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[AUDIO LOGO]

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So we've looked at several
different BGP attributes

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that we can use to change
BGP path selection.

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Now there's another
attribute we need

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to look at here, communities.

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Now, the thing with
communities, they're

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not going to affect BGP's
decision in any way directly.

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Communities are just a way
to tag or mark a prefix.

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The device receiving
that community

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then has to be set to
do something with it.

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What's it going to do?

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Well, that's going to
be for us to decide.

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But one of the
things that can do

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is change one of the attributes
that we've already discussed.

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So maybe it changes
weight, maybe

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it changes local preference.

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And that would affect
the outbound decision

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of the other AS that we're
sending the community

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to, therefore, affecting
our inbound traffic when

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that AS sends traffic to us.

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So effectively what we would
be doing is using communities

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to affect the inbound
traffic, but we're doing it

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indirectly because
really what's happening

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is the other AS is affecting
their outbound policy based

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on the communities we send.

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Now for any of this
to work, the ASs

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would have to communicate the
communities with each other.

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I can't just guess
what community

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to send to a particular
service provider

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to get them to do a particular
thing with my networks.

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So this is information
that's generally

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going to be published somewhere
on each service provider's

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site.

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As a service
provider when you're

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interacting with other
service providers,

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you have to know
their policies just

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as much as you're going
to have to notify them

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of your policies.

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So whatever
communities you decide

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to use to allow them
to influence traffic,

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you have to publish those
probably on a website,

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but the point is
everybody has to be

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aware of these communities
or else nobody can use them.

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So what we're going
to be doing is

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we're going to be sending some
updates between our AS numbers,

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and we're going
to use communities

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to affect the return traffic
coming back to our AS.

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So as always let's take a
look at our topology first,

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put together a game
plan, and then we'll

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jump into the command
line and get communities

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working in our topology.

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So for this demonstration, we're
going to focus on that loopback

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interface on router 8
which is 100.8.8.0/24.

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And what we're
going to be doing is

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having AS 65001
influence the path

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that service
provider 1 or AS 100

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is using to get to that network.

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What should be
happening right now,

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and we will, of course, verify
this, router 1 and router 3

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should be sending traffic
directly to router 7.

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What we want to accomplish is
we want to get both of them

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to switch over and send
the traffic to AS 1000.

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Now we're going to approach
this from the perspective

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that we only have
control over customer A.

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Now what I mean by that is
we'll be doing the configuration

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to influence the path
selection on router 7.

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We will, of course, have to
do configuration on router 1

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and router 3 to get this
to work to begin with.

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The service provider routers for
the neighboring AS, of course,

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have to be configured.

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We also have to choose how
we're going to do this.

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What we need to
remember is this prefix

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that we want to use is
being learned from router 6

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to router 1, and there's no
local preference or weight

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or anything being set on that.

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Therefore, it's going to have
the default local preference

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of 100.

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And we will use local preference
to influence this path

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selection just because
generally it's a better choice.

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So keeping in mind
that we're going

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to be getting that default
local preference, what we need

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to do since higher
local preference wins,

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is when router 7 is sending
out its updates to AS 100,

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we're going to have to set the
local preference to something

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different like, for
example, maybe 80.

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So we will have to assign
community strings for this

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to happen.

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I would recommend
that we're probably

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going to use community 100:80.

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The first part of the
community is usually

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the AS number that's assigning
this community string,

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and then the second
part is usually

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something that just
makes sense based

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on what we're trying to set.

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And since we're trying to set
the local preference to 80,

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it would make sense to make
the second part of this 80.

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Technically it
could be any number

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you want, but we'll use 100:80
to get router 1 and router 3

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to lower their local
preference to 80.

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Therefore, they
will choose the path

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with the default local
preference going to router 6.

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So let's begin by, of course,
verifying the current path

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selection.

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We'll jump into the
routers, and we'll

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make sure that
router 1 and router 3

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are currently going
directly to router 7.

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So let's begin with the show
BGP for the prefix we're

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looking for.

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What we see here is that it
has three available paths.

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The first one is
learned from router 3.

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Now this one actually has
an inaccessible next hop

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of 10.7.7.7.

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If we intended to actually
try to use that path,

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we would want to fix this
because that path is not

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usable.

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And particularly for redundancy,
that should probably be fixed.

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However, in this
particular demonstration,

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that's not the path that
we're going to try to choose,

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so it really
shouldn't affect how

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we're going to be changing
our path selection.

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So for right now, we're not
going to worry about that.

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The second path we
can see is the path

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learned through router 6.

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This is the path that we
ultimately want to use,

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and then, of course,
we have path 3,

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which is the one that's
currently the best.

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This is the path learned
directly from router 7

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with router 7 itself
being the next hop.

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Let's verify what we
have on router 3 as well.

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Remember, in this
example, that router 3

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is a route-reflector client
of router 1 and router 2.

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So notice that we're getting the
prefix from both of our route

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reflectors, and both of them are
showing a next hop of router 1.

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Now the other path we have
here, of course, the one that's

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currently best is, again, the
one directly from router 7

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with router 7 as the next hop.

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So again, these are
things that we're

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going to want to change
through using local preference

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and communities.

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So the first thing we have
to do to get this to work,

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we have to start by setting
up router 1 and router

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3, our service provider
routers, in order

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to accept the
communities and do what

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we want with those communities
to effect the BGP updates.

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So let's go ahead and
configure that first.

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We'll start on router 1.

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First thing we'll need
is a community-set.

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We'll call this
CS_FROM_CUSTOMERS.

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What we're going to
match on is the 100:80

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that we said we were going
to use in our game plan.

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We'll end the set, and then
we'll write our route policy.

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We'll also call
this FROM_CUSTOMERS.

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And for our logic, we're
going to say if community.

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We'll do a matches-any,
meaning it will accept anything

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in the list.

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It doesn't particularly
matter since there's

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only one item in our
list, but here we'll

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put our community-set.

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Then what we're going to do
is set the local preference,

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and we're going to set the
local preference to 80 exactly

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like we said in our game plan.

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We'll then forward the route.

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And again, we're not
trying to filter,

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so we will pass all
the other routes

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as well if they don't
match our community-set.

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We'll exit out of our if.

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We'll exit out of our policy.

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Go into BGP for our neighbor.

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This would be towards router 7.

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Into our address family, and
we'll assign our route policy.

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This will be an inbound
policy because we're

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going to be applying this
as the router coming in

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from the customers.

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We'll commit our changes
and exit configuration.

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Router 3's
configuration is going

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to be identical
to router 1 here,

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so we'll just copy and paste.

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This is what we're going to
be pasting in on router 3.

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As you can see, it's exactly
what we just typed on router 1.

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So let's go ahead and paste that
in and the exit configuration.

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Our next step is going to
be configuring router 7

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to send the community to router
3 and router 1 so that it

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influences the path selection.

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So now let's go configure
this on iOS XE on router 7.

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So the first thing we're
going to need is a route map.

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We'll call it RM_TO_SP1.

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And what we're going to do
is simply set the community

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to the 100:80.

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Now if you haven't
seen this before,

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iOS XE does something
interesting with community

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strings and how
it displays them.

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If we were to say
do show route-map,

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we can see here that on
the route map, RM_TO_SP1,

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it's displaying the
community as 6553680.

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Not exactly the 100:80
as we typed it in.

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This is the older way of
displaying community strings

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in decimal format.

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This is still the
default on iOS XE.

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If we want to see this in the
newer format like the way we

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entered it, we have to
put in one configuration

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command on the router.

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Let's go ahead and
take a look at it.

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So we have to type
ip bgp new-format.

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Now this will, of
course, take us out

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of route-map configuration
mode because this is actually

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a global command.

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But if we look at
our route-map again,

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we can see that now the
community is listed nicely

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as 100:80.

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Keep in mind this just
affects how it's displayed.

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This doesn't actually
affect the functionality

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of the communities in any way.

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However, it is much nicer.

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So now that we
have that in there,

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let's go ahead and
configure our router.

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So we're going to
router BGP 65001.

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Now we haven't really looked
at the running configuration

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on router 7.

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If we do a show run section
BGP, what we need to see here

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is we're actually using peer
policy templates and peer

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session templates
here on router 7.

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Now, we could still put
these commands directly

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on the neighbors, but it's
going to make a lot more sense

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to use these templates since
we already have them here.

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So we're going to say
template peer-policy Main.

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Now we're not trying to
apply the route-map to all

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the neighbors, but we are trying
to get it to send communities,

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and we have to tell
iOS XE to do that.

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So we have to say
send-community.

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Then we have to choose
if we want to send

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extended, standard, or both.

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By the way, all of this
is simply about the fact

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that iOS XE does not send
communities by default.

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So since we're going to
write a route-map that's

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setting communities, and
then apply it to a neighbor,

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it would probably help
if we were actually

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sending those communities.

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The default behavior,
like I said, is not to.

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So all we're really
doing is enabling

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a feature, the ability
to send communities.

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It doesn't mean it's going to
start magically doing that.

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The reason I'm saying it this
way is I usually turn on both.

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Some people look at this
and say, well, you're

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not even using extended
communities right now.

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That's true.

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I'm not.

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But on the other hand,
it doesn't really

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hurt to turn them on even
if you're not using them,

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and then you would be ready
to use them in the future

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if you wanted to.

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So when I'm enabling
communities on the router,

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I usually do both
just because it

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doesn't hurt to
have them enabled

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even if you're not using them.

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So we'll just go
ahead and do both.

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Now at this point,
we're probably

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going to want a
peer-policy specifically

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for service provider 1, so
let's go ahead and create one.

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So we'll say
template peer-policy,

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and let's just call
it something like SP1.

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And what we're
going to do is we're

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going to apply our route-map,
so route-map RM_TWO_SP1,

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and that would be
applied outbound.

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And then, of course, just
like the other peer policies,

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we're going to tell
this to inherit.

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Now when you're
inheriting these,

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you have to give it
a sequence number

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because if you're inheriting
more than one policy,

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they have to be inherited
in a specific order.

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Now in this case, we're
only inheriting one policy,

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so it doesn't really matter.

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We'll say to inherit
this one first.

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Now let's configure
our neighbors

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we'll start with 50.0.167.1,
that's router 1's address.

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And what are we going to do?

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We're going to tell it to
inherit peer-policy SP1.

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Then we're just going
to hit the up arrow,

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and we'll change our
neighbor statement.

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This is going to
be for 10.3.3.3.

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So just a quick recap of what
we had to configure here.

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We had to set up our
route-map, which goes ahead

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and sets the community on
the updates being sent out

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to service provider 1.

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Then we updated our
Main peer-policy

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to tell it to send communities
both extended and standard.

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Then we created a
new peer template

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that inherits the main template
and applies our new route map.

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We then configured both
of our neighbors router 1

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and router 3 in service
provider 1 or AS 100 to then

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inherit that new policy that
we put on the neighborship.

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So now we should be able to
clear our outbound updates

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and see the changes
actually happen on router 1

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and router 3.

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00:16:09,130 --> 00:16:12,760
Now before we actually do
that, let's set up a Wireshark

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capture just on router 1, so we
can see this update coming in

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and take a look at the community
strings on that update.

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So I started up that
Wireshark capture.

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00:16:24,370 --> 00:16:26,050
We've seen that
quite a few times.

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00:16:26,050 --> 00:16:28,000
I don't think we need
to see the setup,

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but let's go ahead
and do our clear now.

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So first, let's go to router 1
and see if it got the updates.

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We'll just do a show
BGP for our prefix.

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00:16:38,110 --> 00:16:42,160
And what we see here
is our first path

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is the path through AS 1000.

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00:16:45,040 --> 00:16:48,670
It has the default
local preference of 100,

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and therefore, is
being chosen as best.

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00:16:52,120 --> 00:16:56,920
The route that we did not want
to use, the one from router 7,

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00:16:56,920 --> 00:17:01,450
we can see is now being set to
a local preference of 80 exactly

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as we wanted.

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00:17:02,510 --> 00:17:08,990
And we can see the community of
100:80 attached to this update.

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00:17:08,990 --> 00:17:10,720
So now let's go
take a look at what

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00:17:10,720 --> 00:17:12,819
this looks like in Wireshark.

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00:17:12,819 --> 00:17:15,910
So looking here in
Wireshark, we can

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00:17:15,910 --> 00:17:19,839
see that we've received
this update from router 7.

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00:17:19,839 --> 00:17:22,480
And if we look down
in the update itself,

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00:17:22,480 --> 00:17:26,329
first for the NLRI, we can
see that this is in fact,

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00:17:26,329 --> 00:17:31,540
for the 100.8.8.0/24 prefix.

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00:17:31,540 --> 00:17:34,670
And then here under
the path attributes,

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00:17:34,670 --> 00:17:37,510
we can see quite clearly
where it's carrying

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00:17:37,510 --> 00:17:41,630
that community of 100:80.

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00:17:41,630 --> 00:17:43,760
Now just as one final
verification just

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00:17:43,760 --> 00:17:46,230
to make sure everything's
working as it should be,

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00:17:46,230 --> 00:17:49,880
let's also verify
this on router 3.

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00:17:49,880 --> 00:17:52,880
So on router 3 we'll
just do a show BGP.

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00:17:52,880 --> 00:17:58,850
And what we see is that for
the 100.8.8.0/24 prefix,

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00:17:58,850 --> 00:18:05,090
the next hop of 10.1.1.1 has
been selected going to an AS

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00:18:05,090 --> 00:18:10,040
path of the 10000 65001.

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00:18:10,040 --> 00:18:12,740
So this has also
worked where it's not

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00:18:12,740 --> 00:18:14,840
trying to go
directly to router 7,

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00:18:14,840 --> 00:18:17,960
but rather going up
to router 1 who's

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00:18:17,960 --> 00:18:20,720
then going over to AS 1000.

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00:18:20,720 --> 00:18:23,720
So this is local
preference being

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00:18:23,720 --> 00:18:31,210
manipulated by community strings
working exactly as we designed.

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00:18:31,210 --> 00:18:34,420
In this video, we
took a look at how

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00:18:34,420 --> 00:18:40,690
to use communities to actually
manipulate inbound BGP path

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00:18:40,690 --> 00:18:41,950
selection.

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00:18:41,950 --> 00:18:45,400
And we do this by sending
a specific community

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00:18:45,400 --> 00:18:49,630
to our neighboring AS who
then has a policy implemented

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00:18:49,630 --> 00:18:52,900
to do something with that
prefix, such as change

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00:18:52,900 --> 00:18:55,330
the local preference,
which is what we did.

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00:18:55,330 --> 00:18:57,220
Theoretically, they
could also change

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00:18:57,220 --> 00:19:00,490
the weight or some
other BGP attribute

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00:19:00,490 --> 00:19:04,300
to influence the path
selection, but local preference

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00:19:04,300 --> 00:19:07,010
is a very common
thing to do here.

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00:19:07,010 --> 00:19:11,410
So in that way, we actually
influenced the outbound path

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00:19:11,410 --> 00:19:14,560
selection of our
neighboring AS, which,

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00:19:14,560 --> 00:19:19,310
of course, is effectively
our inbound path selection.

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00:19:19,310 --> 00:19:21,430
I hope this has been
informative for you,

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00:19:21,430 --> 00:19:24,540
and I'd like to thank
you for viewing.

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