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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 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 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 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.
In regards to the access layer, a switch is not
a high-powered device, such as those found at
the core layer.
