Cisco OSPF — configuration and practice labs

Open Shortest Path First is a link-state interior gateway protocol. Routers exchange link-state information, build a common topology database and calculate best paths with the Shortest Path First algorithm.

OSPFv2 carries IPv4 routes. OSPF uses IP protocol number 89 rather than TCP or UDP. Its administrative distance on Cisco IOS is 110.

• Router ID: 32-bit value that identifies an OSPF router.
• Neighbor: another OSPF router discovered on a common link.
• Adjacency: synchronized OSPF relationship used to exchange LSAs.
• LSA: link-state advertisement describing routing information.
• LSDB: link-state database containing the area topology.
• Area: logical OSPF subdivision. Area 0 is the backbone.
• Cost: metric used to select the best path.

The command enters OSPF router configuration mode but does not yet activate OSPF on an interface.

This temporarily removes OSPF adjacencies, so use it carefully in production.

If no manual router ID is configured, IOS generally chooses the highest IPv4 address on an active loopback interface. If no loopback is available, it uses the highest IPv4 address on an active physical interface when the process starts.

Manual configuration is preferred because interface changes can otherwise make identification and troubleshooting less predictable.

The second form is precise and avoids accidentally enabling OSPF on another interface in the same subnet range.

Use either method consistently. Verification commands show the resulting OSPF-enabled interfaces regardless of how they were activated.

The network statement uses a wildcard mask. A zero bit must match, while a one bit is ignored.

A scalable approach is to make every interface passive by default and explicitly enable neighbor formation only on router-to-router links.

Routers normally require the following to form an adjacency:

• Interfaces share an IP subnet and can exchange protocol 89 traffic.
• OSPF is active on both interfaces.
• Both interfaces belong to the same OSPF area.
• Hello and dead intervals match.
• Authentication parameters match when authentication is enabled.
• Stub area flags match.
• Router IDs are unique.
• MTU mismatches do not prevent database synchronization.

The OSPF process IDs and interface IP addresses do not need to be identical.

Common states include DOWN, INIT, 2-WAY, EXSTART, EXCHANGE, LOADING and FULL.

On Ethernet broadcast networks, DROTHER routers may remain in 2-WAY with each other while becoming FULL with the DR and BDR. A persistent EXSTART or EXCHANGE state often points to an MTU or database-exchange problem.

An election is not automatically preemptive. Changing priority does not necessarily replace an existing DR until a new election occurs.

Both ends should use compatible network types and timers.

Configure matching values at both ends. Default timers are normally preferable unless a design requirement justifies changing them.

OSPF selects the path with the lowest accumulated cost. On Cisco IOS, interface cost is derived from reference bandwidth divided by interface bandwidth, unless an explicit cost is configured.

The value is expressed in Mbps. Inconsistent reference bandwidth does not stop neighbors from forming but can produce asymmetric or unexpected path selection.

Use always deliberately because advertising an unusable default can create a black hole.

Common Cisco routing-table codes include:

• O: intra-area route learned inside the same area.
• O IA: inter-area route learned through an Area Border Router.
• O E1: external route including external and internal OSPF cost.
• O E2: external route using the external metric; this is the common default external type.

Configure the interface addresses and verify directly connected pings before enabling OSPF.

R2 forms a neighbor relationship on both transit interfaces and has no end-user LAN in this topology.

R1 should have one full neighbor, R2 should have two and R3 should have one. R1 should learn LAN 30 as O, and R3 should learn LAN 10 as O.

Every non-backbone area should connect to area 0 through an ABR in a normal hierarchical design.

R1 should learn area 0 and area 20 prefixes as O IA. R4 should learn area 0 and area 10 prefixes as O IA.

The default normally appears as an OSPF external route, commonly O*E2 with default settings.

When two routes exist to the same prefix, modify interface cost to see OSPF choose the lower accumulated metric.

Check for shutdown interfaces, different subnets, area mismatch, passive transit interfaces, timer mismatch, authentication mismatch, duplicate router IDs, incompatible network types, ACL filtering of protocol 89 and MTU problems.

Confirm that the local interface is up, OSPF is active on it, the intended area is correct and the prefix appears in the LSDB. Also check whether a better route from another source already exists.

To disable OSPF only on an interface configured with interface-level syntax, remove that interface command instead.

• Using the subnet mask instead of the wildcard mask in a network statement.
• Assuming the process ID must match between routers.
• Reusing the same router ID on multiple routers.
• Assigning opposite ends of a link to different areas.
• Leaving a transit interface passive.
• Forming neighbors on end-user LANs unnecessarily.
• Forgetting that area 0 is the backbone in a multi-area design.
• Changing timers, MTU or network type on only one side.
• Advertising a default without verifying that the exit route works.
• Changing reference bandwidth on only some routers.