they are as follows:
Core Layer -
the core layer is a high-speed switching backbone and should be designed to switch packets as fast as possible. this layer of the network should not perform any packet manipulation, such as access lists and filtering, that would slow down the switching of packets.
ie:
responsible for aggregating distribution layer connections; provides fast backbone services; provides optimal transport between 'sites'.
Distribution Layer -
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 is the place at which packet manipulation can take place.
ie:
responsible for aggregating access layer connections; segregates access from core; provides policy-based connectivity
Access Layer -
the access layer is the point at which local end users are allowed into the network. this layer may also use access lists or filters to further optimize the needs of a particular set of users.
ie:
responsible for host & end user connections; provides user/host access to the network.
please see the following link for more internetwork design basics info:
http://www.cisco.com/en/US/tech/tk1330/technologies_design_guide_chapter09186a00806666f1.html
