OSPF Areas, LSA and Path Selection ( Note-1)

OSPF Areas, LSA and Path Selection 

Flooding domain

• All devices inside the area agree on the topology
• Changes need LSA flooding and Full SPF

Hides Topology Details

• Inter-area routing ( seems like distance vector)
• Changes outside the area ( no need LSA flooding)
• Limit ( impact on router resources)

Two Areas 

• Backbone (Area 0 ) and Non-backbone areas
• Non-backbone ( must have connection to back bone which is area 0)

Router Types

1. Backbone routers ( one link goes to backbone)
2. Internal routers ( All links are in non-backbone area)
3. Area Border Router ( summarize info between area 0 and non-area 0) (LSA Type3)
4.  Autonomous System Border Router  (LSA Type 5 or Type 7)
• one link at least ( OSPF domain)
• one link at least ( OUTSIDE OSPF domain)

LSA Types

1. Type 1 ( Router LSA)
2. Type 2 ( Network LSA) ( Generated by DR) ( broadcast or non-broadcast network types)
3. Type 3 (Network Summary LSA) ( Generated by ABR) 
4. Type 4 (ASBR Summary LSA) (Generated by ASBR)
5. Type 5 (External LSA) ( Generated by ASBR) ( flood to all except stub areas)
6. Type 7 (NSSA External LSA)

Route Types 

• Intra- Area Routes (O) ( LSA Type1 & 2)
• Inter-Area Routes (OIA) (LSA Type 3 & 4)
• External Routes ( E1/E2 - LSA Type 5) (N1/N2 0 LSA Type 7)