OSPF Configuration #

Single Area OSPF #

Configure Loopback Interface #

An always-up interface which you can assign an IP address (Layer 3).

Router(config)# interface loopback 0
Router(config-if)# ip address <ip_address> <subnet_mask>

Enable OSPF #

Router(config)# router ospf <process_id>
Router(config-router)# router-id <router_id> (optional)

Check the Router ID #

Router# show ip protocols

Advertising Networks #

1. Global configuration

   Router(config)# router ospf <process_id>
   Router(config-router)# network <ip_address> <wildcard_mask> area 0
   

2. Per-interface configuration

   Router(config)# interface <interface>
   Router(config-if)# ip ospf <process_id> area 0
   

Verify OSPF Neighbor #

Router# show ip ospf neighbor

Reload OSPF Process #

Router# clear ip ospf process

Set interface priority #

Router(config)# interface <interface>
Router(config-if)# ip ospf priority 255
Router(config-if)# end

Propagate Default Route #

Router(config)# ip route 0.0.0.0 0.0.0.0 <interface>
Router(config)# router ospf <process_id>
Router(config-router)# default-information originate

Configure Passive Interfaces #

1. Passive by default

   Router(config-router)# passive-interface default
   Router(config-router)# no passive-interface <interface>
   

2. Specific passive interface

   Router(config-router)# passive-interface <interface>
   

Adjust Reference Bandwidth #

OSPF uses a reference bandwidth of 100 Mbps for any links.

Router(config)# router ospf <process_id>
Router(config-router)# auto-cost reference-bandwidth 10000 (Bandwidth in Mbps)

Adjust Interface Bandwidth #

All interfaces have default bandwidth values assigned to them.

Router(config)# interface <interface>
Router(config-if)# bandwidth 100000 (Bandwidth in Kbps)

Manually Setting OSPF Costs #

Administrators can also manually modify the cost of each interface.

Router(config)# interface <interface>
Router(config-if)# ip ospf cost 100

Show OSPF Accumulates Costs #

The cost of an OSPF route is the accumulated value from one router to the destination network.

Router# show ip route <ip_address>

Multi Area OSPF #

Integrate with RIPv2 #

Router(config)# router ospf <process_id>
Router(config-router)# redistribute rip subnets
Router(config-router)# redistribute rip metric-type 1

Metric Types for Type 5 LSA #

The metric for external routes can be Type 1 Metric or Type 2 Metric.

1. Type 1 Metric
   The total (internal + external) cost is used when calculating the routing table.
2. Type 2 Metric
   The external cost dominates the total path cost, internal cost is considered only if multiple paths have the same external cost.

Convert to Stub Area #

It must be configured consistently across all routers within the area, which eliminates Type 4 and Type 5 LSA.

Router(config)# router ospf <process_id>
Router(config-router)# area 10 stub

Convert to Totally Stubby Area #

Eliminates Type 3, 4, and 5 LSA and must be configured consistently across all routers within the area.

Router(config)# router ospf <process_id>
Router(config-router)# area 10 stub no-summary

Convert to Not-So-Stubby Area #

Use the nssa keyword instead of stub to prevent Type 5 AS External LSA flooding in a Not-So-Stubby Area.

Router(config)# router ospf <process_id>
Router(config-router)# area 20 nssa

Convert to Totally NSSA #

Further reduce the table by eliminating Type 3, 4, 5 LSA.

Router(config)# router ospf <process_id>
Router(config-router)# area 20 nssa no-summary

Stub Area Comparison #

LSA Types and Information #

LSAs provide OSPF network details like a database record.

Type 1 Router LSA #

All routers advertise their directly connected OSPF-enabled links and forward their network information to OSPF neighbors.

Router# show ip ospf <process_id> database router

OSPF Router with ID (140.113.0.1) (Process ID 42)
Router Link States (Area 0)
LS Type: Router Links
Link State ID: 140.113.0.1
Advertising Router: 140.113.0.1
[...]
Number of Links: 1

Type 2 Network LSA #

Contains the Router ID and IP address of Designated Router.

Router# show ip ospf <process_id> database network

OSPF Router with ID (140.113.0.1) (Process ID 42)
Net Link States (Area 0)
LS Type: Network Links
Link State ID: 10.113.0.2 (address of Designated Router)
Advertising Router: 140.113.0.2
Network Mask: /24
Attached Router: 140.113.0.1
Attached Router: 140.113.0.2
[...]

Type 3 Summary LSA #

Generated by Area Border Router to advertise networks from other areas.

Router# show ip ospf <process_id> database summary

OSPF Router with ID (140.113.0.15) (Process ID 42)
Summary Net Link States (Area 10)
LS Type: Summary Links(Network)
Link State ID: 140.113.0.0 (summary Network Number)
Advertising Router: 140.113.0.10
Network Mask: /24
Link State ID: 10.0.2.0 (summary Network Number)
Advertising Router: 140.113.0.10
Link State ID: 140.113.20.0 (summary Network Number)
Advertising Router: 140.113.0.10

Type 4 Summary ASBR LSA #

Type 3 LSA describes routes to networks, while Type 4 LSA, though similar in format, specifically details routes to AS boundary routers.

Router# show ip ospf <process_id> database asbr-summary

OSPF Router with ID (140.113.0.15) (Process ID 42)
Summary ASB Link States (Area 10)
LS Type: Summary Links(AS Boundary Router)
Link State ID: 140.113.0.1 (AS Boundary Router address)
Advertising Router: 140.113.0.10
Network Mask: /0

Type 5 AS External LSA #

Advertise routes to external destinations of the OSPF routing domain.

Router# show ip ospf <process_id> database external

OSPF Router with ID (140.113.0.15) (Process ID 42)
Type-5 AS External Link States
Routing Bit Set on this LSA
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number)
Advertising Router: 140.113.0.1
Network Mask: /0

Summary LSA Comparison #