OSPF Explained | Step by Step

hey what's up guys welcome to sir pros in this video we're gonna be talking about OSPF okay so oh s PF what is it well OSPF is a widely used and widely supported routing protocol it's an open standard meaning pretty much any route a will support it it's an interior gateway protocol meaning has been designed to be used within a single autonomous system Oh SPF is also a link state protocol so the goal of a routing protocol is of course to learn routes now the way OSPF does this is by learning about every router and subnet within the entire network the result is every router has the same information about the network as each other the wave rooters learn this information is by sending out what's called link-state advertisements or LSAs now these essays contain information about the subnet Ruta and some other network information once a while essays have been flooded OSPF keeps all of this information in a linked state database this is also known as an LSD be the goal here is to have each router with the same information in there LSD Peas so that's the general goal of our SPF let's take a deeper look at how this actually works there are three main steps to this process first becoming OSPF neighbors this is where connecting reuters running OSPF agreed to form neighbor relationships next exchanging database information after becoming neighbors the Reuters will then exchange their LS DP information and finally choosing the best routes once the LST be information has been exchanged each Reuter will then choose the best routes to be added to the routing table this is done by running a calculation code SPF first we'll look at how connect enriches form neighbor relationships okay so the very first thing that needs to happen before a relationship is formed is each Rooter needs to choose a Rooter ID a Rooter ID is a number that can be used to identify an individual Rooter kind of like Thanh OSPF name the Rooter ID or rid is in the format of an ipv4 address but could be set to whatever you like so there are a few ways that you can set a Rooter ID you can either set it manually or you can let the root to decide for itself so here's the logic that Reuters go through when choosing Rooter IDs first check this be manually set if it's been manually set that is the Rooter ID if it's not been manually set then the Rooter would choose the highest up loopback IP address if there are no loopback interfaces on the router it will then choose the highest up non loopback IP address essentially it would choose the highest IP address on the Rooter okay so let's take a look at two Reuters once OSPF is configured and we have our route IDs the Reuters will reach out to potential neighbors with a friendly hello message this message contains some important information including the unique Rooter ID and any already known neighbors hello messages also let their existing neighbors know that they're still alive and kick'n once Richa two receives the hello message it runs some checks it's important to know here that there are some very strict requirements that need to be met when OSPF forms neighbor relationships if you run into problems with our SPF it's highly likely that the problem will be with one of these the area ID needs to be the same this is used when scaling up OSPF and will be covered in another video the connecting links between the two Reuters must be on the same subnet the hello and dead timers must be the same the dead timer is how long a root will wait without hearing a hello message before it just assumes that a neighbor is dead the default hello timer is every 10 seconds for pointer point and broadcast networks if using authentication this must also match and the stub area flag must match as well Reuters will also need unique Rooter IDs okay so once the checks have been made and if all is good then the Reuters can move to what's called the init State Ruta - then sends a hello message of his own this message not only lists its own route ID but also Ruta one's ID as a known neighbor so when ruta one receives this hello message it sees itself as an own neighbor this allows ruta one to move to the two-way state Ruta one then sends yet another hello message listing ruta two as a known neighbor this then allows a ruta to to enter the two-way state as well and after all of that we now have two neighboring rooters before moving on to the step of exchange in database information first we must look at designated Reuters or DRS and backup designated Reuters or BD ours the arse and videos are not elected on point-to-point connections so we need to take a very quick detour in this network we have six Reuters all connected on the same segment let's say we don't have any DRS or videos and overages become for neighbors with each other if one link goes down the effective Rooter will let all the other neighbors know what just happened then each Rooter that receives this update will then tell all of their neighbors very quickly things begin to get a little bit out of control and this would happen every time a change occurs instead ad r and b dr is elected the election of a dr and be dia is done based on the highest OSPF priority now the priority is set to one by default but this can be changed to influence the election if the priority ties then the election is based on the highest Rooter ID when on the same segment Reuters will only become full neighbors with DRS and B Diaz the other neighbors will remain in the two way state we just looked at this means every router will ignore updates unless they come from the dr so let's look what happens now so the link goes down the route of floods of the updates and the other route is ignore it then the Dr reflux the update and now the ever Reuters pay attention ha look at that much better okay so back to our diagram once the exchange process starts both futures enter the ex start State at this point the route to select a slave and master this is based on the Rooter ID the master will control the sequence numbers and start the exchange process thankfully we don't need to know much more than that now we move on to the exchange dates both root to send each other a list of their LSAs this is called a database description or DBD now onto the loading States each route up will look over the DBD and request any information it doesn't already have the reason it does this is to prevent loops or requiring in the route is to request the information rather than just firing out updates so as an example route to one sees that route or two has the LSA for 192.168.0 so it says hmm can you send me over the 3.0 Network and then sends a request this request is good a link state requests or LS are Ruta - responds with sure here you go and sends a linked state update or LSU Ruta one then sends over a link state acknowledgment or LS Act now the same happens again for ruta to ruta to sends a request for the 1 0 subnet Ruta one sends the LSU I'm Ruta to Shenzhen Alice act once the exchange of NSA's has completed both Reuters enter the full neighbor state now remember this is a point-to-point network on a broadcast like we saw earlier they will only form the full neighbor relationship with DRS and be Diaz okay so on to the last step adding the best routes to the routing table the way OSPF chooses the best routes is by using a metric called cost Oh SPF cost is a value given to a link based on the bandwidth of that interface the sum to work out the cost is the reference value which is set on the Rueter itself and is a hundred thousand kilobits per second by default divided by the interface bandwidth okay so don't worry too much about the sum just try and remember these cost values serial interfaces 1544 kilobits per second is a cost of 64 Ethernet 10000 kilobits per second a cost of ten Fast Ethernet 100,000 kilobits per second this is a cost of one now anything faster than fast ethernet will also be a cost of one now you might be thinking that doesn't make much sense and you'd be right the way to change this would be to change the reference value in the Sun we looked at earlier this can be configured by you on the Rooter itself let's go for a quick cost example and see how this works okay so we have a five Reuter Network here in this example Ruta one wants to add the route for 10.0.0.0 to its routing table as you can see it has three options it can go through route two for two or five to make this decision it will do some simple math to calculate the cost of each one first ruta 4 the first link is a 1544 kilobits per second so this is a cost of 64 the next link is also 1544 kilobits per second so we never cost of 64 finally the last link is 10 megabits per second which is a cost of 10 so all of this means the first route has a total cost of 138 next ruta to the first link is 10 megabits per second so cost of 10 again the next link is also 10 megabits per second and we already know the last link is ten megabits per second so this means ruta 2 gives us the total cost of 30 last but not least ruta 5 the first link is 100 megabits per second this time this means a cost of one the second link is also 100 megabits per second and the last link as we know is 10 megabits per second the total cost for route 2 5 is 12 so as I'm sure you've guessed already the lowest-cost is the best so as route to five has the best cost to the 10.0.0.0 subnet this route is the winner and it is added to the routing table okay so that is it for our SPF there's so much more we could talk about but I think that's enough for one video if you like this video remember to Like comment and subscribe oh and by the way if you're sat there thinking wait what the is a Rooter well that's just the way us Brits like to say router thank you for watching

Channel: CertBros

Views: 711,159

Keywords: cisco, cisco training, ospf, eigrp, open shortest path first, link-state protocol, link state protocol, link state advertisement, cisco routing, ospf explained, ospf routing protocol tutorial, open shortest path first protocol, how ospf works, open shortest path first (protocol), ospf routing protocol, routing & switching, ccna (field of study)

Id: kfvJ8QVJscc

Length: 13min 49sec (829 seconds)

Published: Wed Sep 19 2018