IP Routing

Summing Up

OSPF can support very large networks -- the OSPF hierarchy allows almost unlimited growth because new areas can be added to the network without impacting other areas. Dijkstra’s SPF algorithm leads to radical improvements in convergence time, and OSPF does not suffer from the routing loop issues that DV protocols manifest.

OSPF exhibits all the advantages of a classless routing protocol. Variable Length Subnet Masks permit efficient use of IP addresses. Discontiguous networks can be supported since LSAs carry subnet mask information, and routes can be summarized at arbitrary bit boundaries. Summarization reduces routing protocol overhead and simplifies network management.

Furthermore, OSPF does not tie up network bandwidth and CPU resources in periodic routing updates. Only small hello packets are transmitted on a regular basis.

These OSPF benefits come at a price:

• OSPF is a complex protocol requiring a structured topology. A haphazard environment, without a plan for network addresses, route summarization, LS database sizes, and router performance, will yield a real mess.

• A highly trained staff is required to engineer and operate a large OSPF network.

• OSPF maintains an LS database that requires sizeable memory, and the SPF algorithm can hog CPU resources if the size of the topology database has grown out of bounds. Splitting an area to reduce the size of the LS database may not be straightforward, depending on the topology of the area.

• OSPF assumes a hierarchical network topology ...