Hierarchy helps us
understand where things belong, how things fit together, and what functions go
where. It brings order and understandability to otherwise complex models.
Hierarchy has many
of the same benefits in network design that it does in other areas of life.
When used properly, it makes networks more predictable. It helps us define at
which levels of hierarchy we should perform certain functions. Likewise, you
can use tools such as access lists at certain levels in hierarchical networks
and avoid them at others.
large networks can
be extremely complicated, with multiple protocols, detailed configurations, and
diverse technologies. Hierarchy helps us summarize a complex collection of
details into an understandable model.
Then, as specific
configurations are needed, the model dictates the appropriate manner to apply
them.
The Cisco
hierarchical model can help you design, implement, and maintain a scalable,
reliable, cost-effective hierarchical internetwork. Cisco defines three layers
of hierarchy, as shown in Below Figure, each with specific functions.
The following are the three layers:
_ The Core layer
_ The Distribution
layer
_ The Access layer
The Core Layer
The core layer is
literally the core of the network. At the top of the hierarchy, the core layer
is responsible for transporting large amounts of traffic both reliably and
quickly. The only purpose of the network’s core layer is to switch traffic as
fast as possible. The traffic transported across the core is common to a
majority of users. However, remember that user data is processed at the
distribution layer, which forwards the requests to the core if needed.
If there is a
failure in the core, every single user can be affected. Therefore, fault
tolerance at this layer is an issue. The core is likely to see large volumes of
traffic, so speed and latency are driving concerns here. Given the function of the
core, we can now consider some design specifics. Let’s start with some things
we don’t want to do.
_ Don’t do
anything to slow down traffic. This includes using access lists, routing
between virtual local area networks (VLANs), and packet filtering.
_ Don’t support
workgroup access here.
_ Avoid expanding
the core when the internetwork grows (i.e., adding routers). If performance
becomes an issue in the core, give preference to upgrades over expansion.
Now, there are a
few things that we want to do as we design the core. They include the
following:
_ Design the core
for high reliability. Consider data-link technologies that facilitate both
speed and redundancy, such as FDDI, Fast Ethernet (with redundant links), or
even ATM.
_ Design with
speed in mind. The core should have very little latency.
_ Select routing
protocols with lower convergence times. Fast and redundant data-link
connectivity is no help if your routing tables are shot!
The Distribution Layer
The distribution
layer is sometimes referred to as the workgroup layer and is the
communication point between the access layer and the core. The primary function
of the distribution layer is to provide routing, filtering, and WAN access and
to determine how packets can access the core, if needed.
The distribution
layer must determine the fastest way that network service requests are handled;
for example, how a file request is forwarded to a server. After the
distribution layer determines the best path, it forwards the request to the
core layer. The core layer then quickly transports the request to the correct
service.
The distribution
layer is the place to implement policies for the network. Here you can exercise
considerable flexibility in defining network operation. There are several items
that generally should be done at the distribution layer. They include the
following:
_ Implementation
of tools such as access lists, of packet filtering, and of queuing
_ Implementation
of security and network policies, including address translation and firewalls
_ Redistribution
between routing protocols, including static routing _ Routing between VLANs and
other workgroup support
functions
_ Definitions of
broadcast and multicast domains
Things to avoid at
the distribution layer are limited to those functions that
exclusively belong
to one of the other layers.
The Access Layer
The access
layer controls user and workgroup access to internetwork resources. The
access layer is sometimes referred to as the desktop layer. The network
resources most users need will be available locally. The distribution layer
handles any traffic for remote services. The following are some of the functions
to be included at the access layer:
_ Continued (from
distribution layer) access control and policies
_ Creation of separate
collision domains (segmentation)
_ Workgroup
connectivity into the distribution layer
Technologies such
as DDR and Ethernet switching are frequently seen in the access layer. Static
routing (instead of dynamic routing protocols) is seen here as well.
As already noted,
three separate levels do not imply three separate routers.
It could be fewer,
or it could be more. Remember, this is a layered approach.
MPLS Benefits
IP Networking Exam Simulator
Who needs BGP?
BGP and RIP/EIGRP Split-Horizon Rule.
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