In the previous video, we configured IBGP
neighborships between R11, R12, and R13 as shown here.
And in this video, we're gonna walk through
configuring external BGP neighborships
between R11 and R21 and R11 and R31.
Now the configuration for setting up
an external BGP neighborship is quite shocking
when you compare it to what it looks like
for an internal BGP neighborship.
And here's the syntax.
We're gonna configuration mode,
we'll type in router, space, BGP, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
we'll type in router, space, BGP,
we'll specify our local autonomous system number.
And then now that we're in router,
BGP configuration mode, we're gonna use the command neighbor,
space, the IP address of the reachable neighbor,
space, the keyword remote dash AS, and the AS number.
So if we were configuring a BGP neighborship
between R11 and R31, we would specify that the target,
the destination is 23 to 10 to 30 dot.
And each router is last octet ends with its number.
So the interface here, four slash one, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So the interface here, four slash one,
interface addresses is 23 to 10 to 30 dot 31.
So that'd be this address here.
And then the remote AS that this router is in is 30.
So the only difference between an internal
and an external BGP neighborship and setting it up,
initially is that we're pointing to an AS,
this different than ours.
So R11 is an AS 10, R31 is an AS 30.
And as a result, once we set it up on both sides,
we'll have an external BGP neighborship.
So that's the syntax.
Now with external BGP, it's expecting the peer WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Now with external BGP, it's expecting the peer
is on a common link that's like no more than one hop away.
So in the event that you have to do appearing to loopbacks
or non-directly connected networks,
there is some additional configuration for external BGP.
And I'll point one of those options out as we go through,
however, in our topology,
because the external neighbors are directly connected
on the same link, we won't actually need to use them.
But I will show you what the syntax is for that as well.
So using the syntax, we're gonna go to R11
and configure an external BGP neighborship for R31, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
and configure an external BGP neighborship for R31,
and we'll also configure an external BGP neighborship
for R21.
Now both those routers, R21 and R31,
they are already pre-configured
as are the routers here down in AS 40.
So all we need to do is set up the external BGP neighborship
from R11 to those two routers.
And if we do so correctly,
we should be able to build our external BGP neighborships
between R11 and those respective routers.
So let's go over to R11 and get to work.
All right, so back in the lab environment here,
we are gonna go to R11, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
we are gonna go to R11,
and let's first set up the appearing over to R21.
Now R21 is at 23.10.20.21,
we can confirm that real quick,
just by going to R21 and doing a show IP interface brief,
just to confirm its address.
And indeed, it is 23.10.20.21.
I also use the middle two octets
to indicate the two AS numbers it was going between.
So if you see 10.20,
it's gonna be the network between a Thomas system 10
and a Thomas system 20.
If you see the 2040, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
If you see the 2040,
that would be the link between a Thomas system 20
and a Thomas system 40.
So I'm gonna go ahead and simply highlight that
to copy that and put it in my buffer.
And let's go over to R11 and configure 23.10.20.21,
which is R21 as appear.
So we'll go back to R11.
We'll go into configuration mode for router, BGP10.
And we'll say neighbor.
Now I'll put in the neighbors address of 23.10.20.21,
which is R21's address on its FA4/0 interface. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
which is R21's address on its FA4/0 interface.
Space will type in remote dash AS.
And R21 is in AS20.
So boom, we've just configured an external BGP neighborship.
And because R21 is already configured,
we should here in a moment have a console message
indicating that we have that neighborship.
So let's do the same treatment, but this time,
let's go ahead and do it for our external BGP neighborship
between R11 and down to R31 that's in AS30.
So let's just check R31's interface address
on its FA4/1. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
on its FA4/1.
So if we go to R31,
there's an output of show IP interface brief right here.
And its FA4/1 address is this 23.10.30.31.
So I'll copy that.
We'll go back to R11 and we'll type in neighbor.
One more time here in router BGP10 configuration mode.
It's both type in neighbor.
I'll paste in the IP address that's reachable there
on R31 and I'll say remote AS.
And it's in the tone system 30.
Let's just check our diagram real quick.
So sure enough, R31 is in AS30.
So we'll put a 30 there, press enter. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So we'll put a 30 there, press enter.
And because R31 is already configured here in a moment
as they negotiate their session,
we should have a BGP neighborship.
So now if we did a show IP BGP summary and press enter,
it'll show us our four neighbors.
We have two internal neighbors.
That's there are two IGP sessions over to R12 and R13.
And the second two are both external
because we're going to a top system 20.
R21 is and also to a top system 30 where R31 is. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
R21 is and also to a top system 30 where R31 is.
And that's shown right here in this column
referring to the top system that peers in.
Also, we have learned some routes.
Check this out.
This is showing us that we have learned three prefixes.
That's a fancy way of saying routes in the world of BGP and routing.
But anyway, we learn three routes from the peer at .21,
which is R21 up in AS20.
We've also learned three routes from R31,
which is in a time system 30.
So if we want to see those routes that we've learned
or should be six of them, we can do a show IP BGP. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
or should be six of them, we can do a show IP BGP.
And that's going to show us the routing information base for BGP.
What it's learned about.
So press enter and there are the six routes.
So we've learned about the 2222 network,
the 3232 network and the 4343 network.
Twice, once from the next top here of .31, which is R31.
And also from the next top here of .21, which is R21.
And that is true for those other two routes as well.
And if we take a look at these two right here,
this little greater than symbol here represents WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
this little greater than symbol here represents
is the best route.
Now why in the world did R11 choose this route
with the next top of 2310, 2021, which is R21?
Why did it choose that as the best route over that same route
that was learned from 2310, 3031, which is R31?
And we talked in the overview about the path selection process.
And one of those properties that we looked at is the AS path.
So check it out the path here.
The path for this route is going through autonomous system 20. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
The path for this route is going through autonomous system 20.
And the route that we learned from .31, who's an AS 30,
that route had to go all the way through autonomous system 20,
and then 40, and then 30.
And based on the shortest AS path
and the other previous qualities all being equal,
including weight.
That's why R11 chose this route,
the one with the icon right here of the best path,
using the next top of 23.10.20.21, and not the other one.
And in this case, it's because of the shorter AS path.
So if we were to do a show AP route, this route,
a 22222222, with the next top of .21, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
a 22222222, with the next top of .21,
should be in the routing table.
So we'll do a show IP route.
Back down into a show IP route for just BGP learned routes
to be very easy to see that way.
And here we have our BGP learned routes.
So that next top is 23.10.20.21.
And that's because up here, that is the next top
for its path selection process,
would it consider to be the best route,
which once again, boil down to the lowest AS path,
with 20 being less than 30, 40, and 20.
Now, I would like to show you one other really fascinating thing WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Now, I would like to show you one other really fascinating thing
about this route that our 11 is learned about
for the 22 network.
And that is, it has shared that route with R13 and R12.
And if we were to take a look at R12,
and see what its opinion is of that route,
it's gonna be enlightening.
So let's go ahead and go to R12.
And on R12, we'll do a show IP BGP.
Notice there's a route here for that 22 network.
However, it doesn't have this little symbol next to any one of them.
It's not considering any of these routes to be best.
And as a result, R12 is not putting those routes, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And as a result, R12 is not putting those routes,
any of those routes in the routing table.
So in the next video, I'd like to walk you through why here on R12,
it's not using that route to 22222,
and also share with you how you can fix it.
So I'll see you in the next video for exactly that.