Switching Basics and Intermediate
Routing CCNA 3
Chapter 5
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LAN Design
Introduction
• 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 Design
LAN 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 Design
LAN 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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LAN Design
LAN Design Considerations
• 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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LAN Design
LAN Design Considerations
• 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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LAN Design
LAN Design Considerations
Servers Are
Typically
Placed at a
Point of
Convergence
in the
Network,
Such as
Within an IDF
or MDF
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LAN Design
LAN Design Considerations
• 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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LAN Design
LAN Design Considerations
Collisions Increase Multiplicatively with the
Number of Hosts
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LAN Design
LAN Design Considerations
• 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:FF
Single Broadcast Domain
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LAN Design
LAN 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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LAN Design
LAN Design Methodology
– 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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LAN Design
LAN Design Methodology
• 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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LAN Design
LAN Design Methodology
• 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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LAN Design
LAN Design Methodology
• 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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LAN Design
LAN Design Methodology
• 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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LAN Design
LAN Design Methodology
• 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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LAN Design
LAN Design Methodology
Logical Design Includes Name and Address Schemes
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LAN Design
LAN Design Methodology
• 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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LAN Design
LAN Design Methodology
Cut Sheet for IDF Location – Room XXX
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LAN Design
LAN Design Methodology
VLAN Logical Design
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LAN Design
LAN Design Methodology
IP Networks Are Displayed in a Layer 3 Logical Map
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LAN Design
LAN Design Methodology
Address Maps Provide a Detailed View of IP
Addresses for Key Devices and Interfaces
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LAN Design
Layer 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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LAN Design
Layer 1 Design
• 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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LAN Design
Layer 1 Design
• 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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LAN Design
Layer 1 Design
• 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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LAN Design
Layer 1 Design
HCC Connects Layer 1 Cabling to Layer 2
Switch Ports
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LAN Design
Layer 1 Design
• 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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LAN Design
Layer 1 Design
IDFs
Connect
via the
MDF
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LAN Design
Layer 1 Design
• 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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LAN Design
Layer 1 Design
VCC Interconnects IDFs to the MDF
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LAN Design
Layer 1 Design
• 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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LAN Design
Layer 2 Design
• 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 Design
Layer 2 Design
LAN Switches Provide Microsegmentation
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LAN Design
Layer 2 Design
• LAN switches allocate bandwidth on a perport 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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LAN Design
Layer 2 Design
• 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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LAN Design
Layer 3 Design
• 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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LAN Design
Layer 3 Design
• 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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LAN Design
Layer 3 Design
Logical Addressing Mapped to the Physical
Network
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LAN Design
Layer 3 Design
Logical Network Addressing Map
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LAN Design
Layer 3 Design
Physical Network
Maps Ease
Troubleshooting
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LAN Design
Layer 3 Design
• 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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LAN Design
Layer 3 Design
VLANs Are Essentially Switch Port Groupings
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LAN Switches
Introduction
• Early LAN switches did not support VLANs
• Second-generation switches supported
VLANs, but relied on routers for interVLAN 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 Switches
Switched 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 Switches
Access 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 Switches
Access Layer Overview
The Access Layer is an Entry Point to the Network,
Particularly for End Users
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LAN Switches
Access 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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LAN Switches
Access Layer Switches
• 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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LAN Switches
Access Layer Switches
Features of Access Layer Switches
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LAN Switches
Access Layer Switches
• 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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LAN Switches
Access Layer Switches
The Catalyst 4500 Series Is Often Used at
the Access Layer in an Enterprise Network
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LAN Switches
Distribution 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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LAN Switches
Distribution Layer Overview
• 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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LAN Switches
Distribution Layer Overview
The Distribution Layer Is Typically Where Policy Is Incorporated
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LAN Switches
Distribution 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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LAN Switches
Distribution Layer Switches
• 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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LAN Switches
Distribution Layer Switches
The Catalyst
6513 Is About
as Good as it
Gets for a
Distribution
Layer Switch,
Circa 2006
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LAN Switches
Core 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 Switches
Core Layer Overview
Core Layer is Devoted to High-Speed Switching of
Numerous Packets
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LAN Switches
Core 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 Switches
Core 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
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