Configuring and verifying single-area OSPF

The goal

Single-area OSPFv2 means every OSPF interface is in area 0. OSPFv2 is the version for IPv4 (OSPFv3 is used for IPv6). You need to do three things on each router: start the process, tell it which interfaces take part, and check that neighbours reach Full and routes appear.

Starting the process and the router ID

R1(config)# router ospf 1
R1(config-router)# router-id 1.1.1.1

Always set the router ID by hand. Then it never changes by surprise, and show commands are easy to read because R1 is 1.1.1.1, R2 is 2.2.2.2 and so on.

Method 1: network commands with wildcard masks

R1(config-router)# network 10.0.12.0 0.0.0.3 area 0
R1(config-router)# network 192.168.1.0 0.0.0.255 area 0
R1(config-router)# network 10.0.13.1 0.0.0.0 area 0

A wildcard mask is the inverse of a subnet mask: 0 bits must match, 1 bits can be anything. For a /24 (255.255.255.0) the wildcard is 0.0.0.255; for a /30 (255.255.255.252) it is 0.0.0.3. A quick way: subtract each octet of the mask from 255. A wildcard of 0.0.0.0 matches exactly one interface address, as in the third line.

Important: the network command does not "advertise a network" directly. It finds interfaces whose IP matches, enables OSPF on them (they send hellos), and OSPF then advertises those interfaces' subnets.

Method 2: on the interface

R1(config)# interface GigabitEthernet0/1
R1(config-if)# ip ospf 1 area 0

Passive interfaces, default route, cost and priority

R1(config)# router ospf 1
R1(config-router)# passive-interface GigabitEthernet0/0
R1(config-router)# auto-cost reference-bandwidth 100000
R1(config-router)# default-information originate
R1(config)# interface GigabitEthernet0/1
R1(config-if)# ip ospf cost 50
R1(config-if)# ip ospf priority 0
R1(config-if)# ip ospf network point-to-point

• passive-interface: the LAN subnet is still advertised, but no hellos are sent on that interface. Use it on every LAN facing PCs: no router lives there, and it stops anyone on the LAN from forming a fake neighbour. passive-interface default makes all interfaces passive; then use no passive-interface on the router links.
• default-information originate: shares this router's default route with all OSPF routers. They see it as O*E2 0.0.0.0/0. The router must already have a default route (for example a static one to the ISP).
• ip ospf cost: sets the cost of this interface directly, overriding the bandwidth calculation.
• ip ospf priority: changes the DR election priority on this interface (0 to 255; 0 means never DR).
• ip ospf network point-to-point: on an Ethernet link with only two routers, skips the DR/BDR election, so the adjacency comes up faster. Set it on both ends.

Verification

R1# show ip ospf neighbor
Neighbor ID   Pri   State      Dead Time   Address      Interface
2.2.2.2         1   FULL/DR    00:00:36    10.0.12.2    GigabitEthernet0/1

R1# show ip ospf interface brief
Interface  PID  Area  IP Address/Mask    Cost  State  Nbrs F/C
Gi0/1      1    0     10.0.12.1/30       1     BDR    1/1
Gi0/0      1    0     192.168.1.1/24     1     DR     0/0

R1# show ip route ospf
O     192.168.2.0/24 [110/2] via 10.0.12.2, 00:03:11, GigabitEthernet0/1
O*E2  0.0.0.0/0 [110/1] via 10.0.12.2, 00:03:11, GigabitEthernet0/1

show ip protocols summarises it all: the router ID, the networks or interfaces in OSPF, passive interfaces, the reference bandwidth and the AD (Distance 110). Use it first when you take over a router you did not configure.

Why neighbours do not come up