Network Cartage Models
To architecture and body a acknowledged campus network, you charge accretion a absolute compassionate of
the cartage generated by applications in use, additional the cartage breeze to and from the user
communities. All accessories on the arrangement will aftermath abstracts to be transported beyond the
network. Each accessory could absorb abounding applications that accomplish abstracts with differing patterns
and loads.
Trunk
links
VLANs 1/2/3 VLANs 4/5/6
Campus Arrangement Models 29
Applications such as cyberbanking mail, chat processing, printing, book transfer, and best web
browsers accompany about abstracts cartage patterns that are anticipated from antecedent to destination.
However, newer applications such as videoconferencing, TV or video broadcasts, and IP
telephony accept a added activating user base, which makes cartage patterns difficult to adumbrate or
model.
Traditionally, users with agnate applications or needs accept been placed in common
workgroups, forth with the servers they admission best often. Whether these workgroups are
logical (VLAN) or concrete networks, the abstraction is to accumulate the majority of cartage amid clients
and servers bound to the bounded arrangement segment. In the case of the switched LANs connected
by routers mentioned earlier, both audience and servers would be affiliated to a Layer 2 switch
in the adjacency of the workgroup. This affiliation provides acceptable achievement while
minimizing the cartage amount on the baffled arrangement backbone.
This abstraction of arrangement cartage patterns is accepted as the 80/20 rule. In a appropriately designed
campus network, 80 percent of the cartage on a accustomed arrangement articulation is bounded (switched). No
more than 20 percent of the cartage is accepted to move beyond the arrangement courage (routed).
If the courage becomes congested, the arrangement ambassador will apprehend that the 80/20 rule
is no best actuality met. What recourses are accessible to advance arrangement achievement again?
Upgrading the campus courage is not a adorable option, due to the amount and complexity.
The accomplished abstraction abaft the 80/20 aphorism is to accumulate cartage off the courage in the aboriginal place.
Instead, the ambassador can apparatus the afterward solutions:
• Reassign absolute assets to accompany the users and servers afterpiece together.
• Move applications and files to a altered server to break aural a workgroup.
• Move users logically (assigned to new VLANs) or physically to break abreast their
workgroups.
• Add added servers, which can accompany assets afterpiece to the corresponding workgroups.
Needless to say, befitting avant-garde campus networks to the 80/20 aphorism has become difficult
for the arrangement administrator. Newer applications still use the client/server model, but server
portions accept been centralized in best enterprises. For example, databases, Internet and
intranet technologies, and cyberbanking mail are all accessible from centralized servers. Not alone do
these applications absorb beyond amounts of data, they additionally crave a greater allotment of
traffic to cantankerous a arrangement courage to ability accepted destinations—quite a abandonment from the
80/20 rule.
This new archetypal of campus cartage has become accepted as the 20/80 rule. Now, alone 20 percent
of the cartage is bounded to the workgroup, while at atomic 80 percent of the cartage is accepted to travel
off the bounded arrangement and beyond the backbone.
This about-face in cartage patterns puts a greater accountability on the Layer 3 technology of the campus
backbone. Now, because cartage from anywhere on the arrangement can be destined for any other
part of the network, the Layer 3 achievement alluringly should bout the Layer 2 performance.
30 Chapter 2: Campus Arrangement Architecture Models
Generally, Layer 3 acquisition involves added processing assets because the abstracts packets must
be advised in greater depth. This added ciphering amount can actualize bottlenecks in the
campus network, unless anxiously designed.
Likewise, a campus arrangement with abounding VLANs can become difficult to manage. Before,
VLANs were acclimated to logically accommodate accepted workgroups and accepted traffic. With the
20/80 rule, end accessories charge to acquaint with abounding added VLANs. Measuring traffic
patterns and redesigning the campus arrangement becomes too bulky aloof to accumulate up with the
20/80 aphorism model.