Make a backup route using another area in OSPF

[MitsYara]

Hi everyone!

First, sorry for my English is not good.

I have a problem about backup route between 2 OSPF area. Let's say that I have a topo like picture below, how can I make the connection between R4 and R5 be the backup route for PC7 in case I lost the connection between R4 and R2?

Thank you guys!

[Joseph W. Doherty]

One approach, would be to make R4 and R5 ABRs.

[MitsYara]

Thank you for your help. Do U mean I have to create a new area between R4 and R5? Or just one, for example, make the interface on R4 facing R5 join R5 area? If so, it did not work.

[Julio E. Moisa]

Hi

I think you could create a Tunnel with higher cost into the Area 0 to interconnect both routers. I will lab it. 

[Joseph W. Doherty]

Yes, that would one way to get R4 and R5 to become ABRs. The other would be to use a virtual link.(?)

[MitsYara]

Thank you for your help. The problem is I only have access on both end routers R4 and R5 only, so creating a virtual link or tunnel is impossible.

[Julio E. Moisa]

Hi

If you have access to R4 and R5 only you should be able to create the tunnels like: 

R4

int tunnel 0

ip add 192.168.45.4 255.255.255.0

tunnel source 10.45.8.2

tunnel destination 10.45.8.1

ip mtu 1476

ip tcp mss-adjust 1436

ip ospf 1 area 0

no shutdown

R5

int tunnel 0

ip add 192.168.45.5 255.255.255.0

tunnel source 10.45.8.1

tunnel destination 10.45.8.2

ip mtu 1476

ip tcp adjust-mss 1436

ip ospf 1 area 0

no shutdown

Your output should be something like:

1.0.0.0/32 is subnetted, 1 subnets
C 1.1.1.1 is directly connected, Loopback0
2.0.0.0/32 is subnetted, 1 subnets
O IA 2.2.2.2 [110/1001] via 192.168.45.5, 00:00:00, Tunnel0
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.45.8.0/24 is directly connected, FastEthernet0/0
L 10.45.8.1/32 is directly connected, FastEthernet0/0
192.168.45.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.45.0/24 is directly connected, Tunnel0
L 192.168.45.4/32 is directly connected, Tunnel0

Also what about if you set up the point to point between R4 and R5 under the Area 0, it should work. 

:-)

[Joseph W. Doherty]

Julio, that would create a partitioned area zero, wouldn't it?

[MitsYara]

Also what about if you set up the point to point between R4 and R5 under the Area 0, it should work. 

Yes, I also tried that method (in Packet Tracer), but sadly it didn't work.

I also tried to redistribute area 1 to area 5, it works, but what's the point of doing so if you divide 2 areas then put it back together, right? Also, if I'm going to redistribute that, the others routers that connect to the R3 also using my route to get to router R2, and the boss didn't want that.

Hope that you understand what i'm trying to say. Thanks guys!

[Joseph W. Doherty]

"The problem is I only have access on both end routers R4 and R5 only, so creating a virtual link or tunnel is impossible."

Then it gets difficult. Do you have access to R2 and R3? If you do you could extend area1 and area 5 topologies into the other area. Then, although OSPF won't logically jump between those areas, without using area zero, each router would "know" of the other topology and use whatever it considered the best path. Or, you could do some with a bunch of static routes. If you're truly only concerned about PC7 and loosing R4<>R2, your static route needs would be much less.

[MitsYara]

What do you mean by "extend area1 and area 5 topologies into the other area"?
I'm not so good at english, could you make an example, please? Thank you!

[Joseph W. Doherty]

I mean you have all your area 1 and area 5 routers be in both areas, concurrently. It would be much like your idea of redistribution of area 1 into area 5 and the converse. Either gets all the routers to "know" all the other area's routes.

Both are rather "ugly", and yes, either is rather like combining the two areas into one area, which also might be done, but then you need to make changes on the ABR R1.

Your options are limited if you cannot makes changes to any router in the topology.

[MitsYara]

Thank you so much for your help!

I have tried all I know, but it seem impossible. Still hope someone knows a way out. xD