OSPF Areas/LSAs

• Areas
  • Add scalability.
  • Area is a flooding domain.
  • All OSPF nodes within area agree on that area’s topology.
  • Full SPF occurs within an area when there is a change.
• Inter-Area routing
  • Hides topology details from one area to the next.
  • Saves router resources by having multiple areas.

• Backbone Area:
  • Area 0
  • All inter-area traffic must pass through the backbone.
    • Loop prevention mechanism.
  • All non-area 0 areas need to have direct connection to area 0.

• ABR:
  • Area Border Router
  • Link in area 0 and non-backbone area.
  • Where summarization occurs.
• ASBR:
  • At least one link in OSPF domain.
  • At least one link not in OSPF domain
    • ie. another routing protocol AS.
  • Performs redistribution between domains.

LSA Types:

• Intra-Area Routes
  • Labeled ‘O’ in routing table
  • LSA types 1 & 2
• Inter-Area Routes
  • Labeled ‘O IA’ in routing table
  • LSA types 3 & 4
• External Routes
  • labeled ‘E1/E2’ in routing table
    • LSA type 5
  • labeled ‘N1/N2’ in routing table
    • LSA type 7

• LSA1:
  • Generated by every router.
  • Does not flood out local area.
  • Describes link costs, neighbors.
  • Builds graph inside area.
• LSA2:
  • Generated by DR on multiaccess.
  • Local area significant.
  • Describes who is adjacent to DR and their costs.
  • Reduces flooding.

• LSA3:
  • Used for inter-area routing.

These LSAs can be seen in the OSPF database via ‘Summary Net Link States’ on R5, coming from the ABR, R3.

The ABR (R3) will hide the actual path to these destinations, but continue providing the cost. SPF does not run when advertising routes between areas. Inter-area routes is similar to how distance vector operates.

• LSA4
  • Generated by ABR
  • Flooded between backbone and non-backbone areas.
  • Describes ABR’s reachability to ASBRs in other areas.
  • Inter-area external routing is similar to distance vector as well.
    • SPF is not run, routing by rumor.

LSA4 can be seen via redistribution. In the database its hows up as ‘Summary ASB Link States’

The advertising router is R3 with the router-id 3.3.3.3. Added into the topology is another router called ‘EIGRP_EX’. This new router is advertising the two prefixes 10.100.1.0 and 10.50.1.0 with EIGRP over to the ASBR, ‘R1’. R1 is then redistributing these into the OSPF domain.

The LSA4 is for area border routers, and LSA5 will be for autonomous system border routers.

• LSA5
  • Flooded to all non-stub areas.
  • Comes from ASBR – redistribution
  • Metric types show up as E1 or E2.
    • E2 is default, will keep same metric.
    • E1 will change metric per hop like normal OSPF.
    • E1/Type 1 is preferred over E2.

Type 5s will show up in the database called ‘Type-5 AS External Link States’.