|  | 3.3 | How to Identify and Select Networking Capabilities
| 3.3.6 | Distribution-layer functions
The distribution layer of the network is the demarcation point between the access and core layers and helps to define and differentiate the core. The purpose of this layer is to provide boundary definition, and it is the layer at which packet manipulation occurs. In the WAN environment, the distribution layer can include several functions, such as the following: Address or area aggregation Departmental or workgroup access to the core layer Broadcast/multicast domain definition Virtual LAN (VLAN) routing Any media transitions that need to occur Security The distribution layer would include the campus backbone with all its connecting routers, as shown in the figure. Because policy is typically implemented at this level, we can say that the distribution layer provides policy-based connectivity. Policy-based connectivity means that the routers are programmed to allow only acceptable traffic on the campus backbone. Note that good network design practice would not put end stations (such as servers) on the backbone. Not putting end stations on the backbone frees up the backbone to act strictly as a transit path for traffic between workgroups or campus-wide servers. In non-campus environments, the distribution layer can be the point at which remote sites access the corporate network. The distribution layer can be summarized as the layer that provides policy-based connectivity.