Table Of Content： (OSPF tutorials are interactive animations.)
    1.1 OSPF basic                       OSPF basic flow: neighbor, LSA, LSDB, routing table, DD, Request, Update.
    1.2 DR election basic            Introduce DR election process: Hello, priority, Router ID.
    1.3 DR Election                        DR election details: Hello, two-way, full, priority, RID, DR candidates, BDR candidates.
    1.4 DR LSDB synch                How DR/BDR develop full adjacency with other routers on the subnet: DD, Request, Update, Ack, LSDB synchronization.
    1.5 Neighbor State                  Visualize how neighbors establish Full Adjacency relationship from Down state, including packets Hello,  DD, Request, Update, Ack.
    1.6 Area Border Router          ABR uses Network Summary LSA (type 3) to announce an interior router's addresses to other areas.
   1.7 AS Border Router             ASBR discover external addresses and use External LSA to flood them to all internal routers.
    2.1 Router LSA                         A router floods its Router LSAs to tell its link IP addresses to other routers.
    2.1a ABR's Router LSA          ABR creates one Router LSA for each area it connects.
    2.2 Network LSA                      DR creates Network LSA to list routers attached to the subnet.
    2.3 Network Summary LSA    ABR creates Network Summary LSA to let routers know paths in other areas.
    2.4 ASBR Summary LSA        ABR creates ASBR Summary LSA to let routers in other areas know how to reach ASBR.
    2.5 External LSA                      ASBR creates External LSA and floods it to the entire OSPFA AS. It helps internal routers learn external addresses outside OSPF AS.
    3.1 Stub Area                           Internal routers in stub area don't keep External LSA.. When accessing external destinations, they use ABR as default gateway.
    3.1a Stub Area lab                  (Lab) A Cisco IOS lab (dynamics). It illustrates stub area activities in real world. (Including PCAP)                                 
    3.2 NSSA                                   In NSSA (Not So Stub Area), ASBR creates Type-7-LSA for external addresses and floods external addresses to other areas.
    3.2a NSSA lab                          (Lab)  Cisco IOS lab (dynamics). It animates NSSA behaviors. 
    3.3 Link Down                           Explain how OSPF handle link failover: Timer,  neighbor list, LSDB synchronization, routing path.
    3.3a Link Down Lab                   (Lab)  A Cisco IOS lab (dynamics). Visualize OSPF link failover.

OSPF characteristics.
1. OSPF is a link state routing protocol. It uses LSA (Link State Advertisement) to describe routers' links and their IP addresses. For example, every router creates its own Router LSA to describe its links.  Router saves its LSAs in LSDB (Link State Database). Routers establish Full Adjacency with neighbors, synchronize their LSDBs, then use LSDB to calculate their routing paths independently, and update their routing tables.

2. If routers are on a broadcast media (e.g., Ethernet), they use Hello protocol to elect a DR (Designated Router) and a BDR (Backup DR). DR has Full relationship with other routers on the subnet. Non-DR routers don't have Full relationship with each other. This simplifies the number of pair wise relationships. For example, if a subnet has 10 routers, there are 45 pairs (10*9/2). With DR, there are 9 pairs. DR also creates a Network LSA to describe a list of  attached routers in the subnet.

3.  When there are several hundred routers, flooding large number of Router LSA could be a big overhead. OSPF groups routers into areas. Area makes OSPF scalable and able to handle more routers. ABR (Area Border Router) connects areas. It creates Network Summary LSA and uses it to notify addresses across areas. 

4. A typical OSPF router may access thousands of external destinations outside of OSPF AS. Tracking them is a burden. OSPF provides ASBR (AS Border Router) to connect an OSPF AS with an alien AS (e.g., RIP). When ASBR discovers an external address from an alien AS, it creates an External LSA (type 5) and floods it to the entire OSPF AS. This way, internal routers can discover external addresses. In this process, ABR also creates an ASBR Summary LSA (type 4) to tell internal routers how to reach the ASBR.  With External LSA and ASBR Summary LSA, internal routers can calculate routing paths to reach external destinations.

5. An OSPF router may deal with thousand external routes. It slows down routers significantly. In order to lighten up internal routers, OSPF provides stub area. Routers in a stub area do not keep external addresses. Their LSDB do not have type 4, 5 LSA. When internal routers need to access external addresses, they simply send packets to ABR. That is, ABR plays the role of a default gateway for stub area routers. In Totally stub area, routers don’t keep type 3 LSAs.

5. If a stub area is connected to a non-OSPF AS, it discovers new external addresses and unable to notify internal routers. This is because stub area routers cannot have External LSA. To solve this problem, OSPF provides NSSA (Not So Stub Area). In NSSA, an ASBR creates type 7 LSA when discovering external addresses. ASBR floods type 7 LSA inside NSSA. When an ABR in the NSSA receives type 7 LSA, it converts type 7 LSA into type 5 LSA and floods type 5 LSA to other areas.

Prerequisite
-  IP address: Class, mask, subnet.
- Router:  Static route, routing table, next hop, default gateway.