Cisco - LAN Design

	Internetworking Technologies Instructor: Prabul, CCNA
	Internetworking Technologies Instructor: Prabul, CCNA

4. LAN Design

Top 10 Graphics:

One of the most critical steps to insure a fast and stable network is the design of the network. This design activity is truly an in-depth process, which includes:

Gathering the users requirements and expectations
Determining data traffic patterns now and in the future based on growth and Server placements
Defining all of the layer 1, 2 &3 devices and along with LAN and WAN topology
Document the physical and logical network implementation

LAN Design Goals:

Functionality - the network must work with reasonable speed and reliability.
Scalability - the network must be able to grow without any major changes to the overall design.
Adaptability - the network must be designed with an eye toward future technologies, and should include no element that would limit implementation of new technologies as they become available.
Manageability - the network would be designed to facilitate network monitoring and management.

Design Methodology

Analyse requirements
Develop LAN structure (topology)
Set up addressing (and naming conventions) and routing

Step 1... - Analyze Requirements (of the network and its users)

Business issues
Technology issues
Administrative issues
Gather Data -
- Corporate Structure
- Business information flow
- Applications in use
- Current topology
- Performance characteristics of current network
- Determine if documented policies are in place
- Mission-critical data
- Mission-critical operations
- Approved protocols and platforms
- Control versus distributed authority
Business requirements
Technical requirements
New applications or business operations
Availability requirements -
- Throughput
- Response time
- Access to resources

Design Rule: First and foremost you must understand the customer.

Design Rule: Find out what availability means to your customer.

Step 1... - Analyse Network Load Requirements

Client/Server applications
Host/terminal applications
Routing protocols
Reqularly scheduled services, such as file backup
Estimate worst-case traffic load during the busiest times for users and during regularly scheduled network services

Design Rule: Before developing an internetwork structure and provisioning hardware, determine the network traffic load.

Design Rule: Evaluate applications that cause traffic problems (congestion).

Step 2... - Develop LAN Topology

LAN topology that will satisfy Step 1 requirements
Star Topology
Extended Star Topology

Star topology -

A LAN topology in which end points on a network are connected to a common central hub/switch by point-to-point links.
A ring topology that is organized as a star, implements a unidirectional closed-loop star (star wired ring), instead of point-to-point links.

Step 3... - Layer 3 Addressing

The router divides subnets and networks
The router structures an internetwork
Logical addressing should be mapped to the physical network
Develop and document the IP addressing scheme to be used in the network

Step 3... - Setting Up VLAN Implementation

Group users by department, team, or application
Provide broadcast containment and security
Routers provide communiocation between VLANs (and security)

VLAN (Virtual LAN) - Group of devices on a LAN that are configured (using management software) so that they can communicate as if they were attached to the same wire (media), when in fact they are located on a number of different LAN segments. Because VLANs are based on logical instead of physical connections, they are extremely flexible.

What problems are you trying to solve?

Media contention
- too many devices, all with a high demand for the network segment
Excessive broadcasts
- client packets looking for services
- too many server packets announcing services
- too many routing table updates
Need to transport new payloads
- voice and video network services
Need for more bandwidth
Overloaded backbone
Network-layer addressing issues
- running out of IP addresses
- need for physically separate subnets
- other issues dependent on the protocols

Ethernet Technology

Segmentation - Bridging and switching are both used for segmentation

Results in multiple collision domains
Still a single broadcast domain
Stations can get dedicated bandwidth

Collision domain (bandwidth domain) - In Ethernet, the network area within which frames that have collided are propagated (everything associated with one port on a bridge or switch). Repeaters and hubs propagate collisions; LAN switches, bridges and routers do not.

Broadcast domain - The set of all devices that will receive broadcast frames originating from any device within the set. Broadcast domains are typically bounded by routers because routers do not forward broadcast frames (everything associated with one port on a router).

All broadcasts from any host in the same broadcast domain are visible to all other hosts in the same broadcast domain. Broadcasts must be visible to all hosts in the broadcast domain in order to establish connectivity.

The scalability of the collision domain depends on total traffic, and the scalability for a broadcast domain depends on total broadcast traffic.

Network Design

The major pieces of this topology design can be broken into three unique categories of the OSI model...

Layer 1 - Physical Layer
Includes wire media type such as CAT5 UTP and fiber-optic cable along with EIA/TIA 568 Standard for layout and connection of wiring schemes.

Design Goal: Build this layer of the OSI model with speed and expansion capabilities.
Layer 2 - Data Link Layer
Includes selection of Layer 2 devices such as bridges or LAN switches used to interconnect the Layer 1 media to for a LAN segment. Devices at this layer will determine the size of the collision and broadcast domains.

Design Goals:
Layer 3 - Network Layer
Includes selection of layer 3 devices such as routers which are used to create unique LAN segments and allow communication between segments based on layer 3 addressing such as IP addressing.

Design Goals:

Layer 1 Media and Topology

The Physical layer controls the way data is transmitted between source and destination node.

Physical layer - Layer 1 of the OSI reference model. The physical layer defines the electrical, mechanical, procedural and functional specifications for activating, maintaining, and deactivating the physical link between end systems.

Media - Plural of medium. The various physical environments through which transmission signals pass. Common network media include twisted-pair, coaxial and fiber-optic cable, and the atmosphere (through which microwave, laser, and infrared transmission occurs). Sometimes called physical media.

Topology - Physical arrangement of network nodes and media within an enterprise networking structure.

Cable Plant types:

Electrical (copper)
- coaxial
- twisted pair
Fiber-optic
- multimode
- single-mode
Wiring configurations
- Star / Extended Star
- Distance limitations

Star Topology using CAT5 (UTP):

Specified by EIA/TIA 568 standard
The MDF is the center of the star
The MDF is the Point of Presence (POP) for outside services from the WAN provider

EIA/TIA-568 - Standard that describes the characteristics and applications for various grades of UTP cabling.

Category 5 (CAT5) cabling - One of five grades of UTP cabling described in the EIA/TIA-568B standard. Category 5 cabling is used for running CDDI and can transmit data at speeds up to 100 Mbps.

In a simple star with only one wiring closet, the main distribution facility (MDF) will include one or more horizontal cross connect (HCC) patch panels. HCC patch cables will be used to connect the Layer 1 'horizontal cabling' with the Layer 2 LAN switch ports. The uplink port of the LAN switch will be connected to the Ethernet port of the Layer 3 router using 'patch cable' also. At this point the end host will have a complete physical connection to the router port. The quantity of horizontal cable run and the size (number of ports) of the HCC patch panels will be determined by the user's requirements.

Design Hint: Review the user’s requirements to determine what the user expects for the number of horizontal cable runs to each room that the MDF or IDF will be servicing in its catchment area.

Extended Star Topology:

Specified by EIA/TIA standards
Creates multiple catchment areas interconnected with vertical cabling
All vertical cabling is connected to the MDF to create a single LAN segment

Vertical cabling - (Backbone cabling) Cabling that provides interconnections between wiring closets, wiring closets and the POP, and between buildings that are part of the same LAN.

Catchment areas - Zone that falls within area that can be served by an internetworking device such as a hub.

MDF - Main Distribution Facility. Primary communications room for a building. Central point of a star networking topology where patch panels, hub, and router are located.

IDF - Intermediate Distribution Facility. Secondary communications room for a building using a star networking topology. The IDF is dependent on the MDF.

Layer 1 Documentation (Logical Diagram)

Logical diagram is a snapshot view of over all LAN implementation
Useful in troubleshooting problems and implementing expansion in the future
Elements of the Logical Diagram Include:
- Exact locations of MDF and IDF's wiring closets
- The type and quantity of cabling used to interconnect the IDFs with the MDF, along with how many spare cables are available for increasing the bandwidth between the wiring closets.
- Detailed documentation of all cable runs, what the identification number is and which port on the HCC or VCC that run is terminated on. This is called a 'cut sheet'.

Layer 2 - LAN Switching

The purpose of Layer 2 (Data Link Layer) devices in the network are to provide 'flow control', 'error detection & correction', and 'reduce congestion' in the network. The two most common Layer 2 devices (other than the NIC, which every host on the network has to have) are Bridges and LAN switches.

LAN switchs (Layer 2 Switch) can allocate bandwidth on a per port basis thus allowing more bandwidth to vertical cabling, uplinks and servers.

Note: The theoretical maximum bandwidth for a Layer 2 Switch is:

               Theoretical     No-of-ports * Bandwidth
               Maximum      =  -----------------------
               Bandwidth                 2

Asymmetric Switching -

Provides switching between unlike bandwidths (10/100 Mbps)
Requires the switch to use memory buffering

Use Switches to reduce congestion:

Avoid congestion on a LAN by using microsegmentation to eliminate collision domains
Cascade switches and hubs
Tailor availability to the needs of the device

Microsegmentation - Division of a network into smaller segments, usually with the intention of increasing aggregate bandwidth to network devices.

By installing LAN switching at the MDF and IDFs we can start to look at the size of our collision domains and the speed for each horizontal cable and vertical cable run. Since the vertical cable will be carrying all of the data traffic between the MDF and the IDFs, the capacity of this run must be larger.

Layer 2 Switch Collision Domains:

In a pure switch LAN environment, the size of the collision domain is between two host.
When using hubs, the size of the collision domain increases and bandwidth is shared.

A shared LAN hub is a multiport repeater and therefore is a collision domain. All hosts connected to the shared LAN hub share the same collision domain and the bandwidth.

Layer 2 Switch with Hubs:

use hubs to supply more connection points for hosts

Layer 2 Migrating to higher bandwidth:

migration to higher bandwidth is as simple as patch to higher speed port or adding additional high speed ports
double the capacity in the vertical cabling by bringing up another link

Layer 3 - Routing

Implementation of Layer 3 (Network Layer) devices such as routers allow for segmentation of the LAN into unique networks both physical and logical. Routers also allow for connectivity to wide area networks (WANS) such as the Internet.

Routers serve as broadcast firewalls
Routers consolidate for scalable internetworks
Network protocol addressing and routing provides built-in scaling

Use Routers to Impose Logical Structure

One router divides subnets and networks
One router structures an internetwork
Routers serve as broadcast firewalls

Routers permit greater scalability because they serve as firewalls for broadcasts (broadcast domains). With bridges and switches, all unknown addresses must be flooded out every port. The router also is the central point in the LAN for traffic destination of the WAN port.

File Servers and Traffic Patterns

One of the keys to designing a successful network is to understand the data traffic network. The Applications (servers) can be categorized into two distinct classes - (1) 'enterprise servers' and (2) 'workgroup servers'. Within the MDF and IDFs, the Layer 2 LAN switches must have high speed (100MBps) ports allocated for these servers.

Enterprise network - Large and diverse network connecting most major points in a company or other organization. Differs from a WAN in that it is privately owned and maintained. Enterprise servers - support all of the users on the network such as E-mail or DNS. Work Group servers - support a specific set of users.

Since everyone on the network needs access to the enterprise servers, it should be connected to the MDF. This way traffic to these type of services only have to travel to the MDF and will not be transmitted across other networks.

Workgroup - Collection of workstations and servers on a LAN that are designed to communicate and exchange data with one another.

The Workgroup servers should be placed in the IDF closest to the users accessing these applications. By doing this, traffic to these servers will only have to travel network infrastructure to that IDF and will not affect other users on that network segment.

Documenting Your Network

What to Document:

Physical Network Maps (cutsheet)
Logical Network Map (IP addressing scheme)
Addressing Maps (snapshot view of network)

A standard should be set in the addressing of important hosts on the network. This addressing scheme should be kept consistent throughout the entire network.

Reasons to Document:

Dramatically decreases problem resolution time
Networks with the most problems, have the least documentation
Networks with good documentation drastically reduce problem load
The more documentation the better

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