Now, the next item is a relatively simple one,
but it's a big one that you need to understand,
and you need to see it in action.
We're talking about EBGP-learned routes
versus IBGP-learned routes.
Let's jump in the virtual lab and see it in action.
To refresh your memory one more time,
we have R2 and R3, who each have EBGP connections up to R4.
They then have an IBGP connection to each other.
So when R4 advertises its loopback address, 4.4.4.4, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So when R4 advertises its loopback address, 4.4.4.4,
it'll advertise it down to R3 as well as to R2.
But then R3 is going to advertise this over to R2 as
well and vise versa.
So R2 will learn about 4.4.4.4 from R3, an IBGP relationship,
as well as from R4, the EBGP relationship.
And the selection algorithm here says
EBGP always wins over IBGP.
We can verify this by jumping in the virtual lab WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We can verify this by jumping in the virtual lab
and taking a look with a favorite show command.
Once we're on the virtual lab and we've jumped over to R4
and we've issued the network statement to advertise
that local loopback address, we can
say show run section BGP, verify that we've got it advertised.
Yep.
It's right there-- network 4.4.4.4.
So I know it's being advertised.
I can jump back to R2 and give it my favorite command--
show BGP IPv4 unicast.
And there is right there.
Now, I know your eyes may have gone towards these i's before, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Now, I know your eyes may have gone towards these i's before,
but that's not the i we're looking for this time.
Now we're looking for the i that's right here.
We can see that this route for 4.4.4.4
has the little caret indicating that it's the best path right
there, but we don't have the little i here indicating
that it was learned internally.
You see, this route was learned from R3, our IBGP neighbor,
and this route was learned externally
from R4, our EBGP neighbor.
And because EBGP always wins over IBGP, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And because EBGP always wins over IBGP,
that's the one that was selected in this case.
Now, again, I called it all earlier
that the local preference being blank, that's what really
threw me for a loop the first time I was learning BGP.
Under the hood, there is an implicit 100 calculated here.
So as it went through all of the other selection criteria--
for instance, the AS path is the same,
the origins both end with i, the metrics here are 0--
so then it falls to EBGP over IBGP.
And that's how the route to 4.4.4.4 was selected. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And that's how the route to 4.4.4.4 was selected.
We can jump over to R3 and verify a similar result. Sure
enough, our externally-learned rule was
selected over our internally-learned route
that we received from R2.
So that's been EBGP over IBGP as rule number 8
in the BGP best path algorithm.
I hope this has been informative for you,
and I'd like to thank you for viewing.