Understanding the network.pdf - Back to Home

OSPF is designed to operate in both large and small-scale networks.
The network diameter (the number of subnets and routers) has a
direct relation to how OSPF is implemented. OSPF has different
configuration options and utilities to address operational issues that
arise in both large and small network environments.
Implementing OSPF requires some forethought and planning; a
poorly implemented OSPF solution can affect the efficiency of the
network. The following are major OSPF elements you need to
understand when designing an OSPF implementation:
• The OSPF area hierarchy
• Router designations
• OSPF network types
• Inter/intra routing
The OSPF Area Hierarchy
OSPF's only limiting aspect is the memory required by the router to
compile the shortest path distances needed for the routing table. On a
small network consisting of 20 or so routers, these computations are
manageable, but on a network of 100 or more, these calculations can
cripple a small router with limited CPU and memory.
To address computational issues, OSPF uses a hierarchical network
segmentation structure. These structures are called areas, OSPF's
convention for logically grouping a collection of networks. Every OSPF
network has at least one area, known as a root or backbone area. The
backbone area, commonly designated as area 0 or area 0.0.0.0, is a
special OSPF area used to connect all other areas of the OSPF network
domain. In a single-area OSPF network (see Figure 8.10), only the
backbone area is used. In a multi-area network (again, see Figure
8.10), all areas must have one interface connected to the backbone
area. Because this is not always possible, OSPF provides the
capability to create virtual links between disconnected areas and the
backbone network.