OSPF Areas:

OSPF organizes a network into areas. An area is a set of routers that will share routing information about one or more networks. Routers are used by OSPF to maintain routing informatioin within an area and to send Link State Advertisements to other areas.

 

Each area is designated by an area number. All other areas are described as having 'stubbiness':

• Not So Stubby Area
• Stubby Area
• Totally Stubby Area (Cisco proprietary)

 

Backbone Area (Area 0)

The first area that will always exist in any network using OSPF for routing is Area 0 (zero). Area 0 is always the backbone area. All other areas are connected to the backbone either directly, or using virtual links that 'tunnel' through other areas. Because areas must be directly connected to the backbone, you cannot chain areas together. All routing information beetween areas is sent through the backbone area (Area 0).

 

Totally Stub Area

A totally stubby area is only connected to the backbone area. A totally stubby / totally stub area does not advertise the routes it knows. It does not send any Link State Advertisements. The only route a totally stub area receives is the default route from an external area, which must be the backbone area. This default route allows the totally stub area to communicate with the rest of the network.

Stub Area

Stub areas are connected only to the backbone area. Stub areas do not receive routes from outside the autonomous system, but do receive the routes from within the autonomous system, even if the route comes from another area.

 

Not-So-Stubby (NSSA)

Frequently, it is advisable to use a separate network to connect the internal enterprise network to the Internet. OSPF makes provisions for placing an Autonomous System Boundary Router (ASBR) within a non-backbone area. In this case, the stub area must learn routes from outside the OSPF autonomous system. Thus, a new type of LSA was required--the Type 7 LSA. Type 7 LSA's are created by the Autonomous System Boundary Router and forwarded via the stub area's border router (ABR) to the backbone. This allows the other areas to learn routes that are external to the OSPF routing domain.

 

Virtual Links

Virtual links are used when you have a network that must be connected to an existing OSPF system, but cannot be physically connected directly to the routers in the OSPF backbone area. You can configure an OSPF virtual link from the area to a backbone router, creating a virtual direct connection to the backbone area. This virtual link acts as a tunnel which forwards LSAs to the backbone via a second intermediate area.

 

Note:

• Hello packet is also called LSA (Link State Advertisement Packet)
• ABR (Area Border Router) connect two different OSPF area.
• ASBR takes update from different AS (using different RP)
• Every non-backbone area must be connected to backbone area either logically or physically.
• Non-DR send update to DR at 224.0.0.6 and DR send update to them at 224.0.0.5

 

DR Selection:

1. Priority (first check priority)
2. Highest IP address

(for DR first priority is checked and then highest IP address, if other router has highest IP address but less priority, it will not be DR)

 

Note:

Router that has highest Priority / IP Address will be DR and another router with highest Priority / IP Address will be BDR.