LAN Pros and Cons

            When designing a local area network (LAN), there are always a near infinite number of choices to be made; what type of wiring media, the types of

hardware at the first three OSI layers, the type and placement of servers, the logical addressing schemes etc.,  With each choice in each of these decisions come

both ups and downs.  Generally, reliability, performance and cost are directly proportional.  An increase in one leads to an increase in the others.  Yet users demand

performance and reliability at a reasonable cost.  Every network designer must comb through the many options that are available and make the choices that best

balance out these three elements.  In designing the Acacia network, decisions were made in all fields of the design that provided fast, reliable performance at as low

a cost as could be found.
 

            The first matter in the design was the physical wiring.  Acacia contains 36 rooms to be wired, as per the district's requests.  Each of these rooms will receive

four runs of CAT 5 UTP cable, each terminated at an RJ-45 wall jack.  This results in a total of 144 cable runs.  One wall jack will attach to a switch within the

room, another will attach to the teacher's workstation, and two will be left over.  The advantages of this wiring scheme are that the two spare cables exist to provide

room for future growth, and also provide an alternative path for data if one of the cables fail.  If, for example, the cable to the teacher's workstation failed, network

administrators could simply unplug the bad cable and use one of the spares.  This would, of course, be a temporary solution to keep the network running while the

broken cable was repaired or reinstalled.  The disadvantage of this wiring scheme comes in cost.  More cabling costs more money, takes more time, and requires

more labor to install.  However, compared to the advantages, this is a trifling matter.
 

            Along the same lines as regular cabling is the school's backbone cabling.  Since backbone cabling moves data between wiring closets, it will hold a heavier

traffic load, and will thus require cabling with greater bandwidth.  The only current viable choices are 100 Mbps multimode optical fiber and 100 Mbps Fast

Ethernet UTP.  Due to its noise immunity, fiber has been chosen.  Both of the wiring closets will contain a fiber-optic patch panel with eight cable runs extending

from it.  This provides two runs for connectivity, with six runs to spare.  As mentioned, the fiber is fast, operating at 100 Mbps.  Fast Ethernet could have run just as

fast, although it would not have provided the same immunity to noise as fiber.  The downside is that the optical fiber costs more and is harder to install; so essentially

the extra money is simply buying noise immunity.  However, the six extra runs are advantageous, as they provide room for future growth.
 

            Two Virtual Local Area Networks (VLANs) will be implemented at Acacia; VLAN 1 will carry the administrative network, and VLAN 2 will carry the

curriculum network.  The primary benefit of this VLAN implementation is the ability to transmit data from both the administrative and curriculum networks through a

single switch while still maintaining the access restriction set forth by the Washington School District.  Communication between VLANs requires the services of the

router; the router contains the access control lists that deny or permit access based on district guidelines.  The VLAN benefits Acacia's LAN in a number of ways:
 

• With VLANs installed, all existing switch ports can be used before buying another switch.  Without VLANs, another switch might be required only for a few more ports.  In this case, the switch for one network could not share any of its extra ports with any other network, even if it had extras available.

• Because each classroom has one connection to the administrative LAN and one connection to the curriculum LAN, the cable drops can be connected to the same switch.  This will allow for a more organized wiring closet and an organized labeling system that reflects the way the two networks are distributed throughout the school.

• VLANs reduce administration costs related to solving the problems that come with moves, additions to the network, and changes in the network.

• VLANs provide workgroup and network security.

As all things, VLAN installation has its downsides as well.  The disadvantages of implementing VLANs in the Acacia school are:
 

• A more complicated initial configuration due to creating VLANs, assigning ports to VLANs, configuring the Spanning Tree Protocol, and configuring the trunk ports.  Trunk ports special ports on switches that allow both VLANs to travel on a single high speed link between switches.

• An increase in bandwidth usage on the network due to BPDU messages generated by the Spanning Tree Protocol algorithm.  These BPDU messages are sent among all the switches in the network for the purpose of detecting loops caused by multiple paths between multiple switches.

• VLAN implementation requires Enterprise software which is more expensive than regular switch software.

            At the heart of the Acacia LAN is a Cisco 2621 modular access router.  This router has one WAN interface module installed with room to install another

one should the need arise.  This is an advantage for two reasons.  First, if the initial interface fails, the second one can be brought up.  Secondly, an optional second

WAN interface provides room for growth; when the need arises, a second link can be installed and the load shared over both paths.  This 2621 modular access

router also contains a built in CSU/DSU device, thus eliminating the need to purchase an separate device.  In addition to the 2621 modular access router, the MDF

in Acacia will contain two Cisco Catalyst 2924 switches, used in the VLAN implementation.
 

            To exchange routing information between routers, Acacia will use Interior Gateway Routing Protocol (IGRP).  IGRP is, as the name implies, designed for

usage on an interior system--one that is controlled by a common administration.  IGRP and RIP are two of the more popular interior protocols; the District has

asked to use IGRP across the entire WAN.  Configuring IGRP is very similar to RIP, with only the addition of an autonomous system number required.  IGRP is

superior because it uses multiple measurements to make decisions on what the best path is: bandwidth, load, delay, and reliability of a link.  RIP uses only hop

count.  With only hop count, a router might send a packet down a slow or unreliable link simply because that link involves fewer hops to the receiving station; IGRP

makes better decisions with the help of its multiple measurements.  There really is no downside to the use of IGRP as a routing protocol other than perhaps a slightly

longer amount of time used to make routing decisions.  Still, this delay is so minor as to be negligible.
 

            Acacia will consist of two LANs, one for student usage and one for administrative usage.  An Access Control List (ACL) will stop student users from

accessing the administrative LAN, but not vice-versa.  An exception to this ACL will be made for only two cases: student users accessing DNS services and those

accessing E-Mail services.  The advantage of the installed ACL is that it will stop students from entering and tampering with all of the important administrative files

while still allowing teachers and administrators to access the same resources as students.  Also, since traffic will not be able to go from the student LAN to the

administrative LAN, traffic load on the administrative LAN will be lightened, thus improving performance.  The disadvantage lies only in the insufficiencies of ACLs.

While the ACLs do provide a small measure of security, they are relatively easy for hackers to overcome.  Ideally, Acacia, as well as the rest of the Washington

School District, would want some additional security measures.  Still, ACLs are better than nothing.
 

            The District has also asked that all users on the curriculum LAN be given access to a workgroup server.  Although the functions of this server are not clear,

the server is to run Novell Netware, so it will use IPX instead of IP.  This is advantageous insofar as this server provides services to the users on the curriculum

LAN.  On the down side, the function of this server isn't entirely clear, so a server exists to strain the network resources without serving any apparent purpose.

Secondly, this server is running Novell Netware 3.12, an obsolete operating system.  This earlier version of Netware requires IPX and is not compatible with

TCP/IP, whereas newer versions are.  It would therefore make more sense to use a more current version of Netware and run it with TCP/IP.  The network would

then run entirely on TCP/IP and be easier to maintain.  The disadvantages of the District's requested implementation outweigh any clear advantages, so the wisdom

of these installations is questioned.  Still, the District wants IPX, so the servers will be installed with the requested version of Netware.
 

            The installation of the network at the Acacia school is just like any other LAN; it has its ups and its downs.  Better reliability and higher speed lead to higher

cost, yet speed must never be too low and cost cannot be too high.  Sometimes, the customer specifies exactly what they want.  When this happens, such as for the

implementation of IGRP, ACLs, and IPX, there is little that can be done to balance cost and performance.  Other times, such as in deciding on equipment and

cabling type to use, more flexibility is available.  In our design, our team has done its best to make the most practical decisions that would keep speed and reliability

as high as possible while keeping a lid on the costs.