Hierarchical Model

A hierarchical model is a modular design for an enterprise network that defines three specific layers that each has network services associated with those layers. The layers are access, distribution and core. The access layer is associated with branch offices that usually run lower speed circuits. These branch offices are aggregated at a distribution office and several distribution offices are aggregated at a core office. Modular designs are more scalable and utilize bandwidth more effectively than flat networks.  

Access Level Services

Static Routing, Encryption, Compression, Load Balancing, Access Control Lists, VLAN's, Proxy Services, Queuing

Distribution Level Services

Summarization, Redistribution, Network Address Translation, Protocol Translation, Quality of Service, Inter-VLAN Routing, Access Control Lists (applications, protocols and services)

Core Level Services

Traffic Optimization, Path Optimization, Encapsulation, Quality of Service, Load Balancing  

Topologies

The following describes the most common network wan topologies implemented in an enterprise environment. 

Hub and Spoke 

Each spoke has one or more links to a hub office that will connect the spoke offices. It is the least expensive and easiest to manage. There isn't any circuit diversity unless backup circuits are provisioned at each office. If a circuit fails at a spoke office and there is no backup circuit, that spoke office is without network access.

Partial Mesh  

With a partial mesh topology any office could be designated as a hub office. This is more expensive than hub and spoke since there are alternate circuits to different offices. If one link fails that doesn't affect any office since a new route will be discovered with the routing protocol. The bandwidth at each circuit will affect what preferred routes are selected under normal operating conditions.

Full Mesh

This is the most expensive topology type since there is any - any connectivity among all offices. It is the most difficult to manage although most reliable with multiple paths to any destination. For companies such as banks that require 99.999% availability, this topology would be employed.

Article Source:http://hubpages.com/