Ospf split areas


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Ospf split areas

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Ospf split areas

  1. 1. OSPF Split AreasWe all know that OSPF requires that all areas connect to area 0. We also know thatwe cannot have partitioned area 0 and expect fully functional OSPF routing. Whathappens if some other area is partitioned? Let’s take a look.To verify what will happen in the case of area other than backbone area beingpartitioned, we’ll build a relatively simple network. I will use the network from thediagram below.This is the relevant configuration on all devices.http://www.router-switch.com/
  2. 2. There is absolutely nothing out of ordinary there. In fact, very basic configuration.Routers R2 and R4 are ABRs for partitioned area 15. Let’s see how our networkbehaves. What we care about here is that R1 can see the route for and havereachability to R5′ s Loopback0.R1It worked like a charm – without us having to do anything! If we examine the entry inthe routing table, we can see that is known as inter-area route (Type 3 LSAcarries this information). Let’s take a quick look at that LSA in the database on R1.R1We can see that this looks just as a regular Type 3 LSA originated by ABR withrouter-id (R2). Nothing special in it. As a comparison, let’s examine the samehttp://www.router-switch.com/
  3. 3. LSA on R3, which is backbone area router.R3:In this output, we can see that this LSA is originated by ABR with router-id I’d like to point out the fact that nowhere in the LSA does it say it belongs to anyspecific area ID. In this output, there is no mention of area 15 anywhere. Thisinformation is not carried in Type 3 LSA and this is why our configuration workedwithout any additional configuration.Things in CCIE lab may not be as easy as this, though.“Repairing” the Split Area Using Virtual-LinksVirtual-links are virtual interfaces that are always in OSPF area 0. How can we thenuse them to “repair” the split area 15? First of all, let’s repeat that there is nothing torepair, as there is no single area 15. There are two areas in our network that happento have the same ID, but they are still separate areas. Therefore, no repair is required.But, if we were told that R1 and R5 need to see each other’s Loopback0 interfaces asinternal OSPF routes, we have some work to do. One of the solutions is to usevirtual-links between R1 and R2 and R4 and R5. Let’s do that and see what happens.Don’t forget that we’ll need to change network statements for Loopback0 interfaceson R1 and R5 to put them in backbone area.http://www.router-switch.com/
  4. 4. As a verification, let’s take a look at the route on R1.R1:This fix worked just fine, but we may have broken the rules of the task. Most notably,Loopback0 interfaces on R1 and R5 are no longer in area 15 – they are in area 0. Let’sexamine an alternative approach.At this point, I will roll-back the latest changes and continue with the originalconfiguration.Using GRE Tunnel to Connect Split AreaWe can, of course use the GRE tunnel between R2 and R4. You may be asking whynot between R1 and R5 – after all these are the routers that we need to have in thesame area. The reason for choosing ABRs for tunnel endpoints is because choosingarea routers, like R1 and R5 would present more complex problem to solve from theperspective of preventing recursive routing. Recursive routing is a situation in whichtunnel’s endpoint is learned via the tunnel interface itself, therefore making itunreachable and causing the tunnel to flap.Let’s see the new solution then.http://www.router-switch.com/
  5. 5. Please note the small trick I used in the above configuration. Since I usedunnumbered interfaces, they would have been in the same area as the interfacesthey inherited IP information from. In this case, that would have put them in area 0. Ineeded to have them in area 15, so I used interface-level OSPF configuration toaccomplish my goal. Let’s take a look at our routing now. First off, R2.R2:Our OSPF is working as expected. We have neighbor on Tunnel24. Let’s take a look atR1.R1:More OSPF Tips:How to Configure OSPF in a Single Area?http://www.router-switch.com/
  6. 6. How to Troubleshoot OSPF?OSPF, How to Configure OSPF in the Cisco IOS?OSPF Areas Types, OSPF Router Types & OSPF Route Typeshttp://www.router-switch.com/