12. OSPF DR and BDR Lab Demo
In this lecture you’ll learn how the OSPF, Dr and BDR designated routers work with a lab demo. You can see the lab topology diagram here. So I’ve got my four routers r six, R seven, R eight and R nine. They’ve each been configured with a loopback interface. So R six has got one in 216-80-6322, r seven is 192-16-8732, R eight is eight, and R nine is nine. And they are all connected to the Ethernet segment in the middle here, that’s on the 170 216 O 24 network. Again, R six is six, R seven seven, r eight is eight and R nine is nine. Right now OSPF has been configured on the routers, but there’s only a network statement for loop back interfaces. So OSPF has not been enabled on those Ethernet interfaces yet.
So what I’m going to do in the lab is, first off, I’ll just verify that everything is set up as expected. And then I’m going to enable OSPF on the Ethernet interfaces. And at that time, because it is a multiax segment, a Dr and a BDR will be elected. So my question for you now is which router will be elected as the Dr and which router will be elected as the BDR? If you need a few seconds to think about it, then pause the video now because I’m about to tell you the answer.
Okay, so the answer is that R nine will be elected as the Dr and R eight will be elected as the BDR. I didn’t say anything about configuring an OSPF priority. So all four routers will have the default priority of one, and it’s the router which has the highest router ID, which will be elected as the Dr on the routers. Here they’ve got loop pack interfaces, so it will be the loopback address which is used as the router ID. So R nine has got 192-1689, that’s the highest address. The second highest is R eight with eight. And then we’ve got our R seven and R six. So R nine is going to be elected as the Dr and R eight is going to be elected as the BDR. Okay, so let’s go and do that and verify that first.
Then I will manipulate the Dr election and make sure that it ends up being R six, which is the Dr, by giving it a higher priority. And we’ll also see failover working as well to the BDR when I shut down the Dr. Okay, so let’s do that now. So I will go on to R six and on here, let’s check, the interfaces are there. So I’ll do a show IP interface brief. And there you can see it’s got the loopback 192-1686 and the interface connected to the Ethernet segment one, 7216, six. I’m not going to show you for every router. You can take my word for it, they’ve all been configured like that. Okay, let’s also check the OSPF configuration. So I will do a show run and pipe that to section OSPF and you can see that OSPF has been globally enabled on the router and it’s got a network statement for one thing 2168 or which is the loopback interface. So right now OSPF has not been enabled on the fast Ethernet interface because I don’t have a matching network statement for 170 216.
And if I do a show IP protocols I can verify there as well that OSPF is running and I can see that the router ID is the router’s loopback interface. Okay, so everything is set up as I expected. Let’s now go and enable OSPF on that Ethernet interface and see what happens. So I will go to global configuration if I can type it with a config tea and then router OSPF one. And over in my other window here, I’ve got the network statement already typed up to save me typing that in.
So I’m going to paste that in so that’s for the Ethernet interface and that will enable OSPs there. The router knows it’s an Ethernet interface, so it knows that a Dr needs to be elected. So that is on R six. I also need to do it on R seven. So enable prompt config t, router OSPF one and I can use the same network statement again because we’re all in the same IP subnet, so that will work just fine. So that’s R six and R seven on next up R eight. Do the same thing on there and paste in the network statement. And then I’ve just got one more to do, which is R nine.
So get the enable prompt config t again, try not to make the same typo again. Router OSPF one and paste in the network statement. Okay, so now if I give it a second to converge and form the adjacencies, I should be able to do a show IP OSPF neighbor and see the neighbors. And I’m on R nine right now and I can see that, yes, it has formed adjacencies with six, seven and eight. I can see that right now they’re showing us all two way the other that’s because it is not completed going through the exchange and the loading states and getting to the full state yet. So now I’ve given it a few more seconds. Let’s try that command again and it is still two way and dear ever. And there we go.
Now I can see that the OSPF loading has gone to full. So now if I do that, sure IP OSPF neighbor. So earlier they were in the two way state with all of the neighbors and now it has completed the loading and this is on R nine. So I know already, we checked this earlier, we’re expecting R nine to be the Dr and R eight to be the BDR and I can see that. So this is a designated router. So it is going to have a full relationship with all other routers on this segment and I can see that they are all fully and I can see that R Eight, I was expecting to be the BDR. And yes, I can see that it is the BDR. R Six and R Seven are not designated routers. So they show up as Dr for designated router other, meaning they’re not the Dr and they are not the BDR. Okay? So that was with the show. Iprspf neighbor command. I can see all that information. I can also do so from any router. Now obviously if you are troubleshooting this kind of thing in a real world network and you’re looking at a segment which does have multiple routers on there, you’re not going to be able to tell at a glance which one has got the highest router ID, which one is going to be the Dr.
So a really easy way you can find out that information from any router on this segment is by saying Show Ipospf interface and I can just hit Enter there or I can get more granular and actually enter the interface. So this was on fastethernet, I can do this on any router on that segment. And I can see there that the designated router, the Dr is one thing, 21689, which is R Nine, and I can see that the BDR is one in 2168. Okay, so that’s the expected output on the Dr. Let’s have a look now on R Eight, which is the BDR, and I will do a Show Ipospf Neighbors on there too.
And I can see that nine is the Dr and six and seven are Dr others. And again both the Dr and the BDR do form full state relationship with all the other OSPF routers on there. Let’s have a look at one of the drugs now. So I’ll go into R seven and I’ll do the same command there. Show. IP. OSPF neighbor. And there I can see it’s gone fool with the Dr at nine, fool with the BDR at eight and six, which is another dream. It is just two way there. So you can see the routers which are not the Dr or the BDR with each other. They just go to a two way relationship. They don’t directly exchange routes with each other. Okay, so that was all of the expected behavior.
Now let’s force one of the other routers to become the Dr. So I can do that on R seven. So on R seven here I’m going to go to Global Configuration and then interface fast Ethernet. And I will say Ipospf priority is going to be 100 and I’ll do an end. Now, it’s not going to become the Dr yet, it’s going to wait until the election is forced again. And the way I can do that is by restarting OSPF or by shutting down and bring up the interface again. So let’s just verify that it’s still not the Dr. So on here I will do a show. IP OSPF interface. And I’ll do that for fast zero. And I can see there that the designated router is still one in 21689 and the BDR is still eight. But I can see now that on this router, the product A has changed to 100. It was one before. So on here, if I do a clear IP OSPF process, this will restart OSPF. I’ll say yes to that. It’s asking me to verify because this is going to be disruptive.
The router is going to drop all its OSPF relationships, and it’s going to lose its OSPF routes while this happens. We have to wait for it. Go back to loading fool again. Okay, and that looks like it’s done it with the other three routers. So now let’s do a Show Iprspf interface again. And now I can see that the designated router has changed. Here it is 192-1687. The BDR stays the same. It is still one nine, 2168. Okay, so r nine does not become the BDR. I’ve just made this one, the Dr. The BDR is going to remain the same unless something changes there. If I went and rebooted all of my routers now, then what would happen would be that R Seven would be the Dr, and then R Nine would be the BDR. But it’s not going to cause a disruptive change to change the BDR from R Eight to R Nine right now.
Okay, so that is how you can force one of your routers to become the BDR. Now let’s see what happens if the Dr goes down. So what I’ll do is on my R Seven here, maybe we do this easily because I’m in packet tracer, I will just power off the router. So I’ve just powered off the dr. So now let’s see what’s happening on my other routers. So let’s go on to R Eight, which was the BDR, and see if it’s detected the change yet. Let’s do a show Iposp interface for fast zero.
And looking in here, it still sees the designated router as one in 21687. And now I can see, okay, it has just gone down. So if I put that command in again now, then hopefully I can see that the BDR has now transitioned to the Dr because it saw that the Dr went down. And the next best router at this point in time is going to be R Nine. So I can see that it transitions to the BDR. Okay, so that’s everything thing. That is how the Dr and the BDR election process works on your multi access segments. See you in the next.
13. OSPF Areas
This lecture, you’ll learn about OSPF areas. As you learned already with our link state routing protocols, every router learns the full picture of its part of the network, including every router, its interfaces and what they connect to. And this can cause issues in large networks because every router knows about every link link, there’s going to be a lot of routes in its routing table and that can take up too much memory. Also, if there’s a change in the network, like a new link being added or a link going down, that causes all routers to reconverge, which takes time and CPU resources. And the larger the network is, the bigger impact that this is going to have. So to mitigate against this, OSPF supports a hierarchical design which segments large networks into smaller areas.
Each router maintains full information about its own area, but only summary information about other areas. So the routers are going to have less routes in their routing table and if a new link comes up or goes down in another area, it’s not going to affect that router. You’ll see how that works later when I show you an example later in this lecture. So with our areas we have a two level hierarchy. At the top level we’ve got the transit area, also known as the backbone or area zero. It does not generally contain end users. When we’ve got a multi area network and then we’ve got our regular areas, also known as our non backbone areas, which hang below area zero.
They’re used to connect end users to the transit area. By default, all transit traffic goes through the transit area. So you see the example diagram here. We’ve got area zero and area one and area two are hanging off of there. Any traffic between other areas not area zero always has to go through area zero. We can’t have traffic going directly between area one and area two. Now, multiple areas are really only required if you’ve got a larger network. If you’ve got a small network, there’s less destination networks in there, there’s less routes. So it’s not a problem if our routers know about all of those different routes. In that case, we don’t need the hierarchical design and all routers can be in area zero. When you do have a single area design, it’s always going to be area zero.
Later on, if your network grows and you want to go to a multi area design, this makes it really easy to migrate. So our configuration for configuring our different areas, the area is configured at the interface level with the network command. You see the example here. We’ve got network ten o, a wildcard Mask 025-5255, area zero. So any interfaces on this router which have got an IP address that falls within that range will be enabled for OSPF and put in area zero. For a router to form an adjacency, its neighbor on the other side of the link must be configured to be in the same area. If you’ve got an area mismatch, the adjacency isn’t going to come up and the routers are not going to share routes with each other. Moving on to our different router types, first off, we’ve got our backbone routers. A backbone router is a router where all its interfaces are in area zero. So this forms part of the transit area with OSPF.
Routers maintain a full link state database of other routers and links in their own area. So your backbone routers will have the full link state database for all of the other links that are also in area zero. And whenever an OSPF route is received from a neighbor and that neighbor is in the same area, it will show up in the routing table as an intra area route. You see in the example show IP route output here. All the different types that are shown in bold are different types of OSPF routes.
We can have intra area routes, which means the destination network is in the same area, an inter area route where it’s in a different area, or an external route, meaning it was redistributed into OSPF. So the example here, I can see we’ve got three routes. The prefix for all of them is O. So these are all intra area routes received from the same area. The next type of router we’ve got is the ABR, our area border router. Routers which have interfaces in multiple areas are ABRs. You see the example here. I’ve highlighted the routers in red. I’ve got a router on the left here which has got one interface in area zero and another interface in area one.
And our other ABR has got an interface in area zero and another interface in area two. The characteristics of the ABRs, they separate the flooding zones. This is what really segregates our network into the different areas and compartmentalizes our network. So if we have a link goes up or goes down in that area, it’s going to keep the information just in that one area. It doesn’t impact our other areas. You’ll see how that works in a second when I show you the example. The ABR is also where we always do our summarization in OSPF. Say that again. In OSPF, summarization is always done on your ABRs.
It functions regularly as the source for default routes for other normal areas, normal routers in our normal areas, and it maintains the link state database for each area with which it is connected. So if I go back a slide, you see the ABR on the left here. It’s going to have the full link state database for area zero. So it knows about all the individual networks in area zero. And it also has the fuel link state database for area one. Our ABR on the right here, it will have the fuel LSDB for area zero and for area two. But it only has summary information for area One ideal design is to have each ABR connected to two areas only the backbone and another normal area. An important point is that your ABRs do not automatically summarize. You need to do this manually and if you don’t manually configure summarization all routes will be flooded everywhere. So the network will basically behave like it was all in one big area. So again, the point of doing our multiple areas is for larger networks, picks up less resources, puts less load on the routers. But you don’t need to just configure the different areas. You also need to configure summarization on ABRs to do we get any benefit from it. So looking at how we’re going to configure it here, r two is our ABR. It’s got an interface in area zero which is fast Ethernet 10 and fast zero is in area one.
At Global config we saved out OSPF One and then network ten 1025-5255 is in area zero. So that will put interface fast 10 into area zero and then network ten odoro 25 two five five goes in area one that is fast Zero. Then to get the benefit of our different areas we need to do summarization on the ABR. So you’ll notice that we’ve got all of our networks over here. On the left. In area zero I’ve got ten 1124, ten one O 24. So all the networks in the left begin with ten one. They’re all ten one Xx 24 networks. So I can summarize those into ten one O 16. The command to do that, the routes are in area zero so I say area zero. Range ten dot 102-55-2550 and over in the right hand side of the network in area one my routes there are ten O 24 100. So all of the networks over on the right, they all begin with 100 and verse 24s. Rather than advertising all of those individual 24 networks over to area zero on the left I’m going to summarize it to a single 16 route. So the command I enter is area one because it’s over in area one. Range ten dot o dot o dot 0255 dot two five five dot o dot O.
The effect that you’ll get from this now is R one over an area zero. Rather than having routes to ten 00:24, ten 124 and 100 224, it just has a single summary route for ten O 16. So there’s less routes on R one so that takes up less resources on the router. The other benefit you get from this is let’s say that the ten 00:24 link goes down. Well, all of my other routers that are in area one will have to reconverge, see if they can find a better path to get there. So R three and R four will have to recalculate. They’ll have to do work to update their routing table about the ten 00:24 network going down. But if you look at it from the point of view of R One. Well, its summary route still stays the same. If any of these individual routes here go down, our Ten route is still good.
So if we have an outage in a different area, it doesn’t affect the routers that are in this area. So less routes in your routers, so it takes up less memory and also outages. The impact is confined to just that one area, so it takes up less CPU resources on your routers as well. So that’s why we want to have multi areas when we’ve got larger networks. When we look in the routing table, our inter area routes. So a route that was learned from an ABR which is in another area will show up as type OIA for inter area routes. You see the example output here. I’ve got a couple of intra area routes.
The bottom two destinations are within the same area as this router. And I’ve got an inter area route which is in another area. And inter area routes are always learned from an ABR. They’re the border between our areas and they’re where we do our summarization. The next type of router is a normal area router. This is a router where all of its interfaces are just in one normal area. You can see highlighted in red and left, I’ve got three routers which are area one routers.
All of their interfaces in area one. And I’ve got an area two router over here on the right. Again, these routers will maintain the full link state database for the areas which they’re a member of. So all the area one routers will have full LSDB for area One. They will have summary routes for area zero and area two that they learned from the ABR. And our area two router will have the fuel LSDB for area Two, summary routes for area One and area Zero. Our last type of router is an Asbr an autonomous system boundary router. So ABR is an area border router.
Asbr is autonomous system boundary router. What an asbr is. It means that on that router it’s running OSPF and we’re redistributing from another source into OSPF. So maybe we’re also running EIGRP or Rip on that router. We’re taking our EIGRP or Rip routes and we’re redistributing them into OSPF. So they’ll also be advertised to our OSPF neighbors. Or maybe it’s a static route that we’re redistributing into OSPF. So again, an ASPR, it just means we’re redistributing into OSPF on that router. Our redistributed routes show up as external routes. So an external route does not mean it’s outside this enterprise, outside this organization. It literally means it was redistributed into OSPF.
And if you look in routing table, this will show up as a type O E, either E One or E Two for an external route. So where it says O, it means it was an intra area route. The destination is in the same area. IA is an inter area route it’s from another area owned by an ABR and E two means it was redistributed into OSPF. Okay, so that is our OSPF areas. I’ll see you in the next lecture for AW Damo.
14. OSPF Areas Lab Demo
Lecture, you’ll learn about multi area OSPF through a lab demo. So in the previous OSPF labs, we’ve had all our routers in area zero, as you can see in the topology diagram here. So we’ve got five routers R One to R Five. All of their interfaces are in IP subnets begin with ten. And previously all the routers were in area zero. And let’s say this scenario now is that our network has been growing and we want to convert it to a multi area network. So we’re going to have the same physical topology, but rather than having all of the interfaces in area zero, we’re going to segment it into an area zero and an area one.
Now, normally you would have multiple normal areas hanging off of area zero, but because we’ve only got five routers here, I’m just going to have an area zero and an area one. So routers R Three and R Four are going to be backbone routers in area zero. Routers R Two and R Five are going to be ABRs with an interface in ada zero. And area one and R One is going to be a normal router in area one. So the routers are already configured all in area zero.
R Three and R Four are going to remain backbone routers in area zero. So we don’t need to change any of the configuration on there, but we need to change R Two and R Five. On R two, we need to put Interface Fast Ethernet zero on the right into area one. And we need to do the same for fast 30 on R Five. And on R One, we need to put all of its interfaces into area one. Okay, so let’s go onto the routers. So I’ve got all my routers configured with OSPF. If I do a show IP route, you see that all the routers have got routes to all of the destinations and they’re all showing up as a type O, which is an intra area route, meaning it’s in the same area.
So that was on R One over on the other side, R four. Its routing table is going to look similar again with intra area routes. So let’s configure our ABRs first. So that was going to be R Two and R five. If I do a show run for section OSPF on R Two, I can see that it’s being configured with process one. And I’ve got a network statement that’s putting all its interfaces into area zero. So let’s have a look at the topology again. And I need network ten one O to be in area zero and ten O to be in area one. So let’s fix that up on R Two. So back onto the command line, I’ll go configt router OSPF One, and I’m going to remove this network statement. So I’ll say no and then copy and paste it in.
And then I’m going to say network ten 1255. So all the interfaces that begin with a ten one and then we don’t care what’s after that are going to go into area zero and then network ten O. And again, I need to be careful with a wild card mask here and make it O dot O dot two five five dot two five five. So anything beginning with ten dot O will go into area one. But I’ve been careful to make sure that my network statements are not overlapping there. Ten one goes in area zero, ten zero X goes in area one. And I’m getting an error message about a mismatched area ID.
The reason for that is if we go back to the network topology diagram again, I haven’t changed R One yet. Its interfaces are still in area zero. So on fast ethernet on R one, it’s area zero. On the other side of that link, on fast zero and R two, it’s area one. So that’s a mismatch right now, until I finish configuring R One as well. That’s why I’m getting the error message. But before I do R One, I’ll do my other ABR, which is R Five. So fast 20 is the ten 116 network and fast 30 is the ten 00:16. So let’s configure that next. So I’ll go on to R Five and I’ll do a do show run for section OSPF and give us a second to give me the output. Then router OSPF One and I’m going to remove this old network statement because it’s putting all of the ten networks into area zero. I don’t want that. So I’ll get rid of that network statement and I’ll put in my new network statements which were for ten one dot or 25525 516 goes into area zero, and network ten dot O DOtwo 55255 goes into area one. And I’ll get the error message again here again coming from R One. There it is. Okay, so that’s my ABRs configured. Lastly, I’ll go into R One. I don’t need to check the topology diagram again because I know that all its interfaces are going to go into area one. So let’s do a show run for section OSPF and I’ll remove the old network statement that is putting the networks into area zero again to router OSPF One no.
And then copy and paste. And the new network statement will be, I want all of the interfaces so they all begin with a ten, so I can do ten 5525-5255 again. And now I’m going to put them into area one. And now, because the area does match on both sides of the link, I see the adjacency is coming up. It forms an adjacency with R Five and it also forms an adjacency with R Two. Now, if you look at our old show IP routes, this is when I brought up earlier on R Four, and you can see that all of my OSPF routes are intra area routes. They all show up as a typo, because they all came from other routers in the same area.
Now that I have configured my multi area OSPF, if I do a Show IP route, I expect that I’ll see the ten dot O networks on r four showing up as an inter area route, because r four is in area zero, but those 100 networks are in area one. So you see the difference. Now they are showing up as inter area routes, but right now I haven’t configured summarization. So before I had my 1234 networks that were coming from 100, and now I’ve still got four networks coming from ten dot zero.
So configuring different areas doesn’t actually do us any good until we do the manual summarization as well. So let’s do that next. So our summarization gets done again on our ABRs. So looking back at the topology diagram on r two, I’m going to summarize all of these individual ten x 24 networks into a ten 00:16 and advertise that into area zero. I’ll do the same thing from right to left on r five as well. Coming back in the other direction, all of the individual ten one x 24 networks, I’m going to summarize that to ten 116 and advertise that into area one.
So let’s do that next. So I’ll jump onto my ABR router, r two. So on r two, the command is area zero. Range on that side is ten one O. And when we put in our network statements, we use a wild card, but when we do our summary addresses, we use a normal subnet mask. So, area zero, I’m going to summarize ten in the other direction. For area one, the range is ten dot o, and again, 255-2550 dot o. And I need to do exactly the same commands on my ABR of r five.
So it’s going to be area zero, range ten one or 25 25 or area one, range ten dot o, dot o, dot 0255 dot two five five dot or what I should see now is that change on r four. So before, when I wasn’t doing summarization for all of the routes that were coming from area one into my r four router in area zero, I was getting an individual 24 route for each of those. If I do a Show IP route now, then there you can see the difference. I don’t have four individual 24s, I’ve just got that 116 summary out.
So it’s taking up less space amount or less memory, taking up less resources on there. That’s the benefit we get from doing our multi area OSP. And just finally, I’ll show you on r one. It’s going to be the same thing. If I do a Show IP route here, I’ll just have a single summary route for the ten one six routes which are over in area zero. Okay, that was it. See you in the next.
