Configuring single-area OSPFv2

Start with the interfaces and the required result

PC-A on 192.168.1.0/24 needs to reach the server at 192.168.3.10. R1 knows PC-A's local network, and R3 knows the server's local network. R2 joins them. Each router needs routes to the remote LANs, and the server needs a return path to PC-A.

OSPFv2 is the IPv4 routing protocol we use here. An area groups interfaces that share detailed OSPF topology information. This small network uses one normal area, area 0. An OSPF neighbour is another router on a local link. PCs and ordinary Layer 2 access switches do not need to become OSPF neighbours.

R1–R3 should learn the three LANs. Only router-to-router interfaces form neighbours. R1 also supplies an external default route. Interface addresses and basic connectivity are already configured; the blocks below start in global configuration mode on their named router.

1 · Select the interface, keep its actual subnet mask

The process ID is the local number for OSPF, here 1. The router ID is a separate unique name, such as 1.1.1.1 for R1. It looks like an IP address, but it does not need to be configured on a port. Set it explicitly so the router's identity is predictable.

A wildcard mask tells IOS which address bits to compare: 0 means match, and 1 means ignore. The wildcard 0.0.0.3 matches the four addresses from 10.0.12.0 through 10.0.12.3. R1's 10.0.12.1 interface is inside that range.

The network command selects local interfaces. A precise match does not create a host route. The interface's actual address and mask determine the connected prefix advertised by this example.

The process number is local: neighbours do not need matching process IDs. Area IDs must agree on the link. On IOS, an interface-level OSPF area command takes precedence over a matching network statement; check both when reading an existing configuration.

2 · Advertise user LANs without forming neighbours there

The LAN must be included in OSPF so other routers can learn it. However, there is no OSPF router to discover beside PC-B. A passive interface advertises its selected subnet without sending Hello discovery packets or forming an adjacency on that interface.

Passive does not mean hidden. The OSPF-enabled LAN stays advertised, but no adjacency forms on it. With passive-interface default, enable neighbour formation only on the intended router links.

3 · Apply the configuration to each router

R1 · network statements and the ISP default

Read R1's block in three parts. The static default points unknown destinations to the Internet service provider (ISP). The interface command makes the R1–R2 link point-to-point: OSPF treats it as a link with one other router, without electing a designated router. The process commands then set identity, cost calculation and the interfaces allowed to form neighbours.

R2 · explicit interface commands

R2 has two router-facing links, so both need no passive-interface. Its third interface is the PC-B LAN and stays passive. This block uses interface commands instead of network statements to show the alternative. You do not need both methods on the same interface.

R3 · exact interface-address matches

R3 uses wildcard 0.0.0.0, which compares every address bit. The two lines therefore select exactly the transit interface and the LAN interface. They still advertise the real /30 transit and /24 LAN networks. The wildcard selects an interface; the configured interface mask describes its subnet.

A new interface remains passive under this policy, but its subnet is advertised only if OSPF is also enabled on it. For a new router link, configure the network type on both ends and remove passive on the intended interface.

4 · Check the route cost from the receiving router

Cost is OSPF's path metric. For this example, each gigabit routed interface costs 10. To reach LAN 1, R3 adds its outgoing cost towards R2, R2's outgoing cost towards R1 and R1's outgoing cost onto LAN 1: 10 + 10 + 10 gives an OSPF route metric of 30. Switch ports on the same LAN do not add a routed OSPF hop.

Reference bandwidth is in Mbps. At 10000 Mbps, gigabit interfaces cost 10; 100 Mbps interfaces cost 100; 10 Gbps interfaces cost 1. Use the same reference across the domain. The default 100 Mbps reference gives all three speeds cost 1.

5 · Originate a default only under the intended condition

A default route, 0.0.0.0/0, is used when no more specific route matches the destination. R1 having a static default does not automatically teach it to R2 and R3. default-information originate asks OSPF to advertise one while R1 has an eligible installed default, under the conditions used in this lab.

On R3 the default next hop is R2, 10.0.23.1. R3 cannot send directly to the ISP's 203.0.113.1 address because that router is not on R3's local link. Each router forwards to its own next hop.

An installed default is not an Internet health check. A failure beyond the ISP next hop can leave R1's static default installed. Plain default-information originate then continues advertising it. The always keyword removes the installed-default requirement; it does not repair upstream connectivity.

6 · Verify interfaces, neighbours and learned routes

First check what OSPF is actually running on. Next check whether the expected router has reached Full, meaning database synchronisation has completed. Finally inspect the route for the destination. These checks answer different questions; one successful neighbour entry is not proof that every LAN is advertised.

R2#show ip ospf interface brief
Interface    PID   Area   IP Address/Mask    Cost  State Nbrs F/C
Gi0/1        1     0      10.0.23.1/30       10    P2P   1/1
Gi0/0        1     0      10.0.12.2/30       10    P2P   1/1

R2#show ip ospf interface GigabitEthernet0/2
Internet Address 192.168.2.1/24, Area 0
Process ID 1, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 10
No Hellos (Passive interface)
Neighbor Count is 0, Adjacent neighbor count is 0

R2#show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
3.3.3.3           0   FULL/  -        00:00:34    10.0.23.2       GigabitEthernet0/1
1.1.1.1           0   FULL/  -        00:00:39    10.0.12.1       GigabitEthernet0/0

R3#show ip route ospf
O*E2  0.0.0.0/0 [110/1] via 10.0.23.1, GigabitEthernet0/0
O     10.0.12.0/30 [110/20] via 10.0.23.1, GigabitEthernet0/0
O     192.168.1.0/24 [110/30] via 10.0.23.1, GigabitEthernet0/0
O     192.168.2.0/24 [110/20] via 10.0.23.1, GigabitEthernet0/0

Test PC-B to server 192.168.3.10 and back, checking host gateways and any filtering. Full neighbours alone do not prove end-to-end traffic works. After verification, save on each router with copy running-config startup-config.

Short lab: prove the LAN advertisement

On R3 run show ip route 192.168.1.0, then ping 192.168.1.10 from the server. In this isolated lab, remove only R1's LAN network statement with no network 192.168.1.0 0.0.0.255 area 0 under process 1. Leave the R1–R2 transit statement intact.

Check R2's neighbour table and R3's route again. R1 can remain Full with R2 while the LAN route disappears after convergence. Restore the original network statement and verify the route and host test recover. This separates a healthy neighbour relationship from a complete set of advertisements.

For a failed check, follow Verifying and troubleshooting OSPF. For a complete hands-on build, use the single-area OSPF lab.

Sources: Cisco network-area command reference, passive-interface behaviour, and default-route origination.

Test yourself on configuring OSPF