Hi, welcome to this CertificationKits CCNA training video on OSPF and Designated Routers. We're going to be talking about what a designated router is. When a designated router is required. How designated routers function; as well as some default settings, how you can specify what router becomes a designated router as well as some CCNA exam trouble shooting tips.
In OSPF a designated router is the router that all the other routers have to get their updates from. Say this is router right here is a designated router. What that means is every other router, router A, B, C and D and all the other routers will get their updates directly from the designated router. They would not exchange information with each other. Now designated routers are needed in certain environment to cut down on amount of traffic. In a LAN environment it is an automatic requirement to have a designated router. Sometimes a LAN environment like this might be the backbone of your router network. So, in that LAN environment you'd like to cut down on traffic as much as possible. If you don’t have a designated router, A would have to exchange information with C and D and B. C would also have to exchange with B and D and the DR even though it won't be called the DR; it maybe called router E, D, B, B. You get the idea. All this excess traffic can slow down your network and it's unnecessary. So that's why a designated router is there so you can get rid of that excess traffic.
A CCNA candidate may ask how does a router become the designated router? First what happens when the router is turned on for OSPF and what’s going to be communicated on this interface? Let's say this interface is E0 and all the routers have one of these E0 interfaces. One of the OSPF processes recognizes this as on LAN interface. So since it’s a LAN interface, an OSPF designated router is required. So OSPF then sends it's hello packets out to elect a designated router. Every router sends these hello packets out and in the hello packet is what is called a priority number. The priority determines the designated router. That is a CCNA question. The default priority is the number one. So, if they all have that default priority of one, they’re all going to match, so it comes down to the router ID. The router ID is used to when the priority number matches. So then it comes down to the router ID. How do the routers get the router ID? I talked about that in a previous CCNA slide. But basically it’s an IP address, the highest IP address on an up and up interface. This guy’s IP might be 192.168.1.254. Everybody else would have a lower IP address 240, 230 or whatever the case maybe. Everybody else has a lower IP address and then this guy becomes the designated router. And what that means is all the other routers are going to exchange topology information directly with him. How that happens is instead of a multicasting their information out to the typical multicast address OSPF, they use a multicast address to send the routing updates and routing information out to 224.0.0.5. That’s the IP address they use to send their information out and normally all OSPF routers would be listening to that IP address. Instead when they are not the designated router, they use the IP address of 224.0.0.6 and the designated router is the only one that’s going to get that information that’s a little bit of a lie.
There is also what’s called a Backup Designated Router BDR since he has second highest IP. It’s likely that he’ll be elected as a backup designated router and he will also listen to all of that update information that’s multicasted at the 224.0.0.6 address. But, he doesn’t reply, he keeps his mouth shut. He won't talk back. This is the only guy that going to reply and send information so they all update with him, He is happy, they are all updated with the designated router and he can talk back. When he talks back and he sends the information back to 224.0.0.5. So if there's is a change on A, he will multicast it out to the designated router and then the designated router multicasts it back to all the other routers so everybody get updated. Unless something happens to this designated router, it keep sending those hellos out and telling everybody he's still the designated router. Then something happens to him, the backup designated router has been listening to all the information and building that topology database and keeping it all up to date. He will take over and start broadcasting out if for some reason this designated router goes bye-bye. For a quick CCNA recap. All of the routers recognize that they have LAN interfaces which required a designated router.
So what they do is they send their hellos out with the purpose of electing the designated router. They all want to be the designated router because they get the information first. If allthe priority numbers match since one is the default priority, if you wanted to and you wanted to make sure that one router couldn’t be the designated router and give that one the priority of 0 and then that means he can never be a designated router. Another option is if you wanted a particular router like this guy to be the designated router you can just give him on higher priority number. Something like the number 2 which is higher than all the other routers. Then he would be the designated router. If you had a router that might be more powerful than the other routers or with less over head on it, you might want to elect that router as the designated router. They can all look at their priority numbers and when they compare it to that one, the one with the highest priority wins. If they are all match, then the router with the highest router ID wins and he becomes the designated router. The second highest router would become the backup designated router. So what happens now is all of the other routers multicast their informational out to multicast address 224.0.0.6. The DR gets the information, processes it and then sends it back out to multicast address 224.0.0.5. Some interfaces don’t require designated router like a point to point link.
So we have point to point here. There is router A, router B here they are just going to exchange information with each other. There is not a bunch of excess traffic involved in that environment so a designated router is not needed in a point to point environment. Frame relay may need a designated router depending on the way it's set up. So in that frame relay environment there is a couple of different ways it can be configured. Some ways need a designated router, others would not need a designated router. If you got a frame really topology setup where it was a full mesh environment so all the routers could talk to each other and each router has a direct connection to every router. This is an environment where you would need a designated router. So this guy right here might become the designated router and again he'd be in-charge of sending the updates and receiving the updates from all the other router and another router might become the backup designated router. If it's not a full mesh and you have multiple point to point connections in frame relay so everybody routes through this one core router right here, the central router; they all have a direct connection with him. There wouldn't be a need for a designated router because it's simply multiple point to point links. So if it's multiple point to point links you would not have a designated router. There is a command called IP OSPF network and that command and the network type. That command allows you to specify the type of interface it is.So an interface again like a LAN interface is automatically going to be specified. But IP OSPF network and then the type of interface and there's 5 options here.Whichwould allow you to go in and specify what type of interface it is and whether or not a designated router would be required. This not something you are going to have to do all the time, but again something that you should be aware of.
I got my CCNA simulator loaded up and now I've configured my routers. I have got 3 routers connected through a center switch and the IP addresses are all 192.168.1.0 addresses. Palaestra 1 has a one has .1 address, Palaestra 2 has a two has .2 address, Palaestra 3 has a .3 address. I've configured OSPF and I've not changed the priorities. All of the priorities are one so based on that information, the router with the highest router ID should become the designated router. Again that’s one that everybody is going to exchange their information with. They will not exchange information with each other. So this guy becomes the designated router. Then typically a router with the second highest router ID would become the backup designated router so Palaestra 2 should be the backup designated router. This guy here is out of luck is called the DROTHER Non Backup Designated Router. DR-OTHER, DROTHER, you will see that term.
So let's go in and take a look at the CCNA simulator and say how we can view that information. So I am on Palaestra 2 and what I can do is I can go show IP OSPF interface and it will show me the information for the OSPF interfaces. This shows me right here that Ethernet is up, line protocol is up. It shows me the IP address and it's a part of Area 0. The router ID network type DROTHER meaning Non Designated Router DR-OTHER. Priority level is 1. Designated router ID is 192.168.1.3. Interface address 192.168.1.2 here. All my hello and designated information and again it shows me here a designated router and 192.168.1.1 he is the one that’s not a part of anything. Now if we go to Palaestra 3 and do the same thing show IP OSPF interface we can see here that Palestra 2 and it actually should shown up on to as a back of designated router again. We can see Palestra 3 is 192.168.3 a part of Area 0. He becomes the DR network type designated router, his state again designated router priorities one shows the backup designated router here as a router with the second highest IP address 192.168.1.2. Hello and dead interval information as well and again the neighbor account. I can also do show IP OSPF neighbor command and get information that way. But it's not going to show me the full information. It's just going to show me the state so it has a full state and what's showing me here is the addresses of the router that his a full stay with meaning fully adjacent, have exchange all their topology information, Let's take a look at Palaestra 1 and do the same command show IP OSPF neighbor and check it out he's full with both other routers. The reason he is full with both other routers is because it wants a backup designated router and one is a designated router, so they will get his information. If it was a non designated router a DROTHER then what would happen is the state here would be two way meaning he knows that router is there but he is not exchanging his routing table information since these are a backup designated router and designated router, the state will be full here but again if it was a non DR, DROTEHR then it would be a two way state you would be seeing here if I threw a fourth router into this mix.
Let’s take a look at how at how we can change these things up by adjusting the priority. Maybe we'll up the priority on Palaestra 1 and make it a designated router. I went into Palaestra 1 and I changed the priority on the interface. If we go on and take a look we can see that I used the command IP OSPF priority 255. And OSPF is still running same area so if I do a show IP OSPF neighbors nothing should have changed at this point. I am still fully adjacent with both other routers again which I am because one's a DR and one is a BDR. A show IP OSPF interface will show me that I am not the designated router even though I have the highest priority. The reason is because I have not reset the OSPF process. Just because I go and change a priority, it doesn’t make all the routers go out and scramble around and do a new election. The only time elections take place is when the designated router goes down. Then the backup designated router becomes the designated router. And then a new backup designated router is elected. Just because you change your priority it doesn’t mean everything is going to go on and re-elect. So what you do on the routers if you wanted to go through a new election process you would clear the IP OSPF process using this command. What it would do is it would reset OSPF and go in and have it do a new election. Now on this CCNA simulator no matter what I do I cannot get this guy to become the designated router.
So that is one of the little problems here with the CCNA simulators. I even got rid of OSPF, reconfigured it set this priority first and then turned on the other routers, still nothing happened. However on an actual router with this priority; if I have this router up first and I set his priority to 255, he will become the designated router. And the command to reset the OSPF process to force an election is clear IP OSPF process. And the command to set the interface priority again is IP OSPF priority and then the number. So that’s how I could manipulate what router actually became the designated router. So for a recap, let's go in and take a look at some commands here. Again important commands are show IP OSPF interface; that command allows us to go in and view our designated router status. This shows us what's going on at our interface. For the backup designated router, designated router or non designated router DROTHER will show us that information as well as hello dead interval and stuff like that for trouble shooting. Show IP OSPF neighbors goes in and shows us our neighbors and what state we are in. Again we will be in a full state with the DR and BDR designated router and non — and backup designated router. If it was a non designated router we would be in a 2 way state meaning we knew the other routers were there, but were not fully adjacent with the router because we are not sharing our information with them. We are sharing our information with the designated router. The backup designated router listens in, but he doesn’t send anything back to us. If there's a particular router that I'd like to be the designated router; maybe I like that router better or it is simply a better router or it’s got less of load on it, whatever the case may be; I can specify that with this IP OSPF priority and then I have got a 0-255. If you specify the priority at 0, it cannot be a DR. not equal to — not equal to DR it won't be at the designated router. If I go in and set a priority of like 255; if I pick one router out whose priority was one before and that is in that environment where a designated router is needed while everybody is at priority one and I changed that and I make its priority 255; they are not going to go in and do a new election.
The only time a new election takes place if somebody takes that(the DR) router out. The backup router becomes the designated router; so he will become the designated router and then this guy maybe thrown other router in there. And then this guy would become the backup designated router and then if somebody takes of that guy out then he finally gets to become the designated router. That’s the long way. Again it’s not recommended to use this command if the network’s up and running because everybody has to resynchronize all their information. But if I wanted it to, I could use to clear IP OSPF process and clear out their OSPF processes and then when they came back up this guy right here would become the new designated router. That is because I'd basically be forcing an election and they'd all have to exchange the topology information again and reset everything. So that’s not recommended in an up and running environment. However you can use the command to reset everything OSPF again. Just use it carefully.
We have talked about OSPF and Designated Routers. What a Designated Router is. When is it required? We have talked about how a designated router functions. Remember those multicast addresses 224.0.0.5 and 224.0.0.6? This is the one — a little smiley face there, this is the router again, there this is the multicast address that everybody sends to the designated router with. Then what goes to the designated router 224.0.0.5 is what comes from the designated router. We discussed default settings and how to specify a DR by changing the priority number and some trouble shooting commands that you can use to check just to see your status, see what state you are in and see which router is actually the designated router. I hope you have enjoyed this CertificationKits CCNA training video on OSPF and Designated Routers.