Why do large OSPF networks use a hierarchical design? (Choose three.)
A.
to decrease latency by increasing bandwidth
B.
to reduce routing overhead
C.
to speed up convergence
D.
to confine network instability to single areas of the network
E.
to reduce the complexity of router configuration
F.
to lower costs by replacing routers with distribution layer switches
Open Shortest Path First (OSPF) supports hierarchical network design. Open Shortest Path First (OSPF) allows the network to be designed in two layer hierarchies. Area 0 at one layer and all other Areas at other layer.
An Open Shortest Path First (OSPF) Area must be configured as a group of contiguous IP networks. This allows Route Summerization at Area level.
An OSPF Area is a collection of OSPF routers and networks that share the same Link State Database. A Router in one OSPF Area doesn’t have detailed information about network topology outside of its area.
Advantages of designing Multi-Area OSPF networks are listed below.
The Routers in same OSPF Area share the same Link State Database (LSDB). The memory and processor requirement for Link State Database (LSDB) in Routers are less.
Whenever a network topology change happens in a network, the Routers need to re-run the SPF algorithm to calculate best routes. The SPF algorithm is required to run when the topology change happens in the same area. Hence OSPF Routers within Area have to run SPF less often.
When a routing protocol process is enabled, every participating router will attempt to exchange information about the topology of the network. The extent of this information exchange, the way it is sent and received, and the type of information required vary widely depending on the routing protocol in use, see e.g. RIP, OSPF, BGP4.
A state of convergence is achieved once all routing protocol-specific information has been distributed to all routers participating in the routing protocol process. Any change in the network that affects routing tables will break the convergence temporarily until this change has been successfully communicated to all other routers.
Convergence time
Convergence time is a measure of how fast a group of routers reach the state of convergence. It is one of the main design goals and an important performance indicator for routing protocols to implement a mechanism that allows all routers running this protocol to quickly and reliably converge