Cisco has defined a hierarchical model known as the hierarchical 
internetworking model. This model simplifies the task of 
building a reliable, scalable, and less expensive hierarchical 
internetwork because rather than focusing on packet construction, 
it focuses on the three functional areas, or layers, of your 
network:

The benefits of the Cisco hierarchical model include:

• High Performance: You can design high performance networks, 
  where only certain layers are susceptible to congestion.

• Efficient management & troubleshooting: Allows you to 
  efficiently organize network management and isolate causes 
  of network trouble.

• Policy creation: You can easily create policies and specify 
  filters/rules.

• Scalability: You can grow the network easily by dividing your 
  network into functional areas.


Core Layer

The core layer is responsible for fast and reliable 
transportation of data across a network. The core layer is 
often known as the backbone network because all other layers 
rely upon it. The factors to be considered while designing 
devices to be used in the core layer are:

• High data transfer rate: Speed is important at the core layer. 
  One way that core networks enable high data transfer rates is 
  through load sharing, where traffic can travel through 
  multiple network connections.

• Low latency period: The core layer typically uses high-speed 
  low latency circuits which only forward packets and do not 
  enforcing policy.

• High reliability: Multiple data paths ensure high network 
  fault tolerance; if one path experiences a problem, then 
  the device can quickly discover a new route.

At the core layer, efficiency is the key term. Fewer and faster 
systems create a more efficient backbone. Examples of core layer 
equipment include high end switches such  as 7000 & 7200, Frame 
relays and ATM networks.


Distribution Layer

The distribution layer is responsible for routing. It also 
provides policy-based network connectivity, including:

• Packet filtering (firewalling): Processes packets and 
  regulates the transmission of packets based on its source 
  and destination information.

• QoS: The router or layer 3 switches can read packets and 
  prioritize delivery, based on policies you set.

• Access Layer Aggregation Point: The layer serves the 
  aggregation point for the desktop layer switches.

• Control Broadcast and Multicast: The layer serves as the 
  boundary for broadcast and multicast domains.

• The distribution layer also allows creating protocol gateways, 
  performs queuing and provides packet manipulation of the 
  network traffic.

It is at this layer where you begin to exert control over 
network transmissions. You will also limit and create broadcast 
domains, create virtual LANs and conduct various management 
tasks, including obtaining route summaries. In a route summary, 
you consolidate traffic from many subnets into a core network 
connection. In Cisco routers, the command to obtain a routing 
summary is:

show ip route summary

You can practice viewing routing information using a 
 free CCNA exam router simulator  
available from http://www.SemSim.com . You can also determine 
how routers update each other’s routing tables by choosing 
specific routing protocols.

Examples of Cisco-specific distribution layer equipment include 
2600 & 4000 series routers.


Access Layer

The access layer contains devices that allow workgroups and 
users to use the services provided by the distribution and core 
layers. In the access layer, you have the ability to expand or 
contract collision domains using a repeater, hub, or standard 
switch. At the access layer, a switch is an advanced version 
of a hub and not a high-powered switch, such as those found at 
the core layer.

At the access layer, you can:

• Enable MAC address filtering: It is possible to program a 
  switch to allow only certain systems to access the connected 
  LANs.

• Create separate collision domains: A switch can create 
  separate collision domains to improve performance.

• Share bandwidth: You can allow the same network connection 
  to handle all data.

• Handle switch bandwidth: You can move data from one network 
  to another to perform load balancing.