31. Describe the issues with interconnecting multiple areas and how OSPF addresses.

32. Explain the differences between the possible types of areas, routers, and LSAs.

OSPF LSA types

1. Router link entry
2. Network link (from a DR about the multi-access it’s a DR of, e.g. a LAN
3. Summary link (from a ABR about it’s area and distributes on the backbone where the other ABRs flood their non-totally stubby areas.
4. summary link (about ASBR reachability)
5. AS external link (from a ASBR about external routes
   1. E1 – includes cost of internal links – use if more than one ASBR
   2. E2 – doesn’t have cost of internal links – no point if there is only one ASBR.

Router types

• Internal Router (LSA Type 1 or 2) – Routers that have all interfaces in the same area. They have identical link-state database and run single copy of routing algorithm.
• Backbone Routers (LSA Type 1 or 2) – Routers that have at least one interface connected to area 0.
• Area Border Router (LSA Type 3 or 4) – Routers that have interfaces attached to multiple areas. They maintain separate link-state database for each area.
• Autonomous System Boundary Router (LSA Type 5) – Routers that have at least one interface into an external internetwork (another autonomous system), such as a non-OSPF network. These routers can redistribute non-OSPF network information to and from OSPF network.

Types of Areas

• Standard area – Can accept link updates and route summaries.
• Backbone area (transit area) – This area is always labeled "0". All other areas must connect to this area in order to exchange and route information.
• Stub area – This area does not accept information about redistributed routes (LSA Type 5)
• Totally stubby area – This area does not accept information about redistributed routes and route summary.