show ip ospf neighbor   ! Adjacency database
show ip ospf database   ! Link-state database
show ip route           ! Forwarding database
show ip protocols       ! Verify OSPF and router ID

Basic OSPFv2 Configuration:

router ospf <process-id>
  router-id <rid>

Router ID Determination

The router ID is explicitly configured using the OSPF router-id <rid> router configuration mode command. The rid value is any 32-bit value expressed as an IPv4 address. This is the recommended method to assign a router ID.
If the router ID is not explicitly configured, the router chooses the highest IPv4 address of any of configured loopback interfaces. This is the next best alternative to assigning a router ID.
If no loopback interfaces are configured, then the router chooses the highest active IPv4 address of any of its physical interfaces. This is the least recommended method because it makes it more difficult for administrators to distinguish between specific routers.

Modifying a Router ID

Clearing the OSPF process to reset the router ID:

! Router ID already exists, modify to 1.1.1.1
router ospf 10
  router-id 1.1.1.1

! Clear process to enable new router ID
clear ip ospf process

Using a Loopback Interface as the Router ID

A router ID can also be assigned using a loopback interface.

The IPv4 address of the loopback interface should be configured using a 32-bit subnet mask (255.255.255.255). This effectively creates a host route. A 32-bit host route does not get advertised as a route to other OSPF routers.

interface loopback 0
  ip address 1.1.1.1 255.255.255.255

Enabling OSPF on Interfaces

network <network-address> <wildcard-mask> area <area-id>

Example:

router ospf 10
  network 172.16.1.0 0.0.0.255 area 0
  network 172.16.3.0 0.0.0.3 area 0
  network 192.168.10.4 0.0.0.3 area 0

Using the quad 0 wildcard mask to specify the interface IPv4 address

As an alternative, OSPFv2 can be enabled using the network <intf-ip-address> 0.0.0.0 area <area-id> router configuration mode command.

Example:

router ospf 10
  network 172.16.1.1 0.0.0.0 area 0
  network 172.16.3.1 0.0.0.0 area 0
  network 192.168.10.5 0.0.0.0 area 0

Passive Interface Configuration

router ospf 10
  passive-interface G0/0

Verify with: show ip protocols

Note: OSPFv2 and OSPFv3 both support the passive-interface command.
As an alternative, all interfaces can be made passive using the passive-interface default command. Interfaces that should not be passive can be re-enabled using the no passive-interface command.

Adjusting Reference Bandwidth

OSPF uses a reference bandwidth of 100 Mb/s for any links that are equal to or faster than a fast Ethernet connection. Therefore, the cost assigned to a fast Ethernet interface with an interface bandwidth of 100 Mb/s would equal 1.

To assist OSPF in making the correct path determination, the reference bandwidth must be changed to a higher value to accommodate networks with links faster than 100 Mb/s.

auto-cost reference-bandwidth <value; default = 100>

auto-cost reference-bandwidth 1000      ! Gigabit Ethernet
auto-cost reference-bandwidth 10000     ! 10 Gigabit Ethernet Links

Check/Verify current OSPFv2 cost:

show ip ospf interface <intf-id>

Adjusting the Interface Bandwidth

Check interface bandwidth with show interfaces
Check clock rate with show controllers
If the bandwidth setting is not accurate, the interface default interface bandwidth of 1,544 kb/s may need to be adjusted.

show interfaces serial 0/0/1 | include BW
show ip ospf interface serial 0/0/1

Adjust bandwidth:

interface <intf-id>
  bandwidth <kilobits>

Example: Given a clock rate of 128000 bits on a serial interface, adjust bandwidth:

interface s0/0/0
  bandwidth 128

Manually Setting the OSPF Cost

ip ospf cost <value>

An advantage of configuring a cost over setting the interface bandwidth is that the router does not have to calculate the metric when the cost is manually configured. In contrast, when the interface bandwidth is configured, the router must calculate the OSPF cost based on the bandwidth. The ip ospf cost command is useful in multi-vendor environments where non-Cisco routers may use a metric other than bandwidth to calculate the OSPFv2 costs.

Verify OSPF Settings

show ip ospf neighbor
show ip protocols       ! Protocol Settings
show ip ospf            ! Process Information
show ip ospf interface
show ip ospf interface brief

OSPFv3 (IPv6)

Steps to Configure OSPFv3

Enable IPv6 unicast routing: ipv6 unicast-routing.
(Optional) Configure link-local addresses.
Enter OSPFv3 router configuration mode by ipv6 router ospf <process-id>, then configure a 32-bit router ID using the router-id <rid> command.
Configure optional routing specifics such as adjusting the reference bandwidth.
(Optional) Configure OSPFv3 interface specific settings. For example, adjust the interface bandwidth.
Enable IPv6 routing by using the ipv6 ospf area command.

Changing the OSPF Router ID

After an OSPFv3 router establishes a router ID, that router ID cannot be changed until the router is reloaded or the OSPFv3 process is cleared.

clear ipv6 ospf process

Doing this forces OSPF on R1 to renegotiate neighbor adjacencies using the new router ID.

Verify OSPFv3

show ipv6 ospf interface brief
show ipv6 ospf interface <intf-id>
show ipv6 ospf neighbor
show ipv6 protocols
show ipv6 ospf

Useful commands related to OSPF: