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[MUSIC PLAYING]

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So in the previous
video, we saw how

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to filter using prefix lists
on a neighbor statement in BGP.

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Now, the only
downside to that is

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that our neighbor's, of
course, still sending

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us all of their updates, and
then we're filtering them.

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This is bad for both sides.

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For the sender, they're
creating and building

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updates that, ultimately,
we're just going to throw away.

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And of course, on our side, we
have to bring the routes in,

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we have to look at them,
compare them to our filter,

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and then block them
off and discard them.

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This would be a lot better if
the neighboring router just

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never sent the
updates to begin with.

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And the cool thing is, we
have a feature for that.

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It's called Outbound
Route Filtering, or ORF.

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Now, the way this works is we
will send our inbound prefix

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list to the
neighboring router, who

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will then apply it outbound.

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Hence the name, Outbound
Route Filtering.

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Now, needless to say,
for this to work,

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it has to be properly
configured on both sides.

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One side has to be configured
to send the prefix list.

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And of course,
the other side has

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to be configured to receive it.

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This is normally done between
a customer and a service

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provider.

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However, keep in mind
that, as a BGP feature,

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it, of course, can be
used anywhere we want to.

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So let's take a look,
first, at the requirements

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to get all this
set up and running.

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Then, of course, we'll
just take a quick look

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at our topology to
know where we're

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going to be implementing it.

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And then we'll jump into the
command line, and get ORF up

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and running.

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So now that we have
Router 9 that's actually

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filtering those routes from
our previous video, now

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we're going to prevent Router
1 from sending those over

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to Router 9.

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So just try to keep
this perspective in mind

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as we go through.

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Whatever routes are
being filtered inbound

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on one router--

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in this case, of course,
we're referring to Router 9--

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would be equivalent to filtering
outbound on another router.

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And again, in this example,
we're talking about Router 1,

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how we're going to
be configuring it.

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This is the principle behind
Outbound Route Filtering.

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As I mentioned during
the introduction,

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this is commonly done
for the customer.

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And it allows them to
control the routes being

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sent to them by the provider.

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Now, of course, this would be
an optional feature or a benefit

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that we could offer
for our customers,

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but a lot of service providers
do offer this feature.

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So it's probably a
good idea to do so,

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unless we have a really
good reason not to.

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Now, a couple of things
to keep in mind here.

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On current code, we only
support prefix lists.

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If you look at the RFCs
for route filtering,

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they actually talk
about including

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other types of filtering.

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However, on Cisco's
IOS at the moment,

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we only support prefix lists.

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It has been that
way for a while.

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So it's likely to stay that way.

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And there are three
overall requirements.

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The first requirement,
of course,

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is that the service
provider, which would be us,

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need to allow or
enable this feature.

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Otherwise, the
customers can send it.

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But of course, we wouldn't
do anything with it.

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That's not enough
though, either.

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The customer also
needs to configure this

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so that their router
actually sends the prefix

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list to the service provider.

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And finally, none of this
will work unless, of course,

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the customer has configured
an inbound prefix list.

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If there's no
inbound prefix list,

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then there's nothing to
send to the other side.

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So all of this said, now that
we know all of the requirements,

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let's jump into the command
line and see how this works.

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First, we'll take a
look at our topology.

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And just picking up right where
we were in the previous video,

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this is going to be configured
on the neighborship between R1

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and R9.

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R9, of course, is going to
be the one sending the prefix

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list.

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And then Router 1 would be
the one filtering the updates

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that we no longer want.

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So we'll have to configure both
of these routers, of course.

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Let's jump into the command
line and get started.

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So let's just start off with
a quick review of the problem

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that we're trying to fix here.

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We'll do a debug IP BGP update.

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And then we'll clear the
BGP session to Router 1.

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We'll turn off
debugging for now.

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And what we're looking
for is this right here,

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where we have these three
prefixes that are being

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denied due to a prefix list.

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Keep in mind, the primary
thing we're trying to fix here

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is that these prefixes
don't get sent at all, not

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that they get sent, and then
discarded by our prefix list.

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So for this, of course, we're
going to be configuring ORF.

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So let's jump over to
Router 1, and let's

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configure what's essentially
acting like the service

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provider side first--

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the ability to receive
ORF prefix lists.

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So we'll go into configuration
mode, into router BGP 100.

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And then we're going to go
into our neighbor configuration

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for 50.1.19.9,
which is Router 9.

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We'll go into the address
family, IPv4 unicast.

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And then we'll say capability.

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And we'll see that there is
a capability here called ORF.

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If we look at the
options after this,

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notice the only choice
we have is prefix.

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As we mentioned before,
doing prefix lists

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is currently the only option
in both IOS XR and IOS XE.

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So, of course, that's what
we're going to be configuring.

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So we'll say prefix.

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And then we have some choices.

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Of course, none would turn
this feature off, which

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is essentially
the default. Or we

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can do send, receive, or both.

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Both, of course,
is just a shortcut

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to saying send and receive.

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Now, normally speaking, again,
a service provider customer

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relationship, we
would probably just

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be receiving these on this
service provider side.

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We're generally not going
to be sending a prefix

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list to the customers.

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Although, of course, any
of this can be used however

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you need to in your environment.

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Now we'll simply
commit that change.

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That's all we need to do
to enable the feature.

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And we'll exit
configuration mode.

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Something else very important
to notice on this output.

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Notice that, right here, we see
the neighborship with Router 9

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going down.

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Now, of course, just below,
it comes right back up.

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But it bounces the
BGP neighborship.

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It does this, of
course, because this

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is a new capability that's been
enabled on this neighborship.

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The main reason
I'm pointing it out

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is we want to make
sure that this

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is happening during a
scheduled maintenance

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window with the customer.

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Or, of course, if we set it
up before the customer is even

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a customer-- so in other words,
during initial configuration,

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of course, it's no issue.

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However, if this is an
existing customer that we're

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adding a feature to,
we do want to make sure

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that it's scheduled
so that we don't

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cause an unexpected outage.

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Because as you see, as soon
as I committed that change,

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it took that neighborship down.

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Came back up.

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But it still bounced
the neighborship.

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So now let's jump
back over to Router 9.

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So we're going to
configure Router 9.

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But before we do, I want
to do two other things.

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First, we're going to
start packet capture

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on the connection to Router
1, so we can actually

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look at this in Wireshark.

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And we're going to turn
our debugs back on here

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on Router 9, so we can see if
those prefixes are coming in

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or not.

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So let's go ahead and take
care of those two things first.

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I'll set up the packet
capture off camera.

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And then we'll look at
it after this is done.

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So I have the packet
capture running.

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Let's turn our debug back on.

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We'll go into configuration
mode, into our BGP process.

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And all we have to do is
configure our neighborship.

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And we're going to
turn on capability.

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Once again, we have ORF.

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Once again, the only choice
is prefix, or prefix list.

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And again, we can
send, receive, or both.

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This, acting as the customer
router, we would just send.

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Now remember, we don't
have to commit here.

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So this is going to happen
pretty much right away.

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We see that even
while I'm talking,

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it already sent the
BGP notification.

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The BGP adjacency has gone down.

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And in just a couple of
seconds, it will come up.

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We can see that it did.

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We'll exit out of configuration.

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Turn off our debugs.

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Now, a lot of stuff went by on
the screen there on the debug.

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However, if you look at
this lower part here,

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where the prefixes are
actually being exchanged,

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you'll notice that the prefixes
that we're trying to filter

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out, specifically the /16, the
20 subnet, and the 40 subnet,

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are not here on this list.

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Normally, of course, I
highlight this sort of thing.

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But I can't highlight
something that's not there.

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So just take a second
to look over this list,

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and see that those prefixes
are not being sent any longer

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here to Router 9.

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Now let's go take a
look at Wireshark.

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So here in Wireshark,
notice that this

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has been filtered to only
show us our BGP packets,

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and to eliminate KEEPALIVES.

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The next thing to notice is
that the first thing we're

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seeing here, as soon as
we applied that change

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to set up ORF, we
sent a notification

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message from Router
9 to Router 1,

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tearing down that neighborship.

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Shortly after that, we
send an open message.

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That open message is
what we want to look at.

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So let's open that packet.

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In this example,
it's packet 110.

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And if we open the BGP
section of that packet,

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we see something here
called optional parameters.

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So if we start
opening these, we'll

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see various different
things here.

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We have multi-protocol
extensions, route refresh

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capability.

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And then we see this one,
called cooperative route

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filtering capability.

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This is actually ORF.

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So if we open this one, a
couple of things to note.

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First, of course,
we see that this

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is type cooperative route
filtering capability,

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specifically for Cisco.

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But most importantly, if
we look down here further,

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we see that the specific type is
the Cisco prefix list ORF type.

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00:11:22,840 --> 00:11:25,980

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So this, of course, is us
negotiating the capability

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to do ORF.

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Now, we still have to
send the prefix list.

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And for that, it actually
uses a route refresh message.

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Notice that our
packet 125 is actually

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a route refresh message going
from Router 9 to Router 1.

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Let's take a look
at that packet.

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We'll open up the BGP section.

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And notice that it even
says right in here,

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that it's ORF information.

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So of course, let's open that.

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What do we see under
the ORF information?

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Well, we see that it's a Cisco
prefix list ORF type 128.

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Then we see the different ORF
entries for the prefix list.

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And if we open
each one of these,

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what we see on
this first entry is

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it's the first entry
in our prefix list.

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We see that it's
flagged as a deny,

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and that it's for the
prefix 100.10.0.0/16.

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The next entry is the deny
for the 100.10.20.0/24.

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Of course, the third entry
is our third deny statement.

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And the last entry
here is, of course,

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going to be our permit any.

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00:12:42,690 --> 00:12:46,320
And we can see that this is a
permit because the deny permit

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flag is set to zero.

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And then, of course, we have
the entry being 0.0.0.0/0 with

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an upper bound of 32.

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00:12:56,540 --> 00:12:59,070

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00:12:59,070 --> 00:13:03,060
So we see here, that this is
literally our prefix list being

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sent line by line to the--
well, in our example, service

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provider router.

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And it's then going to
apply that outbound.

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But we can see that on the
service provider router

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as well.

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So let's hop over to Router
1 and take a look at that.

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Now, this command does get
a little bit long here.

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00:13:23,360 --> 00:13:30,890
Show BGP IPv4 unicast for
the neighbor 50.1.19.9.

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00:13:30,890 --> 00:13:36,080
And then what we want to see
is the received prefix filter.

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00:13:36,080 --> 00:13:39,520
And here, we, of course, can
see that received prefix list.

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00:13:39,520 --> 00:13:41,860
And then this, of
course, is being applied

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outbound towards Router 9.

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And we can see that this is
working exactly as we designed.

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In this video, we saw how to
use ORF to have our neighbor not

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send us the prefixes that we're
just filtering out anyway,

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by sending them our
inbound prefix list,

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and them applying it outbound.

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00:14:08,010 --> 00:14:11,030
This, of course, requires some
configuration on both sides

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00:14:11,030 --> 00:14:12,230
to permit this.

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00:14:12,230 --> 00:14:15,530
Because we are sort of letting
one router control another.

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00:14:15,530 --> 00:14:17,450
And of course, there
could be a little bit

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00:14:17,450 --> 00:14:21,090
of a security concern there
in the wrong situation.

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00:14:21,090 --> 00:14:23,810
So this does, of course, have
to be configured properly for it

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00:14:23,810 --> 00:14:24,660
to work.

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00:14:24,660 --> 00:14:26,510
But once it's
enabled, we can see

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00:14:26,510 --> 00:14:30,290
that this is a very efficient
way of causing prefixes

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00:14:30,290 --> 00:14:32,660
not to be advertised,
that are just

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00:14:32,660 --> 00:14:36,090
going to be discarded
by the neighbor anyway.

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00:14:36,090 --> 00:14:41,000
Now, filtering by prefixes
isn't always the best way to go.

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00:14:41,000 --> 00:14:43,730
Sometimes we might
have to filter

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based on what's in the AS path.

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And that's exactly
what we're going

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to be doing in the next video.

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Until then, 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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