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In the previous video we configured external BGP and we started learning routes

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coming in from

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R21 and also routes that were advertised via BGP from R31. So those are both

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external BGP neighbors

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of R11. And one of those routes was a loopback that's being advertised over

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here in AS20 and that's

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22.22.22. And as that route was advertised by R21, the ASPath attribute simply

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included ASPath 20,

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which had gone through and as it was advertised down here, down here and then

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up to R11 this way.
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up to R11 this way.

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And as that route was advertised up here, the ASPath that included 30 because

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it had gone through AS30

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and also gone to AS40 and also gone through AS20 where it was sourced. So R11

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chose the route that went

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or to R21 because all other things being equal, ASPath was shorter on the route

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it learned from

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R21 compared to the route it learned from R31. And another attribute that

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shipped over when it

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routes are advertised is an attribute called Next Top. So in R21, advertised

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that route for 22, 22,
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that route for 22, 22,

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22, etc. One of the attributes besides the ASPath was Next Top. And so the next

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top in R21

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advertised that would be its own IP address. So the next top value would be 23.

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10.20. And this is 21. So

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it would be .21. Now that's not a problem for R11 because R11 knows how to get

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to that Next Top.

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It's directly connected to that network segment. However, as that route is

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being advertised inside

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of the Atom System 10 here and when R12 learns about 22, 22, 22, 22, that Next

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Top of 23.10.20.21
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Top of 23.10.20.21

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is carried with it. It doesn't change. So here when that route is being

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advertised from R11 to R12,

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it's going to have the ASPath of 20. That's not going to change. It was still

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sourced from AS20.

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It's also going to keep the same Next Top information. So it's going to show

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Next Top of 23.10.20.21.

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That's this interface address right here. And that's going to cause a problem

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here in this

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lab apology because R12 inside the Atom System, it does not have a route for

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that Next Top of 23.10.20.21.
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that Next Top of 23.10.20.21.

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And as a result, because it doesn't have a route for that, it's going to go

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ahead and say, "Well,

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I see the route in BGP and the BGP routing information base, but I'm not going

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to add it to my

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routing table because I don't know how to get to the Next Top." So we can

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verify that by taking a

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quick look at R12. So here on R12, let's go over to R12 and let's do a show IP

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BGP. So there's the 22, 22, 22 network.

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We're missing that symbol, meaning none of these routes are considered best.

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And there's our Next Top of 23.10.20.21.

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So one way of addressing this is we could tell R11. You know what? R11, when
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So one way of addressing this is we could tell R11. You know what? R11, when

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you receive this route from

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an external BGP neighbor and they have the Next Top, instead of just keeping

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that same Next Top as you

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send it into your internal BGP neighbors, go ahead and simply modify that value

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, that Next Top value,

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and just say it's your own address. In this case of R11, there'd be 11 to 11 to

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11 to 11 to 11. And that

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way when R12 receives it and sees the Next Top is 11, 11, 11, 11, 11, 11, they

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'll say, "Oh, you know

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what? I didn't have to reach that." And then it can go ahead and consider that

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to be a valid route
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to be a valid route

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that it can put inside its routing table. And by valid route, meaning it's a

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route that it can use

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and it would put a little symbol next to it indicating it's the best route. So

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to implement this,

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we simply modify the configuration on R11 and just tell it, "Hey, listen, as

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you forward updated

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routes to your peers R13 and R12, which are both IBGP peers, go ahead and

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simply modify the Next Top

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and make it your own IP address." Instead of the original Next Top value that

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you receive. Now,

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that's the only happening when we send it to internal peers. We need to modify

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that because

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when we send that route, for example, from R11 to another external BGP neighbor
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when we send that route, for example, from R11 to another external BGP neighbor

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, we're automatically

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going to go ahead and use whatever our egress interface IP address is and

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modify the Next Top to

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that value. So changing the Next Top value is important for these edge routers

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here before sending

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updates to other internal BGP neighbors. So let's head over to R11. We'll

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modify our

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neighborship to R13 and R12 and specify we're using our own Next Top address

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for the updates that we

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send out. And then we should see the difference show up here on R12 and R13.

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All right. So let's just

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do a quick verification. In fact, I'm going to use this option right here. The
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do a quick verification. In fact, I'm going to use this option right here. The

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little send commands

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to multiple routers at the same time. And I'm going to do a show. And then we

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'll do a few spaces

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there to clear the screen a little bit. Let's do a show IP BGP and press Enter

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and then we'll send

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that to R12 and R13 and we'll send that out. Great, great, great. So there's R

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12. There's the 22

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network with the Next Top of 2310 2021. And if we look at our 13, same thing.

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We have that route

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to 22222222 that BGP information with the Next Top. But also in R13, R13 doesn
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to 22222222 that BGP information with the Next Top. But also in R13, R13 doesn

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't have reach

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really that Next Top. So it's not considering that a valid route. So let's go

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to R11 and change

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the rules. Now, an easy way to do this is to do a show, run, pipe, section. And

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you can do router or

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if you just want to see the BGP, BGP. And then from here, we'll just copy and

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paste router BGP.

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And then we want to modify the neighbor 12, 12, 12, 12. And we're going to say

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Next, Hop,

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self. And we'll do the same thing for 13. So we'll copy and paste neighbor 13
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self. And we'll do the same thing for 13. So we'll copy and paste neighbor 13

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13 13 13 13

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and say Next Top self. And then let's take a look at our config. So we'll do a

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show, run, pipe,

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section, BGP, press Center. So now when we're sending updates to R12 and now

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when we're sending

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updates over to R13, instead of leaving that original Next Top attribute, we're

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going to change it

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to R11's address. And in a production environment, it would be wise not to do

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what I'm about to do.

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But because it's a lab, it'll be okay. I'm going to go ahead and do a clear IP
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But because it's a lab, it'll be okay. I'm going to go ahead and do a clear IP

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BGP

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asterisk. And then I'm going to apply that to 11 12 and 13. And it's going to

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take probably a minute

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or two for the whole thing to converge and to get those routes all ready to go.

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But I want to just

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verify that we have a nice clean config. And we're not hanging on to any older

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information. So if we

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head over to R12 and we do a show IP BGP summary, currently what this says is

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that R12 has not

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learned any routes based on this last column states/prefix received. We haven't

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learned any routes yet.
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learned any routes yet.

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But they're coming. So if we hit that bar of key, we can simply wait. We can

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also show IP BGP.

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And currently there's nothing there. But once we have received the prefixes,

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they should show up

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here as account. And we should also be able to show IP BGP. And that's up there

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as well. But again,

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it may take a minute or two for it to converge. So we'll give it a moment. Let

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the upper key show

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IP BGP. Ah, there they are. So fantastic. So if we hit the upper key a couple

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times and do a show

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IP BGP summary, notice that we have received three prefixes from our peer 11 11
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IP BGP summary, notice that we have received three prefixes from our peer 11 11

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11 11 which is R11.

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So now if we do a show IP BGP right here next to all three routes, we have that

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greater than

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symbol, which means best path. And check it out. The next top is 11 11 11 11,

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which is R 11,

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which we know how to get to. So now these three BGP learned routes. They should

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also be showing up

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in the winner's table, the routing table by doing a show IP route and pressing

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enter.

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And in fact, we go ahead and do a show IP route and filter that to just BGP

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learned routes.

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And there they are right there. The 22 network, the 32 and the 43, which are
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And there they are right there. The 22 network, the 32 and the 43, which are

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the loop backs over on

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router 22, router 32 and router 43. Now, should we be able to ping those

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addresses we have routes?

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Should we be able to ping them and get a response pack? And the answer is no.

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And the reason is

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because we have those routes. So router 12 knows how to forward in that

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direction R 11. But what happens

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though, if we ping the loop backs on R 22 or R 43 or R 32, unfortunately, those

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other autonomous
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other autonomous

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systems have no clue how to get back here to the 10 12 address space or 10 11

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address space

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or the 12 12 12 address space. So if you want to do a ping, for example, from R

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12 and is loop back

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over to AS 20 or AS 40 or AS 30, we also want to make sure we're advertising

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those networks inside

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of BGP so that the other autonomous systems in the routers there would know how

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to forward back.

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So right now, if we went to R 22, for example, and we did a show IP route,

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there's no information here
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there's no information here

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about any networks over at a ton of system 10. So the loop back on R 12 is 12

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12 12 12. It's not

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here. So we sourced the ping from 12 12 12 and it was coming over here to R 22.

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R 22 would not know

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how to respond. And to verify that, let's go to R 12 and let's do a ping over

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to 22. 22. 22. 22. 22. 22.

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We'll source it from our loop back zero. And those pings are reaching the

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destination. But the

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responses are not coming back. So that'll be our next step in the next video is
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responses are not coming back. So that'll be our next step in the next video is

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how do we advertise

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and network in BGP. I'll see you in that next video in just a moment.