KB.15.15

The DR and BDR election process is performed when one of the following events occurs:

• Interface is in a waiting state and the wait time expires

• Interface is in a waiting state and a hello packet is received that addresses the BDR

• Change in the neighbor state occurs, such as:

Neighbor state transitions from 2 or higher•

• Communication to a neighbor is lost

• Neighbor declares itself to be the DR or BDR for the first time

OSPF area types

OSPF is built upon a hierarchy of network areas. All areas for a given OSPF domain reside in the

same AS. An AS is defined as a number of contiguous networks, all of which share the same

interior gateway routing protocol.

An AS can be divided into multiple areas. Each area represents a collection of contiguous networks

and hosts, and the topology of a given area is not known by the internal routers in any other area.

Areas define the boundaries to which types 1 and 2 LSAs are broadcast, which limits the amount

of LSA flooding that occurs within the AS and also helps to control the size of the LSDBs maintained

in OSPF routers. An area is represented in OSPF by either an IP address or a number. Area types

include:

• Not-so-stubby (NSSA)• Backbone

• •Normal Stub

All areas in an AS must connect with the backbone through one or more ABRs. If a normal area

is not directly connected to the backbone area, it must be configured with a virtual link to an ABR

that is directly connected to the backbone. The remaining area types do not allow virtual link

connections to the backbone area.

Figure 40 Example of an AS with multiple areas and external routes

Backbone area

Every AS must have one (and only one) backbone area (identified as area 0 or 0.0.0.0.) The ABRs

of all other areas in the same AS connect to the backbone area, either physically through an ABR

or through a configured, virtual link. The backbone is a transit area that carries the type-3 summary

LSAs, type-5 AS external link LSAs and routed traffic between non-backbone areas, as well as the

OSPF area types 193