Hi there, this is part three of chapter two and in this in this lecture I will be covering ebgp. Basically, as we know our five is in different administrative domain which is a is 200. And our two which is neighboring are five is in a different administrative domain, which is hundred. So let's go ahead and start so anything that the neighboring devices are in two different administrative domains. So that's why the terminology that creates the neighboring adjacencies is known as ebgp, which is external for Border Gateway Protocol. So let's start with our five first and do the configuration so config T, and router BGP, it's actually 100 and what we are advertising here is network five dot zero dot zero dot zero and the neighboring IP address or the interface of the neighboring address is 25 dot one dot one dot one remote as hundred I believe that's all we need to do to our five.
Let's bring up our to our tools here. And we will now as you know our two has already is already running BGP, hundred. So comm t we will just jump into the BGP, hundred and we will add the neighbor which is 25 dot one dot one dot two Which is the interface IP address of the of the router are five and we will say remote as 200. Okay, so that should be it and let's verify if I do show I just see the adjacency come up show IP BGP summary and yes, I can see the adjacency here for 25 dot one dot one dot two. And if I do show IP route, show IP route. I can see there is a there's a network here for five dot zero dot zero dot zero slash one.
And I believe I can ping 50555 And yes, I can ping it from our to IP, BGP. I do have an entry here for for the five dot zero dot zero dot zero network and it shows me the next hop. So that's good. So that is what I wanted to show you in this. And that's how we established the ebgp between r two and R five. Now there's a few things I wanted to highlight actually, because since these four routers are in the same autonomous system, let's say let's assume this is a cooperation usually, BGP is not going to be used within the within the same domain administrative distance.
And we will have to introduce an interior gateway, an interior routing protocol that will Basically form adjacencies for all the routers provide full convergence. So rarely you will see an organization internally running BGP. They have to resolve to run either eg RP or OSPF or rip version two within the organization. And that helps a lot. But for the Border Gateway routers, that is fine, you can run the BGP between, let's say r two and R five, and just to establish a neighboring relationship with with different autonomous system networks, and that's what BGP does actually because you cannot use other routing protocols that are IGP for connected to different autonomous system systems or networks, so you have to use BGP. So that's the message I wanted to clear out.
From here. I wanted to make clear. The other point I wanted to basically highlight is the reason why we have used for example, when I do show run and if I bring up the Okay, the reason why used network two dot zero dot zero dot zero for example, and I never put the mask here is because when I say two dot zero dot zero dot zero with no mask, BGP basically assumes it says slash eight network, right? It's a class a network. And that's the reason I did not put the mask. And the other point is BGP will never advertise a network.
That is not that is not clearly defined. In the interface if the network is not clearly mentioned on the interface with the right subnet mask, it will not advertise that network into the routing table. So that's important to know. So if I do show IP interface brief, and I do show running config interface loopback zero. As you can see, this is a slash eight network. Okay, but if I put router BGP, hundred and I put network two dot zero dot zero slash 255245245 dot zero mask 255 dot 255255 dot zero, BGP will look at its interface and say, Oh, hang on a second, I don't see this interface or this IP address and my interfaces and it will never advertise that interface.
So this is kind of critical to know. And I hope that has that made clear. So I think we achieved our purpose of this lab and I hope you have enjoyed the lab. We will continue next in the next labs and we will introduce and mitigated steps to full mesh. As you know, full mesh in BGP is a time consuming for organizations. But there are ways to advertise all the routes without really doing full mesh adjacencies between all the routers.
Thank you and see you in my next topic.