Internetworking:
The Cisco Three-Layered Hierarchical Model
Copyright © 2004, SemSim.com
http://www.SemSim.com
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.
 SemSim.com
provides training for Cisco certification exams:
CCIE, CCNP, CCNA, CCDP, CCDA.
It offers free learning resources and router simulation labs.
For more information and free trial of the router simulator visit
http://www.SemSim.com
. |
