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So the next thing we'll talk about is the OSP of routerity. Like before, we go ahead with the later-on process. Like previously, we have seen the initialization stage and the two-way stage, and then they establish a neighbor-listening ship before they start exchanging the database. But before we move on to the exchange stay, we also need to understand something called routerity. Now, routerid is the name of a router inside OSPF. Let's say I'm having multiple routers here; every router will be identified by some name, which we call the "routerid." So typically the route rider will be a 32-bit number similar to an IP address, and it uniquely identifies that particular router inside the OSPO database. And in general when we talk about routeridiot when we talk about router, let's saythis example the router two is having threeconnections, one three directions let's say. And if I'm coming from this side, generally we say the name of the router is this IP. If you're coming from this side, we usually say this is the name of the router, which is similar to an IP address. But in case of OSPF the Ospre database insidethe OSPR database, basically the router has to beidentified with only one unique name and that namewe call it as a router. Like in this example, you can see the router has many interfaces, but now the question is: which one will be the router ID? So by default, the preference will be set based on the manual. So we can manually go inside the router and configure the router with a command. So there is a command called routerity. We'll probably verify this configuration and the verifications first because I need to explain a few more things. So, in essence, this is a command that says, "This should be the router of this router one." Let's say this is on the router one. We can manually decide that. And this address can be anything. You can type in any IP address before the address you can write down, like any class D class. Also, you can write any address—it doesn't make any difference. So you can write down any address as an outright, and let's say I don't configure this because most of the time you don't configure the route routine, then the router will automatically decide the highest IP of the loopback interface. Now, basically, the loopback interface is a kind of logical interface. As an example, we can go ahead and create a loopback interface. There is something called logical interface, it's not a physicaland we can view the IP address something any othersyou can type in let's say I'm giving some others If you verify here, show the IP interface in brief. Now we can see that this is my physical interface. These are all my physical interfaces, and they also have IP addresses, which means we can connect these interfaces to other devices, whereas the logical interfaces are like there is no connection to them, so loopback is like a logical interface, which are generally used to kind of simulate additional interfaces or for some kind of BGP neighborhood, or you will see the purpose of this later on in the sections, but generally in case you h Let's say you have, then it will automatically take the highest IP of the loopback interface, as in this example, assuming this is my router 1, which has two physical interfaces. These are two physical interfaces. The red ones and I got two loopback interfaces sothe blue ones are the logical loopback interfaces created likeloopback zero or loopback one any number it can beso it's going to automatically see the highest IP ofthe loopback interface which means out of these two whichis the highest so the first portion eleven or onewhich is the highest one so eleven is the highestso assuming the manual is not configured then automatically therouter will decide the route ready based on the highestloopback interface so which means in this example the routeready will be eleven 1111 of the router one soby default that will be the route riding and let'sassume that if there is no loopback interface also configuredlike assume there is no loop back interface here soI did not configure the manual. Assume we did not configure this because, by default, we may not configure it, and assume there's no loopback interface created also, so we didn't create any loopback because we have generally seen that maybe we don't require it; in that case, what will be the router ID? Now, the route ID will be selected by default based on the highest IP of the physical interface, which means that we have, for example, one and eleven, and if the first portion is the same, it will take the second portion. if thesecond portion is same then it will take the third portionso assuming the loop bags are not present and the manualis not present then this will become the router ID likein my example if you check this example here if youcheck on the router one of course I had to clearthe process because I have cleared a loop later on likein this example if I say show IP protocols you canuse specific commands to verify like show IP protocols or Ican use show Ipost neighbour like in this example anyone routercan go ahead and if I say show IP protocols youcan see in the fourth or fifth line the route IDby default it selects this one basically it's not working herebut normally if you have a low back then you expectthe outrage should be this one. Not this one. OK in packet size doesn't support but on the physicaldevices or on the GNS three tools if you're usingsome advanced so basically here it selects the outrage. In that case, I may need to remove the OSPF and reconfigure, but normally I expect this to be the outrage. But one more thing you need to keep in mind is that you have to clear the process. So, in my case, if I say show IP protocols on router one, I'll verify the router, and here the router is 192, 168, and 100, and if I verify the interfaces, what we have is similar to if I say show IP interface brief, and here I only have two interfaces configured, one is 192-1681. these two, thesetwo are physical interfaces and physical interfaces automatically. The first option is manual. I did not confirm manual. The second option is loopback, and the third option is the highest IP of the physical interface. So in this case, the first two options are not there, so it is automatically selecting the highest type of physical interface. You can also verify this based on the neighbour table. You can see that it shows you the neighbor's addresses. This is the router of the router to the neighbor, which is also the neighbour router. I can go and change the route again; we'll be seeing this later on. I could simply say route riding, but make sure you clear the process and clear the rest of the process. This is required whenever you make any changes. Now you can see the routerity changes to oneor one because remember, whenever you make any changesto OSPF because the process has already started becausethis process has already started and finished. So, when you change the outrage, you must also reset the OSPF so that the person networks work. For example, if you try to change the router ID, as I did in this case. You can see a message saying to reload the router. Of course, I have to save the configs and reload the router to make sure that the new router updates the router as well as the neighbors. But reloading is not a good solution. You can use other options, like the clear IPO issue process. This command will also disconnect the neighbours and reconnect. Just like disconnecting the neighbor, you can see the neighbour go from full to down, and then it will come up. So you can also do that. Here I pose the process: does it say you really want to reset? Yes, I want to do that. Then it will disconnect the neighbours and start the process from the beginning. Beginning means downstage, initialise stage, and two waystage, and so on. Okay, so this is the default process. So always remember that the default will be selected manually if you configure manual. This is the first preference. And if this is not configured, then it will see the loopback interfaces. If there is a loopback, it will automatically select the highest IP of the loopback. That means if you have two, three, or four loopback interfaces, it will see which one is the highest and select from the loopback, and if there is no loopback interface also present, then it will automatically select based on the physical interface addresses. And again, there are a few more things. default, the highest IP of the physical interface So why I said default? Because default, we don't configure default. Most of the time, there are no loopback interfaces most of the time.So that's the reason I said default. It uses this. And one more thing relating to route riding that you need to know is that the router must be unique in the Oscar process, which means it must be unique within the hospital domain. Sorry, it must be. This means that if I'm connecting three routers and running OSPF, make sure you're not repeating the router here. So that means the router of router two should be different, and the router of router three should also be different. Also, when connecting the router, make sure you are not using a routerID that has already been assigned to a specific router. So this must be unique. So in case if I try to give the samerouter ID for two routers, then that will impact theOscar database and that can also impact the communication. So this is wrong. So you had to use a unique route ID—anything, but it had to be unique. The outrage must be unique within the domain. must be unique within the OSPF domain. That's what it says. And also the route riding must be uniquebetween the process on the same router. Again, processing means this is more or less advanced when you're running multiple OSPF instances. We'll talk about this in our OSPF instance. So if you are running two OSPFs on the same router, like OSPF One and OSPF Two, you have to use different routers, like One One and Two Two Route. This is why we run multiple OSPF instances. I talk about this in the configuration examples. But if you're running two Ospfs on the same router,then let's say OS P of one and OSP oftwo, again the route riding should be different. So you cannot use the same route while riding two OSP instances on the same router.
So the next thing we'll talk about is OSPF links to databases. We have seen the OSP of the neighbourhood in previous sections. The routers will send out hello messages in the initialization stage. They do exchange "hello" messages initially, and then basically after that, they will form a neighbour relationship. And also, we have discussed the router concepts. So once the router establishes the neighbors, the next step is that they will exchange the database. That's your second step—or second phase—before it selects the best route. And this database is known as a linked database. First, let's try to understand the overview process of this link in the database. Now, first what happens is once the router establishesthe neighbors, now every router, like in this example,you can see there is a router. Now, this route is going to build a lot of information, like all the information about its own network, like what are the routers it is connecting to and what are the links? like what the IP address is and what the status information is. Probably it will collect all the information, and then it is going to advertise it to the neighbors. So basically, it is advertising to this neighbor, meaning whatever I know, I'm just sending it to my neighbor. Maybe I already know some information about some other networks connected here. And this router is going to send out that information to its neighbors. So likewise, the same thing happens. Everyone will advertise the same thing. The router will advertise its information to its neighbors. The router 2 will also advertise its information to its neighbors. Now, this is what happens initially. So the routers are going to flood the informationout of all the neighbours and that neighbours again,they will pass on to the remote neighbors. So it will go on. So we call this a flooding set. That's the difference between the EHRP protocols. If you have learned some EHRP concepts, basicallythere is no flooding concept in distance vectorprotocols, distance vector or EHRP protocols. But whereas in the LinkedIn protocols, like OSPF, the initial flooding will happen, the router is going to flood out all the information, and all the routers will know this information and will maintain this information. OK? So practically, they are going to advertise each and every detail about the complete network, whatever they know, to each and every other router. So at the end, once they do this, like in this example, let's say router one and router eight, they both have the same information. Like information, it means nothing. But here is the database. What is the router rate? So at the end of the process, we'll see a little bit of detail in the next slide. At the end of the process, whether it's router one and router eight, router two, or router three out of four, every router has the same database. Now. Basically, the database is like information about each and every router or each and every interface they are connecting to in the entire network, and all the routers must have the same exact information, so at the end of the Oscar process, all the routers must have the same database. So basically, the flooding will add some overhead, and that's where we use some area concepts; we'll talk about that later in the OSP of areas, but this is the default process. What happens, and technically, each and every advertisement is referred to as "LSS," as is every router advice, so each and every information, and this group of LSS is referred to as "LSDP," the link state database, so what is LSA? Each LSA is actually a data structure or information about a specific with some information about the topology of your network in that, and this collection of these LSAs is referred to as links to the database, and basically all this link database information or LSA information you'll find inside the OSPF database table, and we can verify that by using the Show Iposp database command. Every router in the network must have the same information about all the devices or all the links.
So previously, we saw the Osteo database table. Let's try to get into more details here. If you go with the process initialization stage, we discussed the downstage where the routers are strangers. They did not exchange the complete hollows. And then, in the initialised stage, they send and receive the hollow messages. And then they come to the two stages where they establish the neighbour relationship and are ready to start or exchange databases. So now the fourth stage is your external stage. Now they move on to the fourth stage, called the Exchange Start or External Stage. Now in this stage, what happens is that the router is going to send out a request database distribution packet. That's what we call it, saying that I will start the exchange. Saying that, okay, I will start the exchange. So let's exchange. But before we exchange, the actual exchange process is the next one, the fifth one. But before the exchange, they will decidewho will start the exchange first. like whether I should send first or whether you will send first. So, just as in the case of master and slave, who is master and who is slave? That's what we call it technically, or in simple words, we can say who will start first. So the router one says, "Okay, I will start the exchange, and my route is this." So and so As in this case, 192-1681 100. But the router too says no, I willstart exchange because my route riding is higher. So in this example, assuming this is the route, just assume that whether it is a manual, automatic, or outrage, it depends. As a result, whichever router has the highest router or number, Like in this example, this is one idea to 168, and this is one eight to 1682, which is the higher. And this will become the masterand this will become the slave. As a result, this router two will send first. So that is decided in the next stage of the process. The next starting stage is the first stage that goes after forming the adjacency, meaning the neighbour relationship. And the routers will identify who is the master or slave for the synchronisation of your database. So once they decide to take the next step, they move to the exchange states. the fifth one. Now, in this exchange state, the routertwo sends out a summary of LSS. So it says, "Okay, here is the summary of my advertisements or links to databases." Because router two is a master, so it will send firstand the router one also will do the same thing. Here is the summary of my database. And then they both send acknowledgement packets. Thanks for the information. Thanks for the information. Like that. Now one thing you need to observe here is that during this exchange state, they will exchange the linkstate information by using some database description packets. Now, once they decide to exchange databases, they don't simply send the entire database. Now one thing you need to observe here is the exchange rate. They will exchange their own databases, but not the entire database. They will exchange only the outlines of information about the LSS. Like, let's say the router One has received ten LSS and the router Two has received, let's say, two LSS. Maybe. Now that the router 2 says that's okay, here is the summary of my LSS. I received two LSS, let's say, and the router says I received ten LSEs. So when they both compare now, the router One realises that the router Two is having two extra LSS, or extra advertisements, which I didn't receive. Maybe after forming the neighbors, there will be a new network added, and this will have been advertised. Maybe there are some new advertisements and I didn't have that information during the process. So now what happens? Now they are exchanging that illness. As a result, they only exchange summary information. So if they're having the same illness, then there's no problem. So basically, this relates to the next step. That is what the next step is. It is referred to as a loading stage and the full stages. So in the initial process, they don't exchange the entire database. They just exchange the summary of the list of illnesses, like how many LS I have and how many LSS you have, but not all the details not complete.So we can compare this with the summary topic, summary points at the end of the chapter, and the total chapter. So here they are not exchanging the entire chapter, they are just exchanging summary points. As an example, I have this and they will compare. Both must have the same number, like in this example. Assuming both have the same number of LS, in that case it's okay, but if not, then they will move on to the next stage called the loading stage. Like I said, here the router is going to check the LSS, which it already has. And if the router one realises that the router two has two LSS, whereas I have received only ten, then it is going to send out a query and then get those two remaining two LS.Also, because, as you may recall, all routers must share the same links database, So the database must be the same on all the OSPF routers. And that is only possible when you have the same number of LSS again, okay? So the next thing we'll do is move onto the third phase, like selecting the best routes. Is there a slight process that comes before that? Like in the previous discussion, we will discuss something called the XRT stage, where they will decide who will start the exchange first.
So now what happens is that, as I said, every router is going to check the number of LSS it received from the neighbor. As an example, consider this neighbor, or two neighbors. And this router has two LSS. whereas the neighbour is advertising, let's say 15 LSS. So if that means there are some additional LSS on the neighbor's side and maybe this neighbour has two LLCs, then there's no problem. So whenever a router realises that the neighbour has some additional information or additional LSS, then the router is going to immediately jump into the loading stage. and in the loading stage, it's going to compare. Of course, a comparison is already done, and it's going to send out a request. And the request is, like, I have only two LSS. But from what I have observed in the previous step, you have said that you have 15 LSS. Let's say now I need information about those three remaining LSS. Or maybe, just like an example, it says I don't have this entry in my network or in my database, but I have seen that in that list. What you have said is that you have this additional information, which I don't have. Can you please pass on that? So this is just like a request that is being sent by the router asking for extra LSS. And then that router will probably respond with the entry. Now we can see—okay, this is the complete information. So in the previous stage, they exchanged only the summary of the list. But now it is complete. And again, it is not sending a complete message. Like I said, twelve here, 15 here. So it is sending only the remaining three, which are complete. So the remaining three are complete. In this loading stage, no needto send all the 15, right? So it's going to send out only the remaining three, and then the router says, "Okay, thanks for the information." Now, similarly, if the router realises this additional information, then it will also request and get the information. Now this is what happens in between to ensure that all the routers have the same database. because that is like a condition before it decides the best route. So all the routers must have the same LSS. So each router can check which LS is already there, and it can ask other routers for only the specific LSs that are not present. So in the end, every router on the Internet must have the same exact information about the complete network. Internet work means a complete network. Finally, it arrives with the full stage. Full stage means the database is synchronised and both the routers have the same database. Like I said, in the loading stage, they send out a request asking for the most recent LS that has been discovered, but I have not received it in the exchange rate, probably. And then they will go to the full stage. In the full stage they become the neighbors. The neighbours are already done. The neighbours are adjacent in the two way stageand then now they become to the full stagemeans both the routers have the common synchronised database. Now that the links to the database are on both routers, it will apply an algorithm like the "shortest path first" algorithm and try to figure out how to reach this network. Maybe there are two or more routes; maybe there are three routes. Let's say it will try to calculate the best route and write down the best route in the routing table, and it is going to use that particular route to follow the traffic. More on this best route calculation with the help of metrics we'll talk about specifically like OSPF metrics, we'll see that, but basically in this process, adding the best route is completely based on assuming that the neighbours have established the full stage and then calculating the desktop and writing down that desktop in the routing table. So that's what you can see if you verify the neighbor's ship. If I say, "Show IPOs of neighbours in," you'll notice that the stage is already full. The full now indicates that the routers two and three have a neighbour relationship. Of course, according to this topology, the routers two and one. If you look at my packet as a topology here, they are at full stage, the database is synchronised between them, and the route is visible in the routing table.
The OSPF will now only exchange updates. how they maintain the neighbor's leadership. Like, router A has a neighbour relationship with both router B and router C, correct? And we can confirm this by using "Show Neighbor's IPOs." That's a command. And this neighbourly leadership is maintained based on the hello messages. By default, the router A will send out a hello message or hello packet to the router B every 10 seconds, which is the default. As a result, the default timer for keeping the neighbourhood is hello seconds. Just like you have a neighbor, you say hello every morning. That's a kind of default to maintain the neighborhood. Similarly, the routers A and B will exchange hello messages every 10 seconds by default, with the router A sending to B and the router B sending to A. And if, for whatever reason, the link fails, the router fails, or there is some other problem, the default is that the maximum time it will wait is four times, or 40 seconds. So after 40 seconds, the router A is going to remove this information, the neighbour leadership, from the neighbour table, and it will start following the traffic from the alternate route. So that is what we call convergence time. So the convergence time is nothing. But what if the best route fails? Assuming this is the best route, if this routefails, then the maximum downtime will be 40 seconds. And after 40 seconds, it will start using the second rule. And again, the OSPF maintenance incremental updates like the next update will be sent only whenever there is a change. Again, I'll come to these updates. Incremental and Periodic Update Differences in OSPO If you just want to verify the neighborhood, we can use the Show IP OSPO Neighbor Command, and here you can see the timer here.So this timer will help you. You can see 37, 36, 35, 34, 30, and then after 31 it will go back to 39, 30 and then 39. As a result, this time indicates that. So whenever I receive a hello again, it will reset the timer to 40 and then 39. Of course, because of the faster hello, you don't normally see 40 over there. So, basically, 38, 37, it will increment and continue. Once it reaches 30, again, it will send a hello. So that's the reason this timer will never reach 29; they never go to 30. But let's say if I shut down the link, so what I'lldo is I'll go to router one and I'll shut down thislink, any one of the links, interface one shut on the link. And if I verify swipe OSPF neighbor, of course, when you shut down the link, it will remove that neighbour completely because the interface is done. But let's say due to some reason, if thisrouter goes down or some issues on the otherside, there is going to wait for the timerbefore it starts using the second route. And of course, I don't have any alternate route here. You can see the entry has been removed when you shut down the link. Maybe I should have shut off the opposite-side interface. Let me try that. When I bring this interface back up, you can see it will begin sending hollows, the hollow process will begin, and you can see the neighbourhood is up and the loading to full interface is visible. And if you verify the neighbour ship, the neighbourhood is back up; if you can see what is on router 3, I removed the OSPF, and the timer is visible here. Basically, you can see the timer here going to 27 again because I removed the OSPO on the opposite side of the router 3, and you can see the timer going like this; once it reaches zero, it will remove the entry from the neighbour table and begin using the alternate roots anyway. So this is what generally happens at the back end—how the OSPF is going to maintain the neighbour relationship—and that is purely based on the hollows. So if the interface fails, the router fails, or if you remove the OSPF, anything happens. Basically, the router is not going to receive the Hello messages, and based on that, it will remove the neighbour entry from the neighbour table. And one more thing: we need to know the convergence time. The updates by default updates will be incremental. Incremental means that router B has now exchanged its database linked to the database during the exchange rate, and they have exchanged whatever additional information is required during the loading stage. Now they don't exchange anymore. So there is nothing like periodicity. Periodic will be there for 30 minutes. I'll come to this. So the next update will only come whenever there is a change. So whenever this router, let's say, is connecting to a new router and it starts receiving the new LSA, then this router will now send out this LSA to its neighbors. So that is what we call an incremental update. So the updates will be sent only whenever there is a change, whenever any new router is added or any specific network change happens, like the interface going down or any specific network going down or something like that. And apart from the incremental updates, OSPF also maintains that this is common in EHRP or OSPF. This is more like a similar process of incremental updates whenever there is a change. But with OSPF, there is one thing: It will also do periodic updates every 30 minutes. That is, let's say router B sends this information about some LSS, let's say some LSA about printed out network, as an example, and it was sent around 09:30 p.m. Let's say and after 30 minutes againit will send the same information after30 minutes, like around 10:00 p.m. So like that, there will be a duration for sending the periodic advertisement every 30 minutes. So, even if there is no change, every router will reflut the same LSF every 30 minutes that there is a default timer. even though there are no changes occurring here. But still, it will reflect the same advertisement. After every 30 minutes, there is a default, but again, you might be thinking. Okay. So if that happens Then that will again increase the conversions, but again, the timer will be separated for each LSA. Like let's say about this network,it has flooded around 09:30 p.m. Maybe some other maybe sending 945 like that. So the timer for each LSA will be different. So there is no common time like that. So basically, you don't need to really worry about the overload process in general because normally many people just assume there is a lesser referral for everything. 30 minutes, which may increase the.
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