neighboring AS

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has to support you doing
AS path prepending.

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Now, again, most do.

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However you might run into a
few service providers out there

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that you are trying to pair
up with that maybe don't.

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Also, we have to be concerned
about the number of times

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we need to prepend our AS.

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Generally speaking,
going a little bit higher

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is better particularly
if we're looking

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for a backup and
primary type scenario.

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If you are actually looking for
a load balancing scenario here,

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then you want to start
playing with numbers

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that are pretty close to
what the other AS is getting.

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So, again, if they're
receiving your prefixes

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with an AS of say seven
ASes in the path, then maybe

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you want to prepend say six
or maybe even five times,

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maybe four times, something
to allow some traffic

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to come in that link.

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And you have to
adjust those numbers

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and watch your links to
make sure they're getting

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fairly even distribution.

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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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