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8/12/2019 06 - Building Ethernet LANs With Switches
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1 www.asghars.blogspot.com
ICND I - 01 Ethernet LANs & Switches
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By Muhammad Asghar Khan
06 - Building Ethernet LANs with
Switches
Reference: Cisco CCENT/CCNA ICND1 100-101 Official Cert Guide By WENDELL ODOM
Cisco Press
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Agenda
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Progression from Hub, Bridges & Switches
Switching Logic
Processing on Cisco Switches
LAN Design Consideration
Choice of using Hub, Switch or Router
Virtual LANs (VLAN)
Choosing Ethernet Technology for a Campus LAN
Ethernet LAN Media & Cable Lengths
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Progression from Hub, to Bridge,to Switch
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10Base2 & 10Base5
Ethernet started with standard that used physical buscreated with coaxial cabling (10Base2 & 10Base5)
These standards were vulnerable to single point of failure
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Progression from Hub, to Bridge,to Switch
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10BaseT
Next came the 10BaseT Ethernet, it allowed for the use ofUTP cabling, and a shared hub, removed single point offailure
But still a single device can send at a time as:A collision still occur
A broadcast is heard by all devices
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Progression from Hub, to Bridge,to Switch
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In 10BaseT network starts suffering from collision, to
reduce it bridges were added, bridge create a separatecollision domain, thus doubled the bandwidth of the10BaseT network
10BASE-T Network beforeAdding a Bridge
10BASE-T Network Segmented
Using a Bridge
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Progression from Hub, to Bridge,to Switch
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LAN switches perform the same basic functions of bridges
Like bridges, switches segment a LAN into separate parts,with separate collision domain
Switches have potentially large numbers of interfaces,with optimized hardware
Each interface creates a separate collision domain, thusswitch multiply the available bandwidth
One collision domain per interface is called micro-segmentation
Figure shows, all interfaces are running at 100 Mbps, withfour collision domains. Note each interface also uses fullduplex
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Progression from Hub, to Bridge,to Switch
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Switching Logic
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Switching logic is based on the source & destination MAC
address in each frames Ethernet address IEEE defines three categories of Ethernet MAC addresses:
Unicast address; identify a single LAN interface
Broadcast address (FFFF:FFFF:FFFF); implies all devices on theLAN should receive the frame
Multicast address; all dynamic subset of devices on a LAN tocommunicate
Switch uses the dynamic MAC address table that lists MACaddresses & outgoing interfaces
MAC address table is also called switching table or bridgingtable or even Content Addressable Memory (CAM)
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Switching Logic
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The three main actions performed by the switch are:
Step 1:To decide when to forward a frame or when tofilter a frame, based on destination MAC address
a) If the destination address is a broadcast, multicast, orunknown destination unicast (not listed in the MAC table),
the switch floods the frameb) If the destination address is a known unicast address
(found in the MAC table):
i. If the outgoing interface listed in the MAC address table is
different from the interface in which the frame was received,the switch forwards the frame out the outgoing interface.
In LANs with multiple switches, each switch makes anindependent forwarding decision based on its own MACaddress table
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Switching Logic
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Switching Logic
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ii. If the outgoing interface is the same as the interface in which
the frame was received, the switch filters the frame, meaningthat the switch simply ignores the frame and does notforward it
Step 2:To learn MAC addresses by examining the source
MAC address of each framea) For each received frame, examine the source MAC
address and note the interface from which the framewas received
b) If they are not already in the table, add the addressand interface, setting the inactivity timer to 0
c) If it is already in the table, reset the inactivity timer forthe entry to 0
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Switching Logic
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Processing on Cisco Switches
Almost all recently released switches use store-and-forwardprocessing
With store-and-forward, the switch receive the entire framebefore forwarding the first bit of the frame
Cisco switch can use a couple of different types of internalprocessing variations
Because the destination MAC address occurs very early in theEthernet header, a switch can make a forwarding decision long
before the switch has received all the bits in the framesBased on this logic Cisco offers two other internal processing
methods for switches:
Cut-Through
Fragment-Free
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Switching Logic
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Cut-Through
With cut-through processing, the switch starts sending the frameout the output port as soon as possible
This reduce latency, but also propagates frames with errors as FCSis in the Ethernet trailer
Fragment-FreeWorks like cut-through logic, but it wait to receive the first 64
bytes before forwarding a frame
First 64 bytes are considered due to the fact that CSMA/CD logic
detects a collision in the first 64 bytes of a frame This has less latency then with store-and-forward logic and slightly
more latency than with cut-through
Also frames with errors as a result of collisions are not forwarded
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Switching Logic
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With high speed links and faster application-specific
integrated circuits (ASIC), todays switches typically usestore-and-forward processing, bcz the improved latencyof the cut-through and fragment-free is negligible at thesespeeds
Table below summarizes the switch internal processing
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LAN Design Consideration
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Here we take a broader look at how to design medium to
large LANWhen building a medium to large LAN, you have more
product choices to make:
Such as when to use hubs, switches, and routersWhich LAN switch to choose (switches vary in size, number of
ports, performance, features, and price)
Decide to choice the UTP cabling or fiber optic cabling option
Choice of using Hub, Switch or Router
The terms collision domain and broadcast domain definetwo important effects of the process of segmenting LANsusing various devices
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LAN Design Consideration
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The goal is to define how hubs, switches, and routers
impact collision domains and broadcast domainsCollision Domains
A collision domain is the set of LAN interfaces whose framescould collide with each other, but not with frames sent byany other devices in the network
Figure on next slide illustrtes collision domains
Each separate segment, or collision domain, is shown with a
dashed-line circle in the figureThe switch on the right separates the LAN into different
collision domains for each port
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LAN Design Consideration
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Both bridges and routers also separate LANs into different
collision domainsHub near the center of the network does not create multiple
collision domains for each interface
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LAN Design Consideration
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Broadcast Domains
A broadcast domain encompasses a set of devices forwhich, when one of the devices sends a broadcast, all theother devices receive a copy of the broadcast
Figure depicts the broadcast domains
Router does not forward a LAN broadcast sent by a PC onthe left to the network segment on the right
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LAN Design Consideration
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Thats is why routers are sometime called broadcast
firewallSwitches create a single broadcast domain, as switches flood
broadcasts and multicasts on all ports
Use switch instead of hubs to create different collision
domains and there by enabling the full duplexcommunication
Broadcasts happen, as all hosts need to send somebroadcast to function properly (e.g. IP ARP messages),but broadcast do require all the hosts to spend timeprocessing each broadcast frame
But if 500 PCs connected to switches, the broadcast couldstart to impact the performance of the end-user PCs
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LAN Design Consideration
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However, a design that separated the 500 PCs into five
groups of 100, separated from each other by router,would create five broadcast domains, and thus improvesperformance
Smaller broadcast domains also improve security due to
robust security features in routers
Table lists the benefits of segmenting Ethernet devicesusing hubs, switches and routers
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LAN Design Consideration
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Virtual LANs (VLAN)
A LAN consists of all devices in the same broadcast domain
With VLANs configuration, a switch can put some interfacesinto one broadcast domain and some into another
These individual broadcast domains created by the switch arecalled virtual LANs
Figure shows
sample network
with two broadcast
domains, two switches &
no VLANs
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LAN Design Consideration
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Alternately, you can create multiple broadcast domains
using a single switchFigure shows the same two broadcast domains as in
previous figure, now implemented as two different VLANson single switch
You may also need to use VLAN:To group users by department
To reduce workload for STP
To enforce security, by limiting
sensitive data users to separate
VLAN
To separate traffic from IP phone from traffic sent by PC
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LAN Design Consideration
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Choosing Ethernet Technology for a Campus LAN
The term campus LAN refers to the LAN created tosupport larger buildings, or multiple buildings insomewhat close proximity to one another
When planning and designing a campus LAN, theengineers must consider the types of Ethernet availableand the cabling lengths supported by each type
Also the engineer must consider the type of equipment
that is already installed and whether an increase in speedon some segments is worth the cost of buying newequipment
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LAN Design Consideration
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Cisco-oriented LAN designs use some common
terminology to refer to the designFigure shows a typical design of a large campus LAN,
with the terminology included in the figure
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LAN Design Consideration
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Cisco uses three terms to describe the role of each switch
in a campus design:Access
Distribution , and
Core
Access SwitchesAccess switches connect directly to end users, providing
access to the LAN
Access switches should not be expected to forward traffic
between two other switchesAccess switches tend to be smaller and less expensive
Each of the access switches must use at least two uplinks totwo different distribution switches for redundancy
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LAN Design Consideration
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Distribution Switches
Distribution switches provides an aggregation point foraccess switches, forwarding frames between switches, butnot connecting directly to end-user devices
It provides some cabling advantages and potential
performance advantages, e.g. for 30 access layer switches tobe cabled directly, the LAN would need 435 cables. Instead,by connecting each 30 access switches to two distributionswitches, requires 60 cables
Switches with faster forwarding rates and with two uplinksfrom each access switch to the distribution switch has moreavailability
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LAN Design Consideration
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Core Switches
Core switches aggregates distribution switches in very largecampus LANs
Core switches provide extremely high forwarding ratesthese days into the hundreds of millions of frames per
second
Medium to smaller campus LANs often forego the conceptof core switches
Ethernet LAN Media & Cable LengthsAn engineer must consider the length of each cable run
and then find the best type of Ethernet and cabling typethat supports that length of cable
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LAN Design Consideration
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The three most common types of Ethernet today (10BASE-T,
100BASE-TX, and 1000BASE-T) have the same 100-meter cablerestriction, but they use lightly different cables
Several types of Ethernet define the use of fiber-optic cables
Optical cables support a variety of much longer distances than
the 100 meters supported by Ethernet on UTP cables
Switches can use lasers to generate the light, as well as light-emitting diodes (LED)
The maximum distances of optical cable (single-mode ormultimode) is dependent up on the use of laser switches(often with single-mode fiber)or LED switches (often withmultimode fiber)
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LAN Design Consideration
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Table lists the more common types of Ethernet and their
cable types and length limitationsMost engineers simply remember the general distance
limitations and then use a reference chart
(such as the table ) to remember each specific detail
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