Ospf split areas

1. OSPF Split Areas
We all know that OSPF requires that all areas connect to area 0. We also know that
we cannot have partitioned area 0 and expect fully functional OSPF routing. What
happens 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 being
partitioned, we’ll build a relatively simple network. I will use the network from the
diagram below.
This is the relevant configuration on all devices.
http://www.router-switch.com/

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 network
behaves. What we care about here is that R1 can see the route for and have
reachability to R5′ s Loopback0.
R1
It worked like a charm – without us having to do anything! If we examine the entry in
the routing table, we can see that 5.5.5.5/32 is known as inter-area route (Type 3 LSA
carries this information). Let’s take a quick look at that LSA in the database on R1.
R1
We can see that this looks just as a regular Type 3 LSA originated by ABR with
router-id 2.2.2.2 (R2). Nothing special in it. As a comparison, let’s examine the same
http://www.router-switch.com/

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 4.4.4.4
(R4). I’d like to point out the fact that nowhere in the LSA does it say it belongs to any
specific area ID. In this output, there is no mention of area 15 anywhere. This
information is not carried in Type 3 LSA and this is why our configuration worked
without any additional configuration.
Things in CCIE lab may not be as easy as this, though.
“Repairing” the Split Area Using Virtual-Links
Virtual-links are virtual interfaces that are always in OSPF area 0. How can we then
use them to “repair” the split area 15? First of all, let’s repeat that there is nothing to
repair, as there is no single area 15. There are two areas in our network that happen
to 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 as
internal OSPF routes, we have some work to do. One of the solutions is to use
virtual-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 interfaces
on R1 and R5 to put them in backbone area.
http://www.router-switch.com/

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’s
examine an alternative approach.
At this point, I will roll-back the latest changes and continue with the original
configuration.
Using GRE Tunnel to Connect Split Area
We can, of course use the GRE tunnel between R2 and R4. You may be asking why
not between R1 and R5 – after all these are the routers that we need to have in the
same area. The reason for choosing ABRs for tunnel endpoints is because choosing
area routers, like R1 and R5 would present more complex problem to solve from the
perspective of preventing recursive routing. Recursive routing is a situation in which
tunnel’s endpoint is learned via the tunnel interface itself, therefore making it
unreachable and causing the tunnel to flap.
Let’s see the new solution then.
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5. Please note the small trick I used in the above configuration. Since I used
unnumbered interfaces, they would have been in the same area as the interfaces
they inherited IP information from. In this case, that would have put them in area 0. I
needed to have them in area 15, so I used interface-level OSPF configuration to
accomplish 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 at
R1.
R1:
More OSPF Tips:
How to Configure OSPF in a Single Area?
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6. How to Troubleshoot OSPF?
OSPF, How to Configure OSPF in the Cisco IOS?
OSPF Areas Types, OSPF Router Types & OSPF Route Types
http://www.router-switch.com/