Cost, equal paths and default routes

Cost

OSPF's metric is cost. Each interface has a cost; a route's cost is the total of the outgoing interfaces along the path. By default:

cost = reference bandwidth / interface bandwidth, with a reference of 100 Mb/s

With the default reference, every link of 100 Mb/s or faster costs 1, so OSPF can't tell them apart. Raise the reference on every router: auto-cost reference-bandwidth 10000 (in Mb/s). Or set a cost on one interface: ip ospf cost 50.

Equal-cost paths

If two paths have the same total cost, OSPF installs both and shares the traffic (up to 4 paths, maximum-paths). Here R1 reaches R3's LAN directly (cost 2). Make that link expensive and the path goes round through R2:

In NetLab

▶ R1
R1>enable
R1#show ip route 192.168.3.10
Routing entry for 192.168.3.0/24
  Known via "ospf 1", distance 110, metric 2, type intra area
  Last update from 10.0.13.2 on GigabitEthernet0/2, 00:00:02 ago
  Routing Descriptor Blocks:
  * 10.0.13.2, from 3.3.3.3, 00:00:02 ago, via GigabitEthernet0/2
      Route metric is 2, traffic share count is 1
R1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#interface g0/2
R1(config-if)#ip ospf cost 50
R1(config-if)#end
*Mar  1 00:00:06.000: %SYS-5-CONFIG_I: Configured from console by console
R1#show ip route 192.168.3.10
Routing entry for 192.168.3.0/24
  Known via "ospf 1", distance 110, metric 3, type intra area
  Last update from 10.0.12.2 on GigabitEthernet0/1, 00:00:02 ago
  Routing Descriptor Blocks:
  * 10.0.12.2, from 3.3.3.3, 00:00:02 ago, via GigabitEthernet0/1
      Route metric is 3, traffic share count is 1

Sharing a default route

R1 is the only router with an internet link, and it has a static default route to the ISP. default-information originate tells OSPF to advertise that default route to everyone. The others learn it as O*E2 (an external route, type 2):

In NetLab

▶ R1
R1>enable
R1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#router ospf 1
R1(config-router)#default-information originate
R1(config-router)#end
*Mar  1 00:00:05.000: %SYS-5-CONFIG_I: Configured from console by console
▶ R2
R2>enable
R2#show ip route ospf | begin Gateway
Gateway of last resort is 10.0.12.1 to network 0.0.0.0

O*E2  0.0.0.0/0 [110/1] via 10.0.12.1, 00:00:03, GigabitEthernet0/0
      1.0.0.0/32 is subnetted, 1 subnets
O        1.1.1.1 [110/2] via 10.0.12.1, 00:00:07, GigabitEthernet0/0
      3.0.0.0/32 is subnetted, 1 subnets
O        3.3.3.3 [110/2] via 10.0.23.2, 00:00:07, GigabitEthernet0/1
      10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
O        10.0.13.0/30 [110/2] via 10.0.12.1, 00:00:07, GigabitEthernet0/0
                      [110/2] via 10.0.23.2, 00:00:07, GigabitEthernet0/1
O     192.168.1.0/24 [110/2] via 10.0.12.1, 00:00:07, GigabitEthernet0/0
O     192.168.3.0/24 [110/2] via 10.0.23.2, 00:00:07, GigabitEthernet0/1
R2#ping 8.8.8.8
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 8.8.8.8, timeout is 2 seconds:
..!!!
Success rate is 60 percent (3/5), round-trip min/avg/max = 1/1/1 ms

Remember: Lower cost wins. Cost = reference / bandwidth, so raise the reference on all routers. default-information originate shares a default route; add always to send it even when the router has none itself.