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OSPF Routing

OSPF automatically exchanges routes between routers - routers learn the network topology and calculate the best path using cost (based on bandwidth).

Why Use OSPF?

Section titled “Why Use OSPF?”

You need OSPF when:

  • You have a medium to large network (more than 5-10 routers)
  • You want automatic route discovery and failover
  • Your network topology changes frequently
  • You need fast convergence (quick recovery from failures)
  • You need a hierarchical design with multiple areas

When to use OSPF vs EIGRP: OSPF is an open standard (works with any vendor). EIGRP is Cisco proprietary (but faster convergence).

Enable OSPF

Section titled “Enable OSPF”
Router(config)# router ospf 10
Router(config-router)# router-id 1.1.1.1

Parameters:

  • 10 - OSPF process ID. Can be 1-65535 (only matters on this router).
  • 1.1.1.1 - Router ID. Replace with a unique ID for each router (use loopback IP or sequential IDs like 1.1.1.1, 2.2.2.2, etc.).

Process ID (10): Locally significant only - doesn’t need to match other routers. Can be 1-65535.

Router ID: Uniquely identifies this router in OSPF. Can be any IP format, but typically uses loopback or highest IP.

Add Networks to OSPF

Section titled “Add Networks to OSPF”

Using network statements with wildcard mask:

Router(config-router)# network 10.1.1.0 0.0.0.255 area 0
Router(config-router)# network 192.168.10.0 0.0.0.255 area 0

Format: network [network address] [wildcard mask] area [area ID]

Add a specific IP address (interface-based):

Router(config-router)# network 10.1.1.5 0.0.0.0 area 0
Router(config-router)# network 192.168.10.1 0.0.0.0 area 0

Wildcard 0.0.0.0 means “exact match” - only that specific IP.

Add Networks via Interface

Section titled “Add Networks via Interface”

Alternative method - configure OSPF directly on the interface.

Router(config)# interface GigabitEthernet0/0/0
Router(config-if)# ip ospf 10 area 0

Passive Interfaces

Section titled “Passive Interfaces”

Stop OSPF from sending hello packets on interfaces where no OSPF neighbors exist (like LANs with only end devices).

Make specific interface passive:

Router(config-router)# passive-interface GigabitEthernet0/0/0

Make all interfaces passive, then enable only needed ones:

Router(config-router)# passive-interface default
Router(config-router)# no passive-interface GigabitEthernet0/0/1
Router(config-router)# no passive-interface Serial0/1/0

OSPF Cost

Section titled “OSPF Cost”

OSPF uses cost to determine the best path. Lower cost = better path.

Default formula: Cost = Reference Bandwidth / Interface Bandwidth

Change reference bandwidth (for Gigabit+ networks):

Router(config-router)# auto-cost reference-bandwidth 10000

Value in Mbps. Default is 100 (Mbps), which makes Fast Ethernet and Gigabit both cost 1. Use 10000 for accurate Gigabit costs.

⚠️ Set the same reference bandwidth on all routers in the OSPF domain.

Manually set cost on interface:

Router(config-if)# ip ospf cost 50

Hello and Dead Intervals

Section titled “Hello and Dead Intervals”

OSPF routers send hello packets to detect neighbors. If hellos stop, the neighbor is considered dead.

Default timers:

  • Hello: 10 seconds
  • Dead: 40 seconds

Change timers:

Router(config-if)# ip ospf hello-interval 5
Router(config-if)# ip ospf dead-interval 20

⚠️ Timers must match on both sides of the link, or neighbors won’t form.

Default Route Propagation

Section titled “Default Route Propagation”

Advertise a default route into OSPF (useful for internet connectivity).

First, create a default static route:

Router(config)# ip route 0.0.0.0 0.0.0.0 203.0.113.1

Then propagate it into OSPF:

Router(config-router)# default-information originate

Now all OSPF routers learn the default route pointing to this router.

DR/BDR Election (Multi-Access Networks)

Section titled “DR/BDR Election (Multi-Access Networks)”

In networks with multiple routers on the same segment (e.g., Ethernet LAN), OSPF elects a Designated Router (DR) and Backup DR (BDR) to reduce OSPF traffic.

Election criteria:

  1. Highest priority (default is 1)
  2. Highest Router ID (tiebreaker)

Change interface priority to influence DR election:

Router(config-if)# ip ospf priority 255

Priority 0 = never become DR/BDR.

Force DR election (after changing priorities):

Router# clear ip ospf process

Verify Configuration

Section titled “Verify Configuration”
Router# show ip ospf neighbor
Router# show ip ospf interface brief
Router# show ip ospf interface GigabitEthernet0/0/0
Router# show ip ospf database
Router# show ip route ospf
Router# show ip protocols
Router# show ip ospf

Troubleshooting Common Issues

Section titled “Troubleshooting Common Issues”

Neighbors not forming? Check:

  • Subnet masks match on both sides
  • Hello/Dead timers match
  • Area IDs match
  • Network types match (broadcast, point-to-point, etc.)

Verify neighbor states:

Router# show ip ospf neighbor

Neighbor states:

  • FULL = Fully adjacent (good)
  • 2-WAY = Normal for DROTHERs on multi-access networks
  • INIT, EXSTART, EXCHANGE, LOADING = Transitional states (should resolve quickly)

Example Full Configuration

Section titled “Example Full Configuration”
Router(config)# router ospf 10
Router(config-router)# router-id 1.1.1.1
Router(config-router)# network 10.1.1.0 0.0.0.255 area 0
Router(config-router)# network 192.168.10.0 0.0.0.255 area 0
Router(config-router)# passive-interface GigabitEthernet0/0/0
Router(config-router)# auto-cost reference-bandwidth 10000
Router(config-router)# exit


Router(config)# interface GigabitEthernet0/0/1
Router(config-if)# ip ospf hello-interval 5
Router(config-if)# ip ospf dead-interval 20