The Cisco Three-Layered Hierarchical Model
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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. There are various equipments available for the core layer. Examples of core layer Cisco equipment include:
Cisco switches such as 7000, 7200, 7500, and 12000 (for WAN use)
Catalyst switches such as 6000, 5000, and 4000 (for LAN use)
T-1 and E-1 lines, Frame relay connections, ATM networks, Switched Multimegabit Data Service (SMDS)
Distribution LayerThe 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 to create network borders.
- 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.
- Application Gateways: The layer allows you to create protocol gateways to and from different network architectures.
- The distribution layer also performs queuing and provides packet manipulation of the network traffic.
It is at this layer where you begin to exert control over network transmissions, including what comes in and what goes out of the network. You will also limit and create broadcast domains, create virtual LANs, if necessary, 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 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, 4500 series routers
Access LayerThe 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. In regards to the access layer, a switch is not a high-powered device, such as those found at the core layer.
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