OSPF main features

Router Types

• Internal Router - All interfaces reside within the same area
• Backbone Router - A router with an interface in area 0 (the backbone)
• Area Border Router (ABR) - Connects two or more areas
• AS Boundary Router (ASBR) - Connects to additional routing domains; backbone

Area Types

• Standard Area - Default OSPF area type
• Stub Area - External link (type 5) LSAs are replaced with a default route
• Totally Stubby Area - Type 3,4 and 6 LSAs are replaced with a default route
• Not So Stubby Area (NSSA) - A stub area containing an ASBR; type 5 LSAs are converted to type 7 within the area

Link State Advertisements

• Router Link (Type 1): Lists neighboring routers and the cost to each; flooded within an area.
• Network Link (Type 2): Generated by a DR; lists all routers on an adjacent segment; flooded within an area.
• Network Summary (Type 3): Generated by an ABR and advertised among areas.
• ASBR Summary (Type 4): Injected by an ABR into the backbone to advertise the presence of an ASBR within an area.
• External Link (Type 5): Generated by an ASBR and flooded throughout the AS to advertise a route external to OSPF.
• NSSA External Link (Type 7): Generated by an ASBR in a not-so-stubby area; converted into a type 5 LSA by the ABR when leaving the area.

Adjacency States

1. Down: No hello packets have been received from this neighbor yet.
2. Attempt: Router sends unicast hello packets withing dead interfal.
3. Init: Router has received a hello packet where its own router ID is not included.
4. 2-Way: Bi-directional communication has started, both routers have received hello packages with their own router ID included in it. (Designated Router)DR/(Backup Designated Router)BDR Election takes place
5. Exstart: In this state, the routers and their DR and BDR establish a master-slave relationship and choose the initial sequence number for adjacency formation; higher router ID becomes the master.
6. Exchange: OSPF routers exchange database descriptor (DBD) packets.
7. Loading: Routers send link-state request packets; link-state update(LSU)/link-state request(LSR).
8. Full: Fully adjacent; link-state acknowledge(LSA) are exchanged. Database descriptors(DBD) are fully synchronized.

DR/BDR Election

• The DR serves as a common point for all adjacencies on a multiaccess segment.
• The BDR also maintains adjacencies with all routers in case the DR fails.
• Election does not occur on point-topoint or multipoint links.
• Default priority (0-255) is 1; highest priority wins; 0 cannot be elected.
• DR preemption will not occur unless the current DR is reset.

Virtual Links

• Tunnel formed to join two areas across an intermediate
• Both end routers must share a common area
• At leaset one end must reside in area 0
• Cannot traverse stub areas

How to configure OSPF

How to configure routing protocol and directly connected networks.

router# config t
router(config)# router ospf < process number >
router(config-router)# network < ip address > < wildcard subnetmask > area < area number >
router(config-router)# end
router# wr

How to configure interfaces

router> enable
router# config t
router(config)# interface < interface >
router(config-if)# ip address < ip addr. > < subnetmask >
router(config-if)# no shutdown

Additional configuration commands

router(config-router)# redistribute static subnets
router(config-router)# ip ospf authentication message-digest
router(config-router)# ip ospf message-digest-key 1 md5 < keyname >
router(config-router)# ip ospf priority < number >

Troubleshooting

router# show ip { route | protocols }
router# show ip ospf interface
router# show ip ospf neightbor
router# show ip ospf border-routers
router# show ip ospf virtual-links
router# debug ip ospf [...]

Source PacketLife