WEBVTT 00:00:00.000 --> 00:00:26.000 Before we dive into our next discussion, I solved our R1 R2 problem, which I still don't understand why it was happening because here's the deal, the neighborship is now properly formed and I had figured out that they were both trying to use the same router ID. 00:00:26.000 --> 00:00:49.000 Here's the thing. We know that dynamically your router is going to look at your interfaces and choose the router ID based on that. So the best option here would be the highest loopback address on R2 and the best option here on R1 would be this one right here. 00:00:50.000 --> 00:01:11.000 But for some reason, they were trying to use the exact same router ID. I don't know why. I haven't gone that deep. But as soon as I manually configured the router ID, so if we type in do show run pipe to section router BGP, I manually configured the router ID to this on this router. 00:01:11.000 --> 00:01:30.000 We go over to R2. I manually configured the router ID on this router and then the neighborship just worked. But I had done a little quick research on the error message we had received about the BGP wrong identifier. 00:01:31.000 --> 00:01:45.000 The BGP wrong identifier over here and then over the messages we were getting over here was again identifier wrong. So I just did a quick search and it said both routers are trying to use the same BGP router ID. 00:01:45.000 --> 00:02:03.000 So I went ahead and manually set it and everything was fine. But again, why are they trying to do that? I don't know. I don't have an answer to that right now because there's nothing in the IP addressing space that would even convince them to try to use the same router ID when it's all said and done because I don't have duplicate addresses on these routers. 00:02:04.000 --> 00:02:28.000 All right. Problem solved, though. That's the main thing. Yeah. All right. So let's dive into our next topic of the session. Yeah, I know it really right. Like, in my opinion, that's a bug, right? That's a bug that shouldn't have happened. And I've never had that happen before in this demo at all, which just is strange to me when it's all said and done. Yeah. 00:02:29.000 --> 00:02:48.000 Yeah, it keeps us on our toes as IT professionals, makes our lives very interesting. I agree. All right. So now let's dive into our discussion on BGP best path selection process. So the BGP best path selection process is what? Well, let's think about it from an EIGRP standpoint. So EIGRP does what? 00:02:48.000 --> 00:03:13.000 Well, I have a router that learns from all of its neighbors and then based on all the information gathered, I will decide what the best path is to reach a given destination based on what? The metric, right? It's going to be the metric. My EIGRP metric is going to figure out, well, we're using the diffusing update algorithm, I should say, for EIGRP. If I go this way, it's this far. If I go this way, it's this far, this way, this way, this way, this way. 00:03:13.000 --> 00:03:42.000 And I will ultimately choose the one that is the closest based on the metric. Oh, yeah, it's pretty simple. OSPF, we learned about in our OSPF discussion last week, it was based on cost, cost of interfaces, cumulative added together, all of that stuff. And if an area board of routers in there, I mean, we're not going to go over that again. But in the end, again, it was based on what? Bandwidth. So the EIGRP, our metric would be based on things like bandwidth, delay, reliability, load. By default, it's just bandwidth and delay. 00:03:43.000 --> 00:04:12.000 And with something like OSPF, it's purely based on interface costs. Well, BGP's best path selection process doesn't even care about that stuff. None of that stuff. And BGP's best path selection process is going to be based on a whole list of attributes that have to be analyzed from top down. Top down processing immediately execute upon a match. That's it. Once you find that match, that is unique, that is the one you want to use, it's over, it's done, you don't use any of the other ones in the list. 00:04:12.000 --> 00:04:35.000 Kind of like an access control list when it's all said and done. Now, when you think about BGP, and you think about the web of connections that we have between different autonomous systems, our BGP routers are going to receive information about each prefix from different autonomous systems, different peering autonomous systems. 00:04:36.000 --> 00:05:03.000 So when you think about this one here, 192.0.2.0-24, as we were talking about earlier, every autonomous system is going to learn about that multiple times. For example, this autonomous system here is going to learn about it this way, it's going to learn about it this way, it'll also learn about it this way. You're like, wait, how does it learn about that way? Because this autonomous system learns it that way, it's going to share with that one, that one's going to, right, but also what about this way, this way, this way, this way, this way? 00:05:04.000 --> 00:05:22.000 I mean, it's going to be going everywhere. So there has to be some sort of elimination process in order to ultimately what? Determine what is the best path to use in this very specific moment in time under this given scenario with these given conditions. 00:05:22.000 --> 00:05:49.000 So that way, they're the single best path can be chosen. Here's the list. Now, I want you to realize that this is a Cisco specific list. And from other vendors, you might see a slightly different list. For example, this option here is Cisco proprietary, you're only going to find it on Cisco products. It's not something that's going to come into play if you're using non Cisco products running BGP. 00:05:49.000 --> 00:06:13.000 Something to keep in mind. But first and foremost, before we even go through this 11 step process or begin it, first things first, having next top IP address, right, that is reachable, meaning that we're going to ignore any path that we receive, if we can't get to the next top IP address, no matter what. 00:06:14.000 --> 00:06:35.000 So for some reason, you learn something, but you cannot get to the next hop IP address, because you don't have a route in your routing table to get to the next hop IP address, then you are not going to use that path, you are going to ignore it, the rest doesn't even matter. And that makes sense, right? Because you don't want to use something that you can't actually get to the next hop router for. 00:06:36.000 --> 00:07:00.000 The next one is don't use any path. So ignore paths that have a path that contains your own autonomous system number. We talked about that a little earlier. In our last discussion, by default, if you receive information, and that information has your own autonomous system number in it, ignore it by default. 00:07:01.000 --> 00:07:22.000 That is a loop prevention mechanism. That's what that is to make sure that we don't accidentally cause loops. Because think about it this way. If I taught you about this, and then you taught them, and they taught me, and then I went and use them, does that not mean we'd have something like this? Or oops, traffic would do something like this, and we 00:07:23.000 --> 00:07:38.000 Yes, that's exactly what would happen. So that's why this rule exists here. Do not ever, unless overridden by an administrator, accept any path that has your own autonomous system number in it. 00:07:39.000 --> 00:07:51.000 All right, so once those two conditions are checked off, and you're like, okay, I can get to the next hop IP address, it doesn't contain my own autonomous system. Now it's time to go through all of this stuff here. 00:07:52.000 --> 00:08:15.000 So if I type in something like do show IP BGP, we're going to see that stuff right here. You'll see that the next hop information is there. So I can reach these next hops. So that's good. There's metric, there's local preference, there's weight, there's the AS pass. So that's just some of them. 00:08:16.000 --> 00:08:30.000 But this is not fun. Why? Because that's just a small list. So we're going to take you out to that BGP router that's out on the internet. So that way, there, we can go through all of these one by one, learn about them. So that way, there, we can be successful on the exam. 00:08:30.000 --> 00:08:52.000 Because on the exam, they might give you output like this and ask you, why was such and such a path chosen? What was the deciding factor? Which item in this list is the reason why the route that was chosen was chosen? 00:08:52.000 --> 00:09:10.000 So yes, you got to memorize this. Now, typically on the exam, how far do they go? Typically, we're down to here. You typically don't have to go beyond that on the certification exam. So would I encourage you to memorize whatever you see here in bold? Absolutely. That's why it's bolded. 00:09:10.000 --> 00:09:34.000 We love oranges and orange marmalade. Repeat after me. We love oranges and orange marmalade. We love oranges and orange marmalade. I didn't come up with that. 00:09:34.000 --> 00:10:02.000 I didn't come up with that. But my mentor did back in the day. And for some people, it works. For some people, it doesn't. But if it works for you, great. But know the order and memorize whether it's highest or lowest, or local, or shortest, right? Those are the things you got to know at a minimum to get ready for the certification exam. 00:10:05.000 --> 00:10:24.000 All right. So number one, prefer the path with the highest weight. So this one is Cisco proprietary. Like I said, it's only going to be available on Cisco routers running BGP. So every single one of our BGP learned routes is going to have a weight attribute associated with it. 00:10:24.000 --> 00:10:46.000 By default, if you learn it from another neighbor, it is going to have a weight of zero. If you use the network command locally on your device, then the weight will be set to 32,768, as you can see right here. 00:10:46.000 --> 00:11:14.000 So if you learn it, it's set to zero. If you created it, you set it to 32,768. Right? Higher is better. Higher is better. Always remember that. And that's what's listed right here on our page. Higher is better. 00:11:14.000 --> 00:11:35.000 The total range is zero to 65,535. But now the real way to learn this is to go to our BGP router that's out on the internet. Because what has to happen here is that for every single network you learn, you have to go through this process. 00:11:36.000 --> 00:12:02.000 So for example, and now this is going to help you understand this a lot more because there's more here. All of these are for 1.0.0.0 size 24. You see where the next one starts? Down here. 1.0.4.0. So everything between the next one, the next network ID, and that, that's all belonging to that one network. All of it. 00:12:03.000 --> 00:12:28.000 Now remember I said only one of them can be the best path. And it's the one with the greater than sign. Where is the greater than sign? It's right here. This is the one installed in the routing table. That is the best one. The question is why? That's what we're trying to learn. That's what we're trying to understand. That's what we need to be able to do for the certification exam. Why was this one chosen? Why does it have the greater than sign? 00:12:28.000 --> 00:12:48.000 If I type in right now, show IP route, you'll see that it's via 4.68.4.46. 4.68.4.46. Why is that one in the routing table? 00:12:48.000 --> 00:13:13.000 Okay. Well, let's look at weight first. Weight, as we already described defaults to 0 if you learned it. So I learned about 1.0.0.0 size 24 from this one, this one, this one, this one, this one, this one, this one. Now you get the picture. So therefore, since I learned it, weight is 0 by default 0.0.0.0.0.0.0. 0.0.0.0.0.0.0.0. That means we have a tie, doesn't it? We have a tie. 00:13:13.000 --> 00:13:32.000 We have a tie. So as long as two, as long as two or more have the lowest possible value, then you have a tie. So anytime you have a tie, what does that mean? It means you got to move on to the next one. So tied. We now have to move on to the next one. 00:13:32.000 --> 00:13:56.000 Step number two now. But before we get there, let's just go back and understand that if at any point in time, if we manually change this value, we manually change this value, then we could manipulate the decision making process. We'll talk about that later today. 00:13:57.000 --> 00:14:21.000 But just be mindful of that. You might be saying, well, this is silly, Raymond. They're all going to be 0 if I learn it. Yeah, but you can change it, right? You can manipulate that to change the decision making process. Because remember, from that list, weight is first. So if you want your router, let's say, to use this one here instead, then what would you do? Raise the weight of all, no, higher is better, right? Higher is better. Raise the weight of that to, in this case, 1. 00:14:22.000 --> 00:14:34.000 Even though you can go up to 65,000, 35,000, just change that to 1. And guess what? It's now the best. And the greater than sign would move right here. And this is the one that would be installed in the routing table. Why? Higher is better for weight. 00:14:34.000 --> 00:14:52.000 But right now, they're all the same. Since they're all the same, that means we have a tie. When you have a tie, you move on to the next one in the list. All right, so now local preference. This one here is an industry standard attribute, meaning it's going to work on Cisco devices, non-Cisco devices, everything. 00:14:52.000 --> 00:15:14.000 And in this particular case, it is prefer the path with the highest local preference. So local preference defaults to a value of 100. It defaults to a value of 100. And therefore, anything higher than 100 is better. Anything lower than 100 is worse. Higher is better. 00:15:14.000 --> 00:15:32.000 So where do we find that in our table? We're going to find that in our table right over here. So what I want you to pay attention to, especially for the exam, is if it's blank, blank means 100, right? I've got to prove that to you just to make sure you recognize that. 00:15:32.000 --> 00:16:00.000 Let's type in show IPBGP. Oh, got to get out of the way here. Show IPBGP 1.0.0.0 slash 24. All right, you'll see it says right there, local pref 100. We go to the next one, local pref 100. Next one, local pref 100. So you can see blank equals 100 in this table. 00:16:01.000 --> 00:16:26.000 Blank equals 100. So in this case, every one of them is tied again. But keep in mind, right, that if I wanted to have another one of these chosen, could we manipulate local preference? Sure, of course we could. If I gave one of these a value higher than everyone else, give it a highest value, let's say we change it to 105. 00:16:27.000 --> 00:16:42.000 Well, then now it would be the preferred one because it's the only one at 105. Everybody else is at 100. And we win. And so therefore, let's say we it was this one we changed to 105. It is now going to be the one that's installed in the routing table. 00:16:43.000 --> 00:17:02.000 But remember, local preference is only going to be used if weight is tied. If weight is not tied, then the weight is the winner. So if there's a tie in weight, local preferences then check. But if there's no tie in weight, then weight is the winner. Always remember that it's so important, especially for the exam to understand this breakdown. 00:17:03.000 --> 00:17:12.000 So even if you had values here, let's get a little creative here. Let's make a mess. 00:17:12.000 --> 00:17:41.000 OK, so let's say weight here was 50, weight here was 90, weight here was 100, or 1,000. Put in 1,000 for fun. All right. And but let's say local preference over here was 1,005. Let's say local preference here was 92 and 56 and 47. Right. None of these are tied. 00:17:42.000 --> 00:18:01.000 None of these matter because someone over here wins. If you win in weight, then that doesn't matter. But if everybody over here was normal or tied, even if two were tied, let's just make two of them tied. 00:18:01.000 --> 00:18:22.000 So if two of them are tied at 50 and 50, what you now have to realize is all the rest are now out of the game. You're now all out of the game. Since you're all out of the game, these two are the only two that are left. So now we're only comparing these two now. 00:18:22.000 --> 00:18:41.000 And look, this one is 100. This one is 92. So who's the winner? 100 would be the winner here. Why? Because 100 is greater than 92. Just trying to get you in the right mindset of how you break things down in case any of these show up on the exam. 00:18:42.000 --> 00:19:03.000 My pen just died. It had like one scribble left in it today. Going to have to charge it. So now let's say local preference is tied. So if local preference is tied, what does that mean? It means now you have to look at this really interesting attribute called locally originated. 00:19:04.000 --> 00:19:20.000 Prefer the path that was locally originated. All this means is this. Did you inject it into BGP or did you learn it? Did you inject it into BGP or did you learn it? 00:19:20.000 --> 00:19:44.000 So one of the ways you can check this out is based on something we saw on our own routers today, the ones that we were playing around with. And when we were at ISPA, if you recall from ISPA, when we use the network command and I own that route, we got a next top of 0, 0, 0, 0. 00:19:45.000 --> 00:20:08.000 So immediately if you ever see next top 0, 0, 0, 0, you know 100% that it was locally originated. But that is not 100% accurate all the time. And the reason why is because you go to somebody like R2. We use the network command here, but we don't own the network. Somebody else owns the network. 00:20:09.000 --> 00:20:30.000 But this was also locally originated. But for the sake of this certification exam, the easiest way to see if it's locally originated is just look for 0, 0, 0, 0, because you're not going to have the ability to deep dive into all the options on that platform, on that device, on the exam to verify if it's actually truly locally originated or not. 00:20:31.000 --> 00:20:52.000 So here's my exam tip for you. If you see 0, 0, 0, 0 in the next hop, it's locally originated. If you don't see it, assume it's not locally originated because there's like a 99.0% chance that it was not locally originated. So play the odds in that particular case and you'll win on this certification exam. 00:20:53.000 --> 00:21:01.000 So if you see 0, 0, 0, then that means locally originated. If you don't, then that means it's not. So that's the best way to approach that for the exam. 00:21:02.000 --> 00:21:21.000 All right. So in this case, none of them are locally originated. They all were learned in this particular scenario. So that means we have a tie. None were locally originated. They were all learned. So therefore, it's a tie. Now we got to move down to the next one. 00:21:21.000 --> 00:21:42.000 The next one says prefer the shortest AS path. So remember, the AS path is the hop by hop by hop by hop by hop in order to get there. So take it from the perspective of the actual router in this case. This particular router, it has learned about this network from this next hop. 00:21:43.000 --> 00:21:51.000 So this next hop address, this router that has this IP address is in this autonomous system. 00:21:55.000 --> 00:22:03.000 Yes. Therefore, this next hop IP address is in this autonomous system. 00:22:03.000 --> 00:22:22.000 And this one is in this autonomous system. Now, remember what I said earlier, this network lives in this autonomous system. So the router I'm on is in an autonomous system. 00:22:23.000 --> 00:22:42.000 And all of its neighbors belong to the autonomous system that's listed next here. And then this network is in this autonomous system. So this is our AS path. For me to this autonomous system, it's ultimately to the destination. For me to this autonomous system to this one, the ultimate destination. 00:22:42.000 --> 00:23:08.000 Look down here. Now there's a little bit more. Me to this autonomous system, then this autonomous system, then this one. Me to this one, then this one, then this one. Or going down to the bottom, we got five me to this one. then to this one. Then to this one. Then to this one. Then to this one. 00:23:08.000 --> 00:23:28.000 Based on the shortest path. Shortest path. So out of all of these now, out of all of these, the shortest we have is how long, folks? The shortest path is how long? How many ASs? 00:23:28.000 --> 00:23:47.000 Okay, so I see 2. 2, 2, 2, 2, 2, 2, 2. I see 3. 2, 2, 2, 2, 2, 2, 2, 2. 4, 3, 5. So what's the shortest? What's the shortest? What number? It is, it is, chat in. 00:23:47.000 --> 00:24:05.000 That's right, 2. That's it, okay? So but is there a tie? Okay, so we've got a tie, a tie, a tie, a tie, a tie, a tie, right? Okay, so with that in mind now, all the paths that have two AS numbers in it are still in the competition. 00:24:05.000 --> 00:24:30.000 If you have three, you're out. If you have four, you're out. If you have five, you're out. So now we are only caring about these ones here, plus these ones here. And that's it. The rest of them, you don't qualify anymore. You are not good enough. Shortest AS path. 00:24:31.000 --> 00:24:54.000 All right. So because we have a tie, then we got to move on. But if you were on the exam, and all of a sudden you saw that there was only one with two, or one with three, and all the other ones were higher than those numbers, then that's your winner. That is your winner, right? And let's not forget here. 00:24:55.000 --> 00:25:11.000 We're only this far because weight is tied, local preference is tied. It was not locally, none of them were locally originated, so all of them are tied with being learned. And now we're down to the AS path. That's the order. You got to go through the order that way. 00:25:11.000 --> 00:25:29.000 And so now we're looking at the AS path, and we've got a bunch that are tied, and some that are not anymore, so away you go, you're gone. And whatever's remaining, we'll keep working with. 00:25:30.000 --> 00:25:54.000 The next one is prefer the path with the lowest origin code, or origin type, I should say. So what is the origin type? The origin type is based on whether it was IGP, EGP, or incomplete. That's right. This is one of the hardest ones to wrap your heads around. 00:25:55.000 --> 00:26:03.000 Okay, these are your origin codes, that's it. And they don't mean what you think they mean. That's right, they don't mean what you think they mean. 00:26:04.000 --> 00:26:20.000 So I, where am I looking for the I over here? These are where the origin codes show up. Notice that they're all I's. Every single one of them is an I. So obviously we already know we're tied. 00:26:21.000 --> 00:26:35.000 Okay, what are the other options that you can have? You can have an E, and you can have a question mark. So E will never show up. You're not going to see it nowadays. Why? Because E actually stands for EGP, which is the predecessor to BGP. 00:26:36.000 --> 00:26:51.000 The origin of that network is EGP, the predecessor to BGP, and nobody's using it anymore. So you're not going to ever see that code. So in reality, you're going to see this code, or you're going to see this code. 00:26:51.000 --> 00:27:15.000 I is better than E, which is better than question mark. Let me repeat that. I is better than E, which is better than question mark. So if we go back to this page here, notice how it says lowest origin type, lowest. 00:27:16.000 --> 00:27:33.000 Well, lowest origin type, think of it this way. This is the lowest, then this, then this. So I is the lowest. Question mark is the highest. That's the way to think about it. 00:27:34.000 --> 00:27:49.000 So when does I show up? I is going to show up when you use the network command to put the route into the BGP. I'm not joking about that. That's right. I told you it's tough to wrap your head around this one. 00:27:50.000 --> 00:28:06.000 So let's just take a moment here and do a little side discussion by going back to our routers here. So you notice how we have an I here. Why? Because I use the network command. You notice how we have an I here. Why? Because I use a network command. 00:28:06.000 --> 00:28:22.000 Even if you use something like the aggregate address command, you're going to see an I there. When are you going to see the question mark? When you use the redistribute command. Redistribute. So let's see, do I have a connected route here I can redistribute or a static route I can redistribute? 00:28:23.000 --> 00:28:48.000 So for example, let's this is a connected route. Let's redistribute connected and watch what happens. Let's type in redistribute connected. Yeah, that's good enough. Just for fun. Do you show IP BGP? There you go. There's your question marks. That's it. I'm serious, folks. Okay, so on the exam, right? If you see question mark, it simply means that somebody used the redistribute command at some point. 00:28:49.000 --> 00:29:01.000 If you see the I, it means somebody did like network command or the aggregate address command or something like that. That's all. When it's all said and done, right? If you boil it down to those basic things, you're going to be able to answer the questions on the exam relatively easily. 00:29:02.000 --> 00:29:24.000 So the key here is if you see eyes and question marks for the exact same network. So for example, we go back to here. If you saw some eyes here and you saw some question marks, the eyes are better than the question marks, meaning eliminate the question marks and whatever eyes are leftovers, you'll continue using. 00:29:25.000 --> 00:29:26.000 That's it. 00:29:28.000 --> 00:29:34.000 If there's only one eye and the rest are question marks, there is your winner. You're done. You found the answer you were looking for. 00:29:35.000 --> 00:29:54.000 Yon says, does that mean that they're locally sourced since they're 0000? Those ones? Yes. As I was referring to a while ago, we were having. Yes. So we were talking about the originated option. That's what I was referring to when we were having to originate option. This one right here. 00:29:55.000 --> 00:30:12.000 And so you see the best way to know if it's a locally originated is 0000. If the next half is 0000, that's not. But there are other times that it could be locally originated, but not show 0000. But for the sake of this exam and for us to say, focus on what we need to know, that's how you're going to do it on the exam. 00:30:13.000 --> 00:30:17.000 If you see 000, that's locally originated. If not, then it's not locally originated. It's that simple. 00:30:19.000 --> 00:30:23.000 And you remember that for the exam, you'll have no problem getting through this, getting through step three, I should say. 00:30:24.000 --> 00:30:27.000 Redistributed always 000. Well, what would just happen in our case? 00:30:29.000 --> 00:30:41.000 OK, in this case, the answer is yes, it was 0000. Why? Because we own those networks. OK, we own those networks. They are directly connected to us. 00:30:42.000 --> 00:30:53.000 That's why they're showing up as 0000. But if it was not something that was directly connected to us, if it was something that we learned, then, no, it would not show up as 0000. 00:30:54.000 --> 00:31:03.000 All right. Let me rephrase that. If you redistributed it and it was something you learned. So if I learned it and then I redistributed it, then it would not show up as 0000. 00:31:06.000 --> 00:31:14.000 So, no, it's not always going to be 0000. Yeah. Right. It's just like, you know, if you have a network that's directly connected to us, it's going to show up as 0000. 00:31:14.000 --> 00:31:19.000 So where are we? Origin type. That's right. And we have a network that's directly connected to us. 00:31:19.000 --> 00:31:31.000 So, no, it's not always going to be 0000. Yeah. Right. It's just like over here on R2. You see how we use the network command here, but it's not 0000. Why? Because I learned it via OSPF. 00:31:31.000 --> 00:31:49.000 So where are we? Origin type. That's right. And we have successfully identified I versus question mark. So know that for the exam. I is better than question mark. 00:31:50.000 --> 00:31:56.000 I is better than question mark. All right. Next one is six. Multi-exit discriminator. So as we are picking our way down through here. 00:32:02.000 --> 00:32:10.000 We are now looking for MED. This is the MED. I know it doesn't say MED, but that is the MED. Okay. That is the MED. The multi-exit discriminator. 00:32:11.000 --> 00:32:25.000 Now, the MED has a couple of different purposes, and one of the purposes is to take whatever, let's say you're redistributing. Let's say you're redistributing from EIGRP into BGP or you're redistributing from OSPF into BGP as an example. 00:32:26.000 --> 00:32:40.000 When you redistribute in that particular case, EIGRP would have a metric. Let's say your metric was 32,852 because that was the metric to get to the destination. Let's say for OSPF, your cost was 37 to get to the destination. 00:32:41.000 --> 00:32:55.000 Well, then when you redistribute into BGP from OSPF or EIGRP, that metric value is retained for this area right here. It's imported as well. Even though that doesn't mean the same thing for BGP, it actually becomes the MED. It becomes the metric in that particular case. 00:32:56.000 --> 00:33:10.000 But that's not only what it's for because we can manipulate this value any way we want, and we could ultimately manipulate the decision-making process as well. But it can be used as a placeholder for the metric of other dynamic routing protocol. 00:33:11.000 --> 00:33:29.000 This is really good for MPLS because if you've ever worked with MPLS, if you have OSPF and you redistribute into BGP, and then your service provider takes that BGP route, shares it all the way over to your branch office from headquarters, and then it sends it into your branch office, and then you redistribute it back into OSPF, let's say. 00:33:29.000 --> 00:33:48.000 If you're using OSPF in headquarters and you're using OSPF in the branch office, now the metric that you have in the headquarters is going to be preserved as it goes through the BGP environment, and then your branch office can pick up where that metric left off in headquarters. 00:33:49.000 --> 00:33:51.000 That's pretty cool when you think about it that way. 00:33:51.000 --> 00:33:53.000 So that's just one example of how that can be used. 00:33:53.000 --> 00:33:59.000 But there are a multitude of different ways that we can use it, and I'll show you one here a little later on when we show you how to manipulate the MED. 00:34:03.000 --> 00:34:06.000 So MED, lower is better. 00:34:06.000 --> 00:34:08.000 Lower is better. 00:34:08.000 --> 00:34:13.000 So when we start looking at this list, you'll see this one has 10, so oh, timed out. 00:34:13.000 --> 00:34:14.000 Let's log back in. 00:34:19.000 --> 00:34:23.000 You'll see here that this one is 10, and we had an AS. 00:34:23.000 --> 00:34:24.000 Oh, what happened? 00:34:24.000 --> 00:34:25.000 Oh, I hit a button. 00:34:27.000 --> 00:34:32.000 Okay, this one is 10, but it had an AS path of 2, correct? 00:34:32.000 --> 00:34:34.000 So we were still in the running. 00:34:34.000 --> 00:34:35.000 We were still in the running. 00:34:36.000 --> 00:34:48.000 But now because lower is better when it comes to MED, we are no longer valid because this one is 0, this one's 0, this one's 0, this one's 0, this one's 0, 0, 0, 0. 00:34:48.000 --> 00:34:52.000 So you see there's a whole bunch of other ones that are lower than me, so bye-bye. 00:34:52.000 --> 00:34:55.000 You're no longer valid because lower is better. 00:34:55.000 --> 00:34:57.000 So essentially, who do we have left? 00:34:57.000 --> 00:34:59.000 I think we got this one left. 00:34:59.000 --> 00:35:02.000 We got this one left, this one left, this one left. 00:35:02.000 --> 00:35:08.000 We got this one, this one, this one, this one, okay, no, you're gone, you're gone. 00:35:08.000 --> 00:35:10.000 We got this one left, we've got this one left. 00:35:10.000 --> 00:35:18.000 We got this one left, this one left, this one left, this one left, you're gone, you're gone, you're gone, you're gone, so that's it. 00:35:18.000 --> 00:35:19.000 There we go. 00:35:25.000 --> 00:35:30.000 Oh, and I just realized, I think, I kept saying we had one with five, 00:35:30.000 --> 00:35:32.000 but I was looking at it incorrectly. 00:35:32.000 --> 00:35:36.000 The one with five is actually for 1040-24. 00:35:36.000 --> 00:35:38.000 I'm seeing that now. 00:35:38.000 --> 00:35:42.000 So we didn't actually have one with five for 1000-24. 00:35:42.000 --> 00:35:44.000 We didn't. 00:35:44.000 --> 00:35:45.000 So that's fine. 00:35:45.000 --> 00:35:46.000 It didn't matter. 00:35:46.000 --> 00:35:47.000 It wasn't in the running anyways. 00:35:47.000 --> 00:35:51.000 But, yeah, that would be for 1040 because it's lined up with that one. 00:35:51.000 --> 00:35:52.000 All right. 00:35:52.000 --> 00:35:53.000 I just saw it incorrectly. 00:35:53.000 --> 00:35:55.000 Crooked eyeballs today. 00:35:56.000 --> 00:35:57.000 All right. 00:35:57.000 --> 00:36:00.000 So, as you can see, we're picking the list off, right? 00:36:00.000 --> 00:36:02.000 It's a lot of work. 00:36:02.000 --> 00:36:05.000 Thank goodness BGP can do that relatively fast. 00:36:05.000 --> 00:36:09.000 Now, more than likely, folks, on the exam, that's it. 00:36:09.000 --> 00:36:11.000 You're not going to have to go any further, okay? 00:36:11.000 --> 00:36:22.000 You would have actually found one by now on the exam that was the ultimate winner, either based on weight, local preference, probably not locally originated, probably not. 00:36:22.000 --> 00:36:23.000 Let's be clear about that. 00:36:23.000 --> 00:36:28.000 So more than likely weight or local preference or AS path would have been the winner or even med. 00:36:28.000 --> 00:36:31.000 Probably not originated, probably not origin type. 00:36:31.000 --> 00:36:36.000 If it is origin type, though, it means that one of the paths would have an I and the rest would all have question marks. 00:36:36.000 --> 00:36:38.000 So that would be it. 00:36:38.000 --> 00:36:39.000 All right. 00:36:39.000 --> 00:36:40.000 So keep that in mind. 00:36:40.000 --> 00:36:42.000 Make sure you know that list, the top six minimum. 00:36:42.000 --> 00:36:43.000 You have to. 00:36:43.000 --> 00:36:44.000 All right. 00:36:44.000 --> 00:36:46.000 Now, seven, it says prefer EBGP over IBGP. 00:36:46.000 --> 00:36:53.000 Now, I can't verify all of these because I don't have the rights to just start digging into this router and go deep into it. 00:36:53.000 --> 00:36:55.000 But I can verify number seven. 00:36:55.000 --> 00:36:57.000 How can I verify number seven? 00:36:57.000 --> 00:37:04.000 Seven is actually pretty easy because E versus I would show up on the left-hand side. 00:37:04.000 --> 00:37:05.000 That's right. 00:37:05.000 --> 00:37:07.000 Show up on the left-hand side right here, right in this space. 00:37:07.000 --> 00:37:09.000 You see this space right here? 00:37:10.000 --> 00:37:11.000 Right. 00:37:11.000 --> 00:37:23.000 In that space, you could have an I, not this I, this I, not this I, this I. 00:37:23.000 --> 00:37:26.000 You could have an I in there. 00:37:26.000 --> 00:37:31.000 If you have an I in there, it means IBGP. 00:37:31.000 --> 00:37:35.000 It means you learned it from an IBGP neighbor. 00:37:36.000 --> 00:37:45.000 Whereas if you don't see an I, it means you learned it from an EBGP neighbor. 00:37:49.000 --> 00:37:57.000 If we go to our Cisco pod here we have, let's see if I can just demonstrate that for you real quickly. 00:37:57.000 --> 00:38:02.000 Let's go to R1, and let's just inject a network on R1. 00:38:03.000 --> 00:38:06.000 Let's just find any one we can find in a routing table on R1. 00:38:06.000 --> 00:38:35.000 This may or may not work, but let's just take this for fun, and let's type in network, 00:38:35.000 --> 00:38:39.000 and it might not work only because of administrative distance yelling at me when it's all said and done, 00:38:39.000 --> 00:38:41.000 but we'll give it a try. 00:38:41.000 --> 00:38:45.000 It's 255.255.255.252. 00:38:45.000 --> 00:38:47.000 Do show IBGP. 00:38:47.000 --> 00:38:49.000 All right. 00:38:49.000 --> 00:38:50.000 Is it in there? 00:38:50.000 --> 00:38:55.000 Okay. 00:38:55.000 --> 00:38:56.000 There it is. 00:38:56.000 --> 00:38:59.000 Let's go to R2 now and see if R2 shows it. 00:38:59.000 --> 00:39:01.000 Yep. 00:39:01.000 --> 00:39:02.000 There it is. 00:39:02.000 --> 00:39:03.000 This is all I wanted to show you. 00:39:04.000 --> 00:39:05.000 I. 00:39:05.000 --> 00:39:06.000 There you go. 00:39:06.000 --> 00:39:07.000 IBGP. 00:39:07.000 --> 00:39:08.000 IBGP neighbor. 00:39:08.000 --> 00:39:09.000 That's it. 00:39:09.000 --> 00:39:10.000 All right. 00:39:10.000 --> 00:39:12.000 So that's this I right here. 00:39:12.000 --> 00:39:14.000 Now, notice there's an R. 00:39:14.000 --> 00:39:15.000 Okay. 00:39:15.000 --> 00:39:16.000 What is the R? 00:39:16.000 --> 00:39:18.000 You don't have to worry about this for this exam, but I'll tell you right now, you never know. 00:39:18.000 --> 00:39:19.000 It could show up. 00:39:19.000 --> 00:39:20.000 Rib failure. 00:39:20.000 --> 00:39:21.000 Rib failure. 00:39:21.000 --> 00:39:31.000 So the rib failure is not a Mortal Kombat move, sorry, but I guess it could be. 00:39:31.000 --> 00:39:44.000 Rib failure refers to the fact that we cannot install this in the routing table because there is this route already in the routing table with a better administrative distance. 00:39:44.000 --> 00:39:57.000 So if I say show IBGP rib failure, you'll see it says that these are not installed in the routing table because it has too high of an administrative distance. 00:39:57.000 --> 00:40:15.000 So IBGP routes have an administrative distance of 200, and all of these are learned either because they're directly connected to us or we learn them via OSPF or EIGRP or maybe even exterior border gateway protocol. 00:40:15.000 --> 00:40:20.000 So if you learn about something via EBGP and IBGP, then EBGP is going to be the winner. 00:40:21.000 --> 00:40:25.000 If you learn about something via EBGP and OSPF, EBGP is the winner. 00:40:25.000 --> 00:40:36.000 But if you learn about something via OSPF and IBGP, then OSPF is the winner because IBGP has a higher administrative distance of 200. 00:40:36.000 --> 00:40:40.000 So if there's an I, it means IBGP. 00:40:40.000 --> 00:40:43.000 If there's nothing there, it means EBGP. 00:40:43.000 --> 00:40:45.000 So we come back and we look at this. 00:40:51.000 --> 00:40:58.000 We'll see that there are no I's, no I's at all over here on the left-hand side, left-hand side, left-hand, nothing, nothing. 00:40:58.000 --> 00:41:01.000 So we have a tie in regards to IBGP. 00:41:01.000 --> 00:41:05.000 Sorry, tie in regards to EBGP, EBGP in that case. 00:41:05.000 --> 00:41:09.000 But if all of them are I's except for one of them, then that would be the winner, the one without. 00:41:10.000 --> 00:41:12.000 So seven's not the deciding factor. 00:41:12.000 --> 00:41:13.000 We go to eight. 00:41:13.000 --> 00:41:17.000 Prefer the path of the lowest IGB metric to the EBGP next time. 00:41:17.000 --> 00:41:19.000 So I can't actually verify that on these routers. 00:41:19.000 --> 00:41:22.000 I do not have permissions to do that on these routers. 00:41:22.000 --> 00:41:28.000 But essentially, it's saying, all right, so now that we've gotten this far, what is the IGB path to get there? 00:41:28.000 --> 00:41:38.000 What is the IGB metric, the IGB metric to get there, the IGB metric to get there, the IGB metric to get there, there, there, there, there, there, there, there, there, there, and so on and so forth. 00:41:38.000 --> 00:41:40.000 I can't verify that on these routers. 00:41:40.000 --> 00:41:42.000 I do not have permissions to do that. 00:41:42.000 --> 00:41:48.000 But let's say that in this particular case, two or more are tied. 00:41:48.000 --> 00:41:52.000 So then the next one, if two or more are tied, more than likely on the exam, you never have to do that one anyways, okay? 00:41:52.000 --> 00:41:54.000 Keep that in mind. 00:41:54.000 --> 00:41:57.000 Prefer the older path, okay, the older path. 00:41:57.000 --> 00:42:00.000 Which one has been, which one have we known longer than the other? 00:42:00.000 --> 00:42:01.000 That's right. 00:42:01.000 --> 00:42:04.000 Which one have we known longer than the other? 00:42:04.000 --> 00:42:08.000 Now, for the certification exam, not something that would be easy for you to do. 00:42:08.000 --> 00:42:10.000 So I wouldn't worry too much about that one. 00:42:10.000 --> 00:42:16.000 But if you ever had to verify that, the easiest thing to do is look at your neighborships, okay? 00:42:16.000 --> 00:42:18.000 Now, that's not 100% perfect. 00:42:18.000 --> 00:42:26.000 But typically on an exam, just looking at how long you've been neighbors with somebody will ultimately determine if that is the reason why you won. 00:42:26.000 --> 00:42:29.000 So, for example, six weeks, three days. 00:42:30.000 --> 00:42:31.000 Now, we know. 00:42:31.000 --> 00:42:34.000 We know already that this one is the winner. 00:42:34.000 --> 00:42:35.000 This one is the winner. 00:42:35.000 --> 00:42:38.000 We know that already because there's a greater than sign there. 00:42:38.000 --> 00:42:40.000 We know that. 00:42:40.000 --> 00:42:46.000 So if we look now at how long they've been a neighbor with me, six weeks, three days. 00:42:46.000 --> 00:42:48.000 Anybody else here longer than that? 00:42:48.000 --> 00:42:49.000 Yeah, we do. 00:42:49.000 --> 00:42:50.000 Okay. 00:42:50.000 --> 00:42:58.000 So, so, it is very possible, very possible in this particular situation. 00:42:58.000 --> 00:43:08.000 Since somebody is older than us, we may have won in this case purely based on this one here. 00:43:08.000 --> 00:43:11.000 We may have won using eight in that example. 00:43:11.000 --> 00:43:19.000 But I can't verify that right now because I don't have permissions on that router because this one is actually older than this one. 00:43:21.000 --> 00:43:22.000 And we go down. 00:43:22.000 --> 00:43:23.000 This one's even older. 00:43:23.000 --> 00:43:25.000 Ten weeks, three days, okay? 00:43:25.000 --> 00:43:27.000 And so on and so forth. 00:43:27.000 --> 00:43:32.000 But I guess now the question is, is did we actually learn about that network from this one? 00:43:32.000 --> 00:43:33.000 91. 00:43:33.000 --> 00:43:34.000 So that's another thing you would have to check. 00:43:34.000 --> 00:43:36.000 Is 91 actually on this list? 00:43:36.000 --> 00:43:37.000 It is. 00:43:37.000 --> 00:43:38.000 It's right there. 00:43:38.000 --> 00:43:40.000 Okay, so we did learn about that. 00:43:40.000 --> 00:43:43.000 So why did this one beat this one? 00:43:43.000 --> 00:43:49.000 Well, if we look at this table, it would be the one probably because there's a lower IGP metric to the BGP Next Hub. 00:43:49.000 --> 00:43:53.000 However, think about it this way, too. 00:43:53.000 --> 00:44:07.000 What if, what if the owner of this network sent out an update recently? 00:44:08.000 --> 00:44:27.000 And as that update rippled through the Internet, it arrived at this router first from this neighbor 30 seconds before it arrived from any other neighbor. 00:44:27.000 --> 00:44:46.000 If that's the case, then that would imply that my router has preferred the older path in this case because it learned about it from that router 30 seconds before anybody else. 00:44:46.000 --> 00:44:49.000 So it's older by 30 seconds. 00:44:49.000 --> 00:44:51.000 These are some interesting attributes, isn't it? 00:44:51.000 --> 00:44:55.000 Again, very tough to verify that on routers. 00:44:55.000 --> 00:45:01.000 You don't have the permissions to verify stuff like those numbers on, those statistics on, as well as on a certification exam. 00:45:01.000 --> 00:45:04.000 So the odds of getting this far in an exam are, like, next to none, folks. 00:45:04.000 --> 00:45:06.000 But you can see what's happening here, right? 00:45:06.000 --> 00:45:15.000 The router's getting down to these, like, nitty-gritty things that it's like, okay, well, since I've known you longer, I guess I'm going to use you. 00:45:15.000 --> 00:45:16.000 And then what? 00:45:16.000 --> 00:45:17.000 We got 10. 00:45:17.000 --> 00:45:18.000 If that's tied. 00:45:18.000 --> 00:45:25.000 If two of them are tied in that case, and that can happen, then you got to go with the path with the peering router that has a lower router ID. 00:45:25.000 --> 00:45:29.000 So whoever has a lower router ID now is ultimately going to be the winner. 00:45:29.000 --> 00:45:39.000 So, again, I can't confirm 100% without a doubt in this particular scenario that the preferred the older path is the winner. 00:45:39.000 --> 00:45:47.000 But when you think about it, this would have a lower router ID because if the router is using that as its router ID, then it has the lower router ID. 00:45:47.000 --> 00:45:53.000 So let's say everything else was still tied, then it's the lower router ID that's the winner here in this example. 00:45:53.000 --> 00:45:58.000 And I can verify that by just typing in show IP BGP neighbor. 00:45:58.000 --> 00:46:11.000 And as we go down through the neighbors, let's change this to this. 00:46:11.000 --> 00:46:18.000 We're going to do this and say instead pipe to include BGP neighbor is. 00:46:18.000 --> 00:46:23.000 So pipe to include BGP neighbor is. 00:46:23.000 --> 00:46:28.000 And pipe to include remote router ID. 00:46:28.000 --> 00:46:34.000 And let's see if that gives me what I want. 00:46:34.000 --> 00:46:35.000 It does. 00:46:35.000 --> 00:46:36.000 Perfect. 00:46:36.000 --> 00:46:43.000 So you'll see here, for example, that I have this BGP neighbor using this address and there is its router ID. 00:46:43.000 --> 00:46:46.000 So the address is not exactly the same as the router ID. 00:46:46.000 --> 00:46:51.000 But from a router ID standpoint, there's 4, 12, 37, 216. 00:46:51.000 --> 00:46:54.000 It's still the lowest one when it's all said and done. 00:46:54.000 --> 00:47:01.000 But, again, very, very tough things for you to actually be able to verify when you are provided limited information on the exam. 00:47:01.000 --> 00:47:03.000 But you'll see router ID. 00:47:03.000 --> 00:47:09.000 And if the router ID is tied, and that would happen if, for example, you had peerings between two routers like this. 00:47:09.000 --> 00:47:11.000 So the router IDs would be the same. 00:47:11.000 --> 00:47:16.000 So in that case, if the router IDs are tied because you have dual connections to the router, 00:47:16.000 --> 00:47:20.000 then you've got to prefer the lowest IP address of the neighboring device. 00:47:20.000 --> 00:47:21.000 It's incredible, right? 00:47:21.000 --> 00:47:24.000 Absolutely incredible. 00:47:24.000 --> 00:47:26.000 Yeah, usually the higher router ID is the winner. 00:47:26.000 --> 00:47:28.000 But, yeah, in this case it's lower router ID. 00:47:28.000 --> 00:47:31.000 Incredible, isn't it? 00:47:31.000 --> 00:47:32.000 Incredible. 00:47:32.000 --> 00:47:33.000 Difference. 00:47:33.000 --> 00:47:36.000 Just little minor differences that you can be tested on an exam, little things like that. 00:47:36.000 --> 00:47:37.000 So there you go, folks. 00:47:37.000 --> 00:47:44.000 That is the BGP best path selection process. 00:47:44.000 --> 00:47:54.000 So right now I do not have the perfect, perfect opportunity to tell you exactly why it was chosen 00:47:54.000 --> 00:47:58.000 because I don't have permissions on this BGP router because it doesn't belong to me. 00:47:58.000 --> 00:48:08.000 But, and I can't verify how long it's known that very specific route for, 00:48:08.000 --> 00:48:11.000 that we're talking about 1.0.0.0 size 24. 00:48:11.000 --> 00:48:13.000 So we do have neighbors. 00:48:13.000 --> 00:48:18.000 If, for example, this neighbor has been known the longest out of anybody else, 00:48:18.000 --> 00:48:21.000 then I safely conclude, oh, that's the reason why we won. 00:48:21.000 --> 00:48:24.000 But in this case, I can't today. 00:48:24.000 --> 00:48:25.000 And that's okay. 00:48:25.000 --> 00:48:26.000 That's okay. 00:48:26.000 --> 00:48:28.000 I want to be very clear, though, very clear. 00:48:28.000 --> 00:48:36.000 There will not be anything that gets down to where you can't truly verify it when it's all said and done. 00:48:36.000 --> 00:48:38.000 But that's the gist of it. 00:48:38.000 --> 00:48:43.000 Your ability to walk through it is what I'm trying to teach you here, especially the first six. 00:48:43.000 --> 00:48:59.000 Being able to walk through the first six and figure things out is the key to your success in that case. 00:48:59.000 --> 00:49:01.000 Ooh, great question coming in. 00:49:01.000 --> 00:49:02.000 I really have to think about that. 00:49:02.000 --> 00:49:06.000 So the question is, all right, so we're down to the last two steps. 00:49:06.000 --> 00:49:13.000 What if we're using a loopback address as the neighbor address, which happens to be the router ID we choose? 00:49:13.000 --> 00:49:14.000 Oh, okay, that's simple. 00:49:14.000 --> 00:49:15.000 That's simple. 00:49:15.000 --> 00:49:16.000 That has no effect on us. 00:49:16.000 --> 00:49:17.000 Absolutely not. 00:49:17.000 --> 00:49:26.000 Because if you think about the router ID, if you're using a loopback address as the router ID in this scenario here, okay, 00:49:26.000 --> 00:49:30.000 so you're using a loopback, loopbacks, right? 00:49:31.000 --> 00:49:36.000 So when it comes to the router ID, prefer the path with the peering router that has the lower router ID. 00:49:36.000 --> 00:49:42.000 So in this particular case, right, we have a path through here. 00:49:42.000 --> 00:49:44.000 We have a path through here. 00:49:44.000 --> 00:49:47.000 The router ID is tied. 00:49:47.000 --> 00:49:49.000 So then we go down to 11. 00:49:49.000 --> 00:49:51.000 Prefer the path with the peer who has the lower neighbor address. 00:49:51.000 --> 00:49:54.000 Okay, we're talking about the physical address of the interface. 00:49:54.000 --> 00:49:59.000 Which of these two physical interfaces is lower IP address? 00:49:59.000 --> 00:50:01.000 That's the winner. 00:50:01.000 --> 00:50:06.000 So in the last two steps, what if we're using a loopback address as neighbor, which happens to be the router ID it chooses? 00:50:06.000 --> 00:50:11.000 That's fine, because this is going to be based on this and the router ID, 00:50:11.000 --> 00:50:19.000 and this is going to be based on the actual interface IP addresses. 00:50:19.000 --> 00:50:22.000 This is going to be actually based on interface IP addresses. 00:50:22.000 --> 00:50:34.000 Does that answer your question? 00:50:34.000 --> 00:50:35.000 All right. 00:50:35.000 --> 00:50:46.000 So exam, make sure you know these. 00:50:46.000 --> 00:50:49.000 Would it be the same if there are other routers in between? 00:50:49.000 --> 00:50:54.000 Okay, so if there are other routers in between, remember, we're forming TCP peerings in that case. 00:50:54.000 --> 00:50:56.000 So that's still the same result. 00:50:56.000 --> 00:51:04.000 So if I had a router here and I had a router here and I had a router here and I had a router here, all right, 00:51:04.000 --> 00:51:11.000 and basically what you're saying is here's a loopback, here's a loopback, and I have two interfaces that go out. 00:51:11.000 --> 00:51:14.000 And let's say there's one link here and let's say there's two interfaces there. 00:51:14.000 --> 00:51:15.000 It doesn't matter. 00:51:15.000 --> 00:51:19.000 The fact of the matter is I'm trying to go from here and form a neighborship to what address? 00:51:19.000 --> 00:51:21.000 This address over here. 00:51:21.000 --> 00:51:27.000 From here to this address over here. 00:51:27.000 --> 00:51:36.000 All right, so if we're peering with the loopbacks, okay, then we'd have the same router ID 00:51:36.000 --> 00:51:41.000 and then prefer the path to peer who has the lower neighbor address. 00:51:42.000 --> 00:51:52.000 In this scenario here, neighbor address, yeah, that's now a challenge, isn't it? 00:51:52.000 --> 00:52:04.000 That's a problem because I'm not really going to know this neighbor address because it's way over there. 00:52:04.000 --> 00:52:07.000 We're peering based on this information. 00:52:08.000 --> 00:52:10.000 Oh, and this would be your next hop. 00:52:10.000 --> 00:52:11.000 This would be your next hop. 00:52:11.000 --> 00:52:14.000 Yeah, I never actually thought of that scenario. 00:52:14.000 --> 00:52:15.000 I've never thought of that. 00:52:15.000 --> 00:52:19.000 I'd actually have to sit down and map that up to figure out exactly what would happen. 00:52:19.000 --> 00:52:28.000 Never actually had to do that that way. 00:52:28.000 --> 00:52:31.000 Yeah, that would be slightly different. 00:52:31.000 --> 00:52:34.000 I don't know how 11 would apply in that exact scenario. 00:52:34.000 --> 00:52:36.000 It's a good question, though, and I don't have an answer for that one right now. 00:52:36.000 --> 00:52:42.000 It's just based on the complicated nature of that architecture. 00:52:42.000 --> 00:52:47.000 But that's what you were trying to get at, that type of scenario, right? 00:52:47.000 --> 00:52:48.000 So I directly connected. 00:52:48.000 --> 00:52:51.000 I would know the neighbor's in, but if I'm not, then that would be tough. 00:52:51.000 --> 00:52:59.000 Yeah, that would be interesting. 00:52:59.000 --> 00:53:01.000 Yeah, well, here's the deal, okay? 00:53:01.000 --> 00:53:05.000 I would encourage you never to get this far in the process, first and foremost. 00:53:05.000 --> 00:53:06.000 I would never, never. 00:53:06.000 --> 00:53:07.000 Absolutely not. 00:53:07.000 --> 00:53:08.000 So that's the thing. 00:53:08.000 --> 00:53:12.000 In that type of scenario, you would never want to get this far anyways. 00:53:12.000 --> 00:53:16.000 You would definitely want to control it and get to the point where you can have the decision 00:53:16.000 --> 00:53:24.000 made earlier on when it's all said and done. 00:53:24.000 --> 00:53:25.000 But wait a second. 00:53:25.000 --> 00:53:26.000 Hang on, hang on, hang on. 00:53:26.000 --> 00:53:34.000 Wait, now that I think about it, now that I think about it. 00:53:34.000 --> 00:53:35.000 Okay, no, no. 00:53:35.000 --> 00:53:36.000 No, it wouldn't matter. 00:53:36.000 --> 00:53:37.000 Wait a second. 00:53:37.000 --> 00:53:38.000 Go back to that. 00:53:38.000 --> 00:53:44.000 Because, right, even if this happened, let's think about it this way instead. 00:53:44.000 --> 00:53:50.000 If I'm peering with the loopback, how many peerings do we have, no matter what? 00:53:50.000 --> 00:53:54.000 Okay, if we're peering with the loopback, I have a peering from here to here. 00:53:54.000 --> 00:53:56.000 I only have one peering. 00:53:56.000 --> 00:54:01.000 I only have one peering, even though I have multiple paths we can use. 00:54:02.000 --> 00:54:05.000 So in the end, we win right here. 00:54:05.000 --> 00:54:07.000 We don't even go beyond 10. 00:54:07.000 --> 00:54:09.000 We won't even go beyond 10. 00:54:09.000 --> 00:54:11.000 Because there's only one peering. 00:54:11.000 --> 00:54:13.000 That's it. 00:54:13.000 --> 00:54:15.000 There's just one peering. 00:54:15.000 --> 00:54:19.000 Even though there's multiple paths, there's just one peering. 00:54:19.000 --> 00:54:23.000 This would be no different than, like, inside of our organization, peering with our loopback 00:54:23.000 --> 00:54:25.000 addresses and we have a bunch of different ways to go. 00:54:25.000 --> 00:54:27.000 There's only one peering. 00:54:27.000 --> 00:54:28.000 So yeah, no. 00:54:28.000 --> 00:54:30.000 Prefer the path with the lowest router ID. 00:54:30.000 --> 00:54:32.000 It doesn't matter. 00:54:32.000 --> 00:54:34.000 Because there is only one path anyways. 00:54:34.000 --> 00:54:35.000 We wouldn't even get this far. 00:54:35.000 --> 00:54:37.000 We wouldn't even get this far. 00:54:37.000 --> 00:54:38.000 Yeah. 00:54:38.000 --> 00:54:43.000 You see that now? 00:54:43.000 --> 00:54:49.000 Because we're peering with the loopbacks, there's only one neighborship. 00:54:49.000 --> 00:54:51.000 I'm learning from you. 00:54:51.000 --> 00:54:53.000 It doesn't matter the number of paths in this case. 00:54:53.000 --> 00:54:55.000 So prefer the older path. 00:54:55.000 --> 00:54:58.000 Well, there's only one path, me to you. 00:54:59.000 --> 00:55:01.000 So it's based on neighborship, don't forget. 00:55:01.000 --> 00:55:02.000 Based on neighborship. 00:55:02.000 --> 00:55:08.000 So if we go back to that other example, the other example, so when I was saying router, 00:55:08.000 --> 00:55:16.000 router, okay, router, router, two links, I'd be peering from here to here, here to here. 00:55:16.000 --> 00:55:18.000 We're not using loopback in that scenario. 00:55:18.000 --> 00:55:20.000 That means we have two peerings with each other. 00:55:20.000 --> 00:55:22.000 Two peerings. 00:55:22.000 --> 00:55:24.000 So in this case, I'm learning from you twice. 00:55:24.000 --> 00:55:27.000 So prefer the path with the lower router ID. 00:55:27.000 --> 00:55:29.000 Well, they're using the same router ID. 00:55:29.000 --> 00:55:32.000 So now we break that tie with this. 00:55:32.000 --> 00:55:40.000 But if we were using the loopbacks instead, if we were using the loopbacks, okay, take that out. 00:55:40.000 --> 00:55:45.000 If we're using the loopbacks and we're peering from loopback to loopback, we'll never get beyond 10 anyways. 00:55:45.000 --> 00:55:47.000 Right? 00:55:47.000 --> 00:55:48.000 We're not going to get beyond 10. 00:55:48.000 --> 00:55:49.000 We're done. 00:55:49.000 --> 00:55:52.000 Because we only have one peering. 00:55:52.000 --> 00:55:54.000 We don't have two peerings or more peerings. 00:55:54.000 --> 00:55:56.000 Yes. 00:55:56.000 --> 00:55:57.000 Yes. 00:55:57.000 --> 00:55:58.000 You see the difference there? 00:55:58.000 --> 00:56:03.000 Yeah. 00:56:03.000 --> 00:56:04.000 Good stuff. 00:56:04.000 --> 00:56:05.000 All right. 00:56:05.000 --> 00:56:06.000 Perfect. 00:56:06.000 --> 00:56:07.000 All right. 00:56:07.000 --> 00:56:08.000 So for the exam, that's a lot. 00:56:08.000 --> 00:56:09.000 I know. 00:56:09.000 --> 00:56:10.000 It's one of those things. 00:56:10.000 --> 00:56:11.000 Okay. 00:56:11.000 --> 00:56:13.000 If A is this and B is that, then C is this. 00:56:13.000 --> 00:56:19.000 But wait, if D comes into play, then we're talking about E, F, and G. 00:56:19.000 --> 00:56:23.000 So the key here for you to prepare for the exam is to make sure what? 00:56:23.000 --> 00:56:30.000 You know what you're looking at when you look at this output. 00:56:30.000 --> 00:56:32.000 Oops. 00:56:32.000 --> 00:56:37.000 I'm trying to log into the router on the Internet and I'm, like, typing in all the wrong stuff here. 00:56:37.000 --> 00:56:39.000 It's going to yell at me. 00:56:39.000 --> 00:56:44.000 I accidentally tried to type in with a show IPBGP username. 00:56:44.000 --> 00:56:45.000 Oops. 00:56:45.000 --> 00:56:46.000 Let's type in the right one. 00:56:46.000 --> 00:56:47.000 Our views. 00:56:47.000 --> 00:56:48.000 There we go. 00:56:48.000 --> 00:56:50.000 Show IPBGP now. 00:56:50.000 --> 00:56:56.000 So when we look at this output here, if you were presented with something like this on the exam, be able to know what you're looking at. 00:56:56.000 --> 00:57:04.000 So weight, local preference, originated, and then we've got AS path, and then we've got origin code, and then we've got met. 00:57:04.000 --> 00:57:06.000 So those are the ones. 00:57:06.000 --> 00:57:07.000 Where are they? 00:57:07.000 --> 00:57:08.000 No, higher, lower, this and that. 00:57:08.000 --> 00:57:09.000 You've got to memorize that chart. 00:57:09.000 --> 00:57:11.000 You've got to memorize the first six. 00:57:11.000 --> 00:57:13.000 You've got to know how to read this for the first six. 00:57:13.000 --> 00:57:16.000 And then, more importantly, if you're ever asked, hey, how do you make this one? 00:57:16.000 --> 00:57:19.000 How do you make this one the preferred path? 00:57:19.000 --> 00:57:24.000 Well, based on everything else you see, I mean, I could increase weight, and that would accomplish the objective. 00:57:24.000 --> 00:57:29.000 I could also just simply increase local preference, and that would accomplish the objective in this particular scenario. 00:57:29.000 --> 00:57:31.000 How do you make this one the preferred path? 00:57:31.000 --> 00:57:32.000 Again, I could change weight. 00:57:32.000 --> 00:57:35.000 I could change local preference, and that would make that the preferred path. 00:57:35.000 --> 00:57:38.000 So those are all things to keep in mind on the certification exam. 00:57:38.000 --> 00:57:41.000 But pay attention to the details. 00:57:41.000 --> 00:57:47.000 Pay attention to everything that's going on around that particular scenario to ultimately determine what is going on. 00:57:47.000 --> 00:57:55.000 So if you don't memorize this list, then, I mean, there are a few BGP questions you probably won't get right on the certification exam.