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06 Ethernet LAN Switching Concepts
By Muhammad Asghar Khan
Reference: CCENT/CCNA ICND1 Official Exam Certification Guide By Wendell Odom
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Agenda
Progression from Hub, Bridges & Switches Switching Logic
Processing on Cisco Switches
LAN Design Consideration Choice of using Hub, Switch or Router
Virtual LANs (VLAN)
Campus LAN Design Terminology
Ethernet LAN Media & Cable Lengths
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Progression from Hub, to Bridge, to Switch
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10Base 2 & 10Base 5 Ethernet started with standard that used physical bus
created with coaxial cabling (10Base 2 & 10Base 5)
These standards were vulnerable to single point of
failure
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Progression from Hub, to Bridge, to Switch
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10Base T Next came the 10Base-T Ethernet, it allowed for the
use of UTP cabling, and a shared hub, removed single
point of failure
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 10Base T network starts suffering from collision, toreduce it bridges were added, bridge create a
separate collision domain, thus doubled the
bandwidth of the 10BaseT 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, thus
switch multiply the available bandwidth
One collision domain per interface is called micro-
segmentation Figure on next slide shows, all interfaces are running at
100 Mbps, with four collision domains. Note each
interface also uses full duplex
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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 & destinationMAC address in each frames Ethernet address
IEEE defines three categories of Ethernet MACaddresses:
Unicast address; identify a single LAN interface
Broadcast address (FFFF:FFFF:FFFF); implies all devices onthe LAN should receive the frame
Multicast address; all dynamic subset of devices on a LANto communicate
Switch uses the dynamic built table that lists MACaddresses & outgoing interfaces
MAC address table is also called switching table orbridging table 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 frame
b) 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 isdifferent from the interface in which the frame was received,the switch forwards the frame out the outgoing interface
ii. If the outgoing interface is the same as the interface in whichthe frame was received, the switch filters the frame, meaningthat the switch simply ignores the frame and does notforward it
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Switching Logic
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Figures below depicts the point (b)of previous slide
Switch Forwarding
Decision
Switch Filtering
Decision
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Switching Logic
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Step 2: To learn MAC addresses by examining thesource MAC address of each frame
a) For each received frame, examine the source MACaddress and note the interface from which theframe was 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 timerfor the entry to 0
Step 3: Switches use Spanning Tree Protocol (STP) toprevent loops by causing some interfaces to block,meaning that they do not send or receive frames
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Switching Logic
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Processing on Cisco Switches
Almost all recently released switches use store-and-forward processing
Cisco switch can use a couple of different types of internalprocessing variations
Because the destination MAC address occurs very early inthe Ethernet header, a switch can make a forwardingdecision long before the switch has received all the bits inthe frames
Based on this logic Cisco offers two other internalprocessing 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 theframe out the output port as soon as possible
This reduce latency, but also propagates frames with errorsas FCS is in the Ethernet trailer
Fragment-Free Works 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/CDlogic detects a collision in the first 64 bytes of a frame
This has less latency then with store-and-forward logic andslightly more latency than with cut-through
Also frames with errors as a result of collisions are notforwarded
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Switching Logic
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With high speed links and faster application-specificintegrated circuits (ASIC), todays switches typically
use store-and-forward processing, bcz the improved
latency of the cut-through and fragment-free is
negligible at these speeds 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 designmedium to large LAN
When building a medium to large LAN, you have
more product choices to make:
Such as when to use hubs, switches, and routers
Which LAN switch to choose (switches vary in size,
number of ports, performance, features, and price)
Decide to choice the UTP cabling or fiber optic cablingoption
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LAN Design Consideration
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Choice of using Hub, Switch or Router The terms collision domain and broadcast domain
define two important effects of the process of
segmenting LANs using various devices
The goal is to define how hubs, switches, and routersimpact collision domains and broadcast domains
Collision Domains
A collision domain is the set of LAN interfaces whose
frames could collide with each other, but not with
frames sent by any other devices in the network
Figure on next slide illustrtes collision domains
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LAN Design Consideration
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Each separate segment, or collision domain, is shown with a
dashed-line circle in the figure
The switch on the right separates the LAN into different
collision domains for each port
Both bridges and routers also separate LANs into different
collision domains Hub 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 for
which, when one of the devices sends a broadcast, all
the other devices receive a copy of the broadcast
Figure depicts the broadcast domains
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LAN Design Consideration
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Router does not forward a LAN broadcast sent by a PC
on the left to the network segment on the right
Thats is why routers are sometime called broadcast
firewall
Switches create a single broadcast domain, as switchesflood broadcasts and multicasts on all ports
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LAN Design Consideration
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Use switch instead of hubs , to create differentcollision domains and there by enabling the full
duplex communication
Broadcasts happen, as all hosts need to send some
broadcast to function properly (e.g. IP ARPmessages), but broadcast do require all the hosts to
spend time processing each broadcast frame
But if 500 PCs connected to switches, the broadcast
could start 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 intofive groups of 100, separated from each other by
router, would create five broadcast domains, and thus
improves performance
Smaller broadcast domains also improve security dueto robust security features in routers
Table lists the benefits of segmenting Ethernet
devices using 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 broadcastdomain
With VLANs configuration, a switch can put someinterfaces into one broadcast domain and some into
another
These individual broadcast domains created by the switchare called virtual LANs
Figure shows
sample networkwith two broadcast
domains, two switches &
no VLANs
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LAN Design Consideration
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Alternately, you can create multiple broadcastdomains using a single switch
Figure shows the same two broadcast domains as inprevious slide figure, now implemented as twodifferent VLANs on 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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Campus LAN Design Terminology
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 Ethernetavailable and the cabling lengths supported by eachtype
Also the engineer must consider the type ofequipment that is already installed and whether anincrease in speed on some segments is worth the costof buying new equipment
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LAN Design Consideration
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Cisco-oriented LAN designs use some common
terminology to refer to the design
Figure shows a
typical design of a
large campus LAN,
with the
terminology
included in thefigure
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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 Switches
Access switches connect directly to end users, providing
access to the LAN
Access switches should not be expected to forward trafficbetween two other switches
Access switches tend to be smaller and less expensive
Each of the access switches must use at least two uplinks to
two different distribution switches for redundancy
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LAN Design Consideration
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Distribution Switches
Distribution switches provides an aggregation point for
access switches, forwarding frames between switches,
but not connecting directly to end-user devices
It provides some cabling advantages and potentialperformance advantages, e.g. for 30 access layer
switches to be cabled directly, the LAN would need 435
cables. Instead, by connecting each 30 access switches
to two distribution switches, requires 60 cables
Switches with faster forwarding rates and with two
uplinks from each access switch to the distribution
switch has more availability
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LAN Design Consideration
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Core Switches
Core switches aggregates distribution switches in verylarge campus LANs
Core switches provide extremely high forwardingratesthese days into the hundreds of millions of
frames per second Medium to smaller campus LANs often forego the
concept of core switches
Ethernet LAN Media & Cable Lengths
An engineer must consider the length of each cablerun and then find the best type of Ethernet andcabling type that 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 same100-meter cable restriction, but they use lightly differentcables
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 aslight-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 limitations
Most engineers simply remember the general
distance
limitationsand then use
a reference
chart
(such as the
table ) to
remember
each specific detail
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