Three-Layer Model
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 Fig. 12, each with specific functions. The following are
the three layers and their typical functions:
* The core layer: Backbone
* The distribution layer: Routing
Fig -12
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, 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, here are
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 (i.e., adding
routers) when the internetwork grows. 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:
* 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 functions of the
distribution layer are 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 if
needed. 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. And here are several actions that generally should be done at the
distribution layer:
* Routing
* 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
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.
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