06 - Ethernet LAN Switching Concepts

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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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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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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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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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Switching Logic


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


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


Figures below depicts the point (b)of previous slide

Switch Forwarding

Decision

Switch Filtering

Decision

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Switching Logic


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


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


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


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


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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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06 - Ethernet LAN Switching Concepts