IPv6 Configuration Guide K/KA/KB.15.15
The DR and BDR election process is performed when one of the following events occurs:
• An interface is in a waiting state and the wait time expires
• An interface is in a waiting state and a hello packet is received that addresses the BDR
• A change in the neighbor state occurs, such as:
A neighbor state transitions from 2 or higher◦
◦ Communication to a neighbor is lost
◦ A neighbor declares itself to be the DR or BDR for the first time
OSPFv3 area types
OSPFv3 is built upon a hierarchy of network areas. All areas for a given OSPFv3 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, including the backbone (area 0). Because each area
represents a collection of contiguous networks and hosts, the topology of a given area is not known
by the internal routers in any other area. Areas define the boundaries to which router-LSAs and
network-LSAs are broadcast. This limits the amount of LSA flooding that occurs within the AS and
also helps to control the size of the link-state databases (LSDBs) maintained in OSPFv3 routers. An
area is represented in OSPFv3 by either a 32-bit dotted-decimal address or a number. Area types
include:
• Backbone
• Normal
• Not-so-stubby (NSSA)
• 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 stub and NSSA area types do not allow virtual link
connections to the backbone area.
Figure 26 Example of an AS with multiple areas and external routes
A
B
R
A
S
B
R
NSSA
A
B
R
ABR
A
B
R
Virtual Link
Backbone Area
Normal
Area
Stub
Area
Normal
Area
OSPFv3 Domain
External
(IGP)
Domain
A
S
BR
External
(IGP)
Domain
Normal area
This area type allows inter-area-prefix-LSAs and AS-external-LSAs to and from the backbone area.
(As noted earlier, the backbone area is a special type of normal area.) A normal area connects
252 OSPFv3 Routing