06 - building ethernet lans with switches

8/12/2019 06 - Building Ethernet LANs With Switches

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ICND I - 01 Ethernet LANs & Switches


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By Muhammad Asghar Khan

[email protected]

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


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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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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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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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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Switching Logic


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


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


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


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


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


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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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06 - building ethernet lans with switches

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