OSPF single area and multiple areas configuration

OSPF configuration:

Step 1: Enter OSPF configuration mode for a particular OSPF process using the router ospf process-id global command.

Step 2: (Optional) Configure the OSPF router ID by:

a. Configuring the router-id id-value router subcommand.

b. Configuring an IP address on a loopback interface.

Step 3: Configure one or more network ip-address wildcard-mask area area-id router subcommands, with any matched interfaces being added to the listed area.

Step 4: (Optional) Change the interface Hello and Dead intervals using the ip ospf hello-interval time and ip ospf dead-interval time interface subcommands.

Step 5: (Optional) Impact routing choices by tuning interface costs as follows:

a. Configure costs directly using the ip ospf cost value interface subcommand.

b. Change interface bandwidths using the bandwidth value interface subcommand.

c. Change the numerator in the formula to calculate the cost based on the interface bandwidth, using the auto-cost reference-bandwidth value router command.

Step 6: (Optional) Configure OSPF authentication:

a. On a per-interface basis using the ip ospf authentication interface subcommand.

b. For all interfaces in an area using the area authentication router subcommand.

Step 7: (Optional) Configure support for multiple equal-cost routes using the maximum-paths number router subcommand.

OSPF Single-Area Configuration

OSPF configuration differs only slightly from RIP configuration when a single OSPF area is used.

router ospf has a parameter called the OSPF process-id.  If you run multiple OSPF processes in a single router, the process-id is used to distinguish between the processes.  The process-id does not have to match on each router, and it can be any integer between 1 and 65,535.

The network command tells a router to enable OSPF on each matched interface, discover neighbors on that interface, assign the interface to that area, and advertise the subnet connected to each interface.  The OSPF network command matches interface using an address and a wildcard-style mask like those used with IP ACLs.

The wildcard mask in the OSPF network command works like an ACL wildcard mask, but there is one restriction on the values used.  The OSPF wildcard mask must have only one string of consecutive binary 1s and one string of consecutive binary 0s. works. works. works. does not work.  The wildcard mask of means that all 32 bits must be compared and they must match – so the network commands include the specific IP address of each interface, respectively.

OSPF Configuration with Multiple Areas

Configuring OSPF with multiple areas is simple when you understand OSPF configuration in a single area.

show ip route ospf lists OSPF-learned routes, as opposed to the entire routing table.  show ip route lists all three connected routes, as well as the three OSPF learned routes.