lan design

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Switching Basics and Intermediate Routing CCNA 3

Chapter 5

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LAN DesignIntroduction

• LAN design has become more difficult – Due to multiple media types and LANs– Complexity has increased

• Three aspects of a network that need to be identified before designing a large LAN:– An access layer that connects end users to a LAN– A distribution layer that provides policy-based

connectivity between end-user LANs– A core layer that provides the fastest connection

between distribution points

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LAN DesignLAN Design Goals

• Requirements of most networks designs:– Functionality: the network must work as

intended– Scalability: the network must be expandable– Adaptability: the network must be designed

with a vision toward future technologies– Manageability: the design must facilitate

network monitoring and management to ensure stability

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LAN DesignLAN Design Considerations

• A virtual LAN (VLAN) is a group of devices on one or more LANs that communicate as if they were attached to the same wire

• To maximize bandwidth and performance, address these LAN design considerations:– Function and placement of servers– Collision domain issues– Segmentation issues– Broadcast domain issues

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• Servers are usually dedicated to one function such as email or file sharing– Servers can be one of two types:

• Enterprise servers support all users on the network– e-mail– Domain Name System (DNS), the Internet-wide system

of mapping names to IP addresses

• Workgroup servers support a specific set of users and offers services such as word processing and file sharing

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• Enterprise servers are usually placed in the main distribution facility (MDF)– Traffic to enterprise servers should travel only to the

MDF and not across other networks– Workgroup servers should be placed in the

intermediate distribution facilities (IDFs) closest to the users who access the applications on these servers

• Layer 2 switches in the MDF and IDF should have 1000Mbps (1Gbps) allocated bandwidth

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Servers Are Typically

Placed at a Point of

Convergence in the

Network, Such as

Within an IDF or MDF

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• Ethernet nodes use carrier sense multiple access with collision detection (CSMA/CD)– Each node must contend with all other nodes

for access to the shared medium, or collision domain

• If two nodes transmit at the same time, a collision occurs

• The transmitted frames are destroyed and a jam signal is sent to all nodes on the segment

– Excessive collisions reduce bandwidth

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Collisions Increase Multiplicatively with the Number of Hosts

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• Microsegmentation is when a single collision domain is split into smaller collision domains– Reduces number of collisions on a LAN segment– A broadcast occurs when the destination MAC address

is FF:FF:FF:FF:FF:FFSingle Broadcast Domain

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LAN DesignLAN Design Methodology

• LAN design should be done in a set of systematic steps:– Step 1: Gather the requirements and expectations

• Users• Corporate structure• Skill level of people• User attitudes towards computes and applications• Documented policies of the organization• Business information flow• Data that is mission critical• Protocols allowed on the network

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– Step 1: Gather the requirements and expectations (continued)

• Performance characteristics of current network• Types of desktops supported• Persons responsible for LAN addressing, naming,

topology design, and configuration• Current topology• Human, hardware, and software resources• How resources are linked and shared• Financial resources of organization

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• Documentation of requirements allows for an informed estimate of costs and timelines for implementation

• Availability measures the usefulness of the network– Factors affecting availability:

• Throughput• Response time• Access to resources

– Customers may have different definitions of availability• As a network designer, goal is greatest availability at least cost

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• Step 2: Analyze the requirements of the network and its users– Needs of users change– Need for bandwidth increases

• Voice and video applications

– The network must reliably provide prompt and accurate information

– Information requirements of the users and organization must be met

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• Step 3: Decide on the overall LAN topology that will satisfy user requirements– Star– Extended star (most common)The Star Topology is a Special Case of the Extended Star Topology

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• Step 3: Decide on the overall LAN topology that will satisfy user requirements (continued)– LAN topology design has three unique OSI

model categories:• Network layer (Layer 3)• Data link layer (Layer 2)• Physical layer (Layer 1)

– By looking at the OSI layer, the design engineer can properly incorporate products and technologies

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• Step 4: Document the physical and logical topology of the network– Physical topology: the way the network

components are connected– Logical topology: the flow of data in the

network, and the name and addressing schemes used

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Logical Design Includes Name and Address Schemes

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• Important elements of LAN design documentation:– OSI layer topology map– LAN logical map– LAN physical map– Cut sheets, which show cable runs– VLAN logical map– Layer 3 logical map– Address maps

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Cut Sheet for IDF Location – Room XXX

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VLAN Logical Design

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IP Networks Are Displayed in a Layer 3 Logical Map

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Address Maps Provide a Detailed View of IP Addresses for Key Devices and Interfaces

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LAN DesignLayer 1 Design

• One of the most important design considerations is the cables– Most LAN cabling is based of FastEthernet or

Gigabit Ethernet technology• Both can utilize full duplex technology, giving

concurrent, collision-free, two-way communication• A logical bus topology that uses CSMA/CD can also

be used with standard Ethernet

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• Design issues at Layer 1:– Type of cabling (copper or fiber optic)

• 100BASE-TX specifies Cat5e unshielded twisted-pair (UTP), limited to 100m per segment

• 100BAS-FX specifies multimode fiber with a length limit of 2 km

– TIA/EIA-568-A standard details layout and wiring connection schemes

• Media types: Cat5, Cat5e, Cat6 UTP and shielded twisted-pair (STP) that has shielding around wire pairs and another shield around all the wires in the cable, single-mode fiber, multi-mode fiber

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• Design issues at Layer 1 (continued):– Carefully evaluate strengths and weaknesses of

topologies• Layer 1 issues cause most network problems

– Use fiber-optic cable in the backbone and risers of a network

– Use Cat5e or Cat6 in horizontal runs– Every device should be connected to a central location

with a horizontal cabling run

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• In a simple star topology with only one wiring closet, the MDF includes one or more horizontal cross-connect (HCC) patch panels– HCC patch panels connect Layer 1 horizontal

cabling with Layer 2 switch ports– The uplink port on the LAN switch is connected

to the Ethernet port on the Layer 3 router with a patch cable

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HCC Connects Layer 1 Cabling to Layer 2 Switch Ports

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• When hosts in larger networks exceed the 100m distance limitation for Cat5e UTP, more than one wiring closet is required

• Multiple wiring closets means you have multiple catchment areas

• Secondary wiring closets are referred to as intermediate distribution facilities (IDFs)

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IDFs Connect via the MDF

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• Vertical cabling is also called backbone cabling

• A vertical cross-connect interconnects IDFs to the central MDF– Fiber-optic cable is normally used for the VCC

because cable lengths are longer than the100m limit for Cat5e cable

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VCC Interconnects IDFs to the MDF

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• The logical diagram is the basic road map of the LAN and includes these elements:– Location and identification of MDF and IDF wiring

closets– Type and quantity of cables used to interconnect the

IDFs with the MDF– Number of spare cables that are available to increase

bandwidth between wiring closets– Detailed documentation of all cable runs, identification

numbers, and port on which the run is terminated at the HCC or VCC

– Essential for troubleshooting network problems

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• Purpose of Layer 2 devices is to switch frames based on destination MAC address– Collisions and collision domain size negatively

affect network performance– Devices at Layer 2 (and Layer 3) determine

the size of collision domains– Microsegmentation reduces the size of

collision domains and is implemented through the use of switches

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LAN Switches Provide Microsegmentation

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• LAN switches allocate bandwidth on a per-port basis– This supplies more bandwidth to vertical

cabling, uplinks, and servers– Referred to as asymmetric switching

(provides switch connections between ports of unlike bandwidth)

– Symmetric switching provides switched connections between ports of similar bandwidth

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• Desired capacity of vertical cable runs is greater than that of a horizontal cable run– 100 Mbps is adequate on a horizontal drop– Asymmetric LAN switches allow 100 Mbps and 1

Gbps on a single switch• Next task in the design process is to determine number

of 100 Mbps and 1 Gbps ports needed in the MDF and every IDF– The number of hosts connected to a single port on a

switch determines the size of the collision domain, affects bandwidth available to each host

– Collision domains can be eliminated by using one host per switch port

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• A router is a Layer 3 device– Creates unique LAN segments– Allows communication between segments based on

Layer 3 addresses, such as IP addresses– Allows segmentation of LAN into unique physical and

logical networks– Allows for connectivity to WANs, such as the Internet– Forwards data packets based on destination

addresses– Does not forward broadcasts– Is the entry and exit point for a broadcast domain

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• When to use a router:– If the problem is a protocol issue instead of a

contention issue– If there are excessive broadcasts on the LAN– If a higher level of security is needed

• However, Layer 3 switches can now perform many of these functions at nearly the same cost– Expect Layer 3 switching to become pervasive in 10

years– Layer 3 switches perform wire-speed routing, QoS,

and security functions

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Logical Addressing Mapped to the Physical Network

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Logical Network Addressing Map

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Physical Network Maps Ease

Troubleshooting

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• VLAN implementation combines Layer 2 switching and Layer 3 routing technologies– Limits collision and broadcast domains– Provides security with creation of VLAN

groups that communicate only through a router

– Ports on a switch are assigned to different VLANs

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VLANs Are Essentially Switch Port Groupings

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LAN SwitchesIntroduction

• Early LAN switches did not support VLANs• Second-generation switches supported

VLANs, but relied on routers for inter-VLAN communication

• Third generation switches have the route processors built into the switches– With the exception of access layer switches,

switches are becoming almost indistinguishable from routers

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LAN SwitchesSwitched LANs and the Hierarchical Design Model

• Use of a hierarchical design model makes it more likely to meet the needs of a medium or large organization

• Layers of the hierarchical model:– Access layer: gives users access to the network– Distribution layer: provides policy-based connectivity– Core layer: provides optimum transport between sites;

often referred to as the backbone

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LAN SwitchesAccess Layer Overview

• Entry point to network for user workstations and servers

• Functions include MAC layer filtering and microsegmentation

• Layer 2 switches are used

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LAN SwitchesAccess Layer Overview

The Access Layer is an Entry Point to the Network, Particularly for End Users

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LAN SwitchesAccess Layer Switches

• Access layer switches generally operate at Layer 2 of the OSI model– Provide services such as VLAN

membership

– Main purpose is to connect end users

– Should do this with low cost and high port density

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• Common legacy access layer switches used today– Catalyst 1900 series– Catalyst 2820 series– Catalyst 2950 series (not a legacy switch)– Catalyst 4000 series– Catalyst 5000 series

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Features of Access Layer Switches

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• Catalyst 1900 and 2820 switches were effective for small campus networks

• The 2950 series provides access for users and servers that require higher bandwidth– Use FastEthernet and Gigabit Ethernet ports

• The 4000 and 5000 series include Gigabit Ethernet ports– Effective in large campus networks

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The Catalyst 4500 Series Is Often Used at the Access Layer in an Enterprise Network

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LAN SwitchesDistribution Layer Overview

• The distribution layer is between the access and core layers– Provides a boundary definition where packet

manipulation can take place– Segments layers into broadcast domains– Can apply policies and access control lists to filter

packets– Prevents problems at the access layer from affecting

the core layer– Switches in this layer operate at Layer 2 and Layer 3

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• Distribution layer functions:– Aggregation of the wiring closet connections– Broadcast/multicast domain definition– VLAN routing– Any media transitions that need to occur– Security

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The Distribution Layer Is Typically Where Policy Is Incorporated

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LAN SwitchesDistribution Layer Switches

• Distribution layer switches are the aggregation points for multiple access layer switches– Must be able to accommodate the traffic from access

layer devices– Must have high performance– Use Layer 2 and Layer 3 switching to delineate

broadcast domains (multilayer switches that combine switching and routing functions)

– Combine VLAN traffic– Focal points for decisions about traffic flow

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• Distribution layer often used legacy Cisco switches:– Catalyst 2926G– Catalyst 5000 series– Catalyst 6000 series

• Current distribution layer switches:– Catalyst 4500– Catalyst 4900– Catalyst 6500

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The Catalyst 6513 Is About as Good as it

Gets for a Distribution

Layer Switch, Circa 2006

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LAN SwitchesCore Layer Overview

• The core layer is a high-speed switching backbone– If the core switch does not have a routing

module, an external router is used for the Layer 3 function

– This layer should not perform packet manipulation, such as access list filtering

– Should have redundant paths– Current trend is to use wire-speed Layer 3

switching at the core

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LAN SwitchesCore Layer Overview

Core Layer is Devoted to High-Speed Switching of Numerous Packets

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LAN SwitchesCore Layer Switches

• Core Layer is the backbone of the campus switched network– Switches in this layer can use various layer 2

technologies• Ethernet• ATM cell switching

– The core layer can be a routed (Layer 3) core– Factors such as cost, need, and performance

must be considered when choosing equipment

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LAN SwitchesCore Layer Switches

• Core Layer legacy switches:– Catalyst 8500 series– IGX 8400 series– Lightstream 1010

• Current core layer switches:– Catalyst 6500 series– Cisco 12000 series routers– (occasionally) Cisco CRS-1 (Carrier Routing

System) devices

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Summary

• LAN design typically focuses on:– Functionality– Scalability– Manageability– Adaptability

• Important elements of LAN design documentation:– OSI topology map– LAN logical map– LAN physical map

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Summary

• Important elements of LAN design documentation (continued):– Cut sheets– VLAN logical map– Layer 3 logical map– Address maps

• Layer 1 design issues include:– Types of cables– Overall structure of the cabling– Media types such as Cat5, Cat5e, Cat6, single-mode

fiber, and multi-mode fiber

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Summary

• The logical diagram of a LAN includes– MDF and IDF locations– Type and quantity of cables used to connect IDFs to

MDF– Number of spare cables available to increase

bandwidth between MDF and IDFs

• Layer 2 devices provide flow control, error detection and error correction, and reduce congestion in a network

• Microsegmentation of the network reduces the size of collision domains and reduces collisions

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Summary

• Characteristics of routers:– Layer 3 devices– Create unique LAN segments– Allow communication between segments

based on Layer 3 addresses (IP, usually) – Allow for segmentation of LAN into physical

and logical networks– Allow for connectivity to WANs such as the

Internet

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Summary

• VLAN implementation combines Layer 2 switching and Layer 3 routing technologies– Limit collision domains and broadcast

domains– Originally used to create logical groupings

according to function– Now more often used to group IP subnets– Routers or route processors enable

communication between VLANs

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Summary

• The hierarchical design model includes three layers:– Access layer gives access to users in workgroups– Distribution layer provides policy-based connectivity– Core layer provides optimal transport between sites

• Access layer switches operate at Layer 2 of the OSI model – Offer services such as VLAN membership– Main purpose is to provide connectivity for end users– Should have low cost and high port density

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Summary

• Distribution layer switches delineate broadcast domains– Combines VLAN traffic– Focal point of decisions about traffic flow– Operate at both Layer 2 and Layer 3 of the OSI model

(called multilayer switches)

• Core layer switches provide a high-speed switching backbone– Should not be used for packet manipulation (no

ACLs)– Should have redundant paths

lan design

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