OSPF Area types

Area Types

• Standard area
• Stub area - Will not accept external routes (type 5 LSAs); type 5 LSAs are replaced by a default route
• Totally stubby area - Will not accept LSAs of type 3, 4, or 5; routes are replaced by the ABR with a default route; Cisco proprietary. Полный аналог static default route
• Not-so-stubby area (NSSA) - Stub areas which contain one or more ASBRs; ASBRs in a NSSA generate type 7 LSAs which are then converted to type 5 by the ABR

The greatest advantage of designating stub areas is decreased convergence time.

Stub & Totally stub areas

Not-So-Stubby-Area

area stub

Areas are designated as stubs when the chosen exit ABR is unimportant (or there is only one).

Router(config-router)# area [id]  stub

All routers within a stub area must be configured as such. A stub router will not form an adjacency with a non-stub router in the same area.

area stub no-summary

no-summary further limits a stub area by creating a totally subby area. Totally stubby areas do not receive type 3 or 5 LSAs from other areas.

Router(config-router)# area [id]  stub no-summary

Only ABRs need to be configured with no-summary appended to the stub command. Остальные роутеры в area могут быть просто с флагом stub.

To direct packets outside the stub area, routers rely on a default route advertised by the ABR(s).

The concept of totally stubby areas is Cisco proprietary.

area default-cost

Not-So-Stubby area configuration:
Router(config-router)# area [id] nssa

Used to define the cost of the default route injected by an ABR into a stub area. The default is (cost to the ABR + 1).

This can be used to prefer one stub exit over others.

Router(config-router)# area  default-cost 

Configuration is done only on the ABR.