HP 200 Series Services And Applications page 266

Routing with OSPF
Routing Improvements
Area and Router IDs. Both areas and routers have IDs (identifiers)
that are used by OSPF to build its topological database. Both IDs are
given in dotted decimal notation. This is the same notation used for IP
addresses. The range of the identifiers is thus 0.0.0.0 to 255.255.255.255.
Area identifier 0.0.0.0 is reserved for the Backbone area. Area and router
IDs are not IP addresses, however, and thus such concepts as IP address
classes, subnetting, broadcast addresses, etc., do not apply.
The OSPF standard does not provide guidance on the selection of area identi-
fiers except the Backbone area (0.0.0.0). Several schemes have been
proposed. A unified way to select both area IDs and router IDs is to assign
IDs using a scheme compatible with that of the corporation or organization,
such as AREA.REGION.OFFICE.ROUTER. In figure 1, area A has an area ID
of 1.0.0.0, signifying a particular geographic area—a country, for example.
Router IDs can then be selected based on their location within area 1.0.0.0.
Thus the first router in office 1 of region 1 of area 1 receives the router ID
1.1.1.1. In any event, it is desirable to select identifiers that have some
significance.
Area Sizing. A frequently asked question is: "How large should an
OSPF area be?" Or: "What is the optimal number of routers in an OSPF
area?" An area could contain as few as one router or as many as
hundreds of routers. Carving a network up into smaller-sized areas will
help to minimize the amount of OSPF protocol traffic. The costs associ-
ated with reducing the size of areas include additional complexity in the
OSPF Backbone and the network as a whole. The answer to the question
of area size probably has more to do with organizational structure than
with the OSPF protocol. Generally, areas are suggested by the structure
of the organization(s) responsible for managing the network. Within
many large corporations, responsibility for network management is
distributed. Corporate telecommunications departments may manage
the network in and around the corporate offices or sites, while network
management in other regions is handled by other groups in those
regions. These organizational boundaries are also natural OSPF area
boundaries.
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