Chapter 8 Local Area Networks - Internetworking - มหาวิทยาลัยอีสเทิร์ ...

hellhollowreadingNetworking and Communications

Oct 26, 2013 (3 years and 7 months ago)

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1

Chapter 8


Local Area Networks
-

Internetworking


Objective


สามารถอธิบายเหตุผลในการเชื่อมต่อเครือข่ายท้องถิ


(LAN)
หลายๆวง และการเชื่อมต่อเครือข่ายท้องถิ


(LAN)

กับเครือข่ายบริเวณกว้างแวน
(WAN)


สามารถระบุหน้าที่การท างานและวัตถุประสงค์ของอุปกรณ์ในการ
เชื่อมต่อแต่ละชนิด


สามารถนิยามความหมายของฮับ
(Hub)

และและยกตัวอย่าง
สถานการณ์ที่น าไปใช้


2

Objective (
ต่อ
)


สามารถอธิบายวิธีการเรียนรู้ของบริดจ์
(Bridge)


สามารถนิยามความหมายของสวิตช์
(Switch)
และบอกถึงความ
แตกต่างจากอุปกรณ์อื่นๆ


สามารถอธิบายถึงสถานการณ์ต่างๆและประโยชน์ที่เกิดขึ

นเมื่อน า
สวิตช์
(Switch)
ไปใช้


สามารถนิยามความหมายของเราเตอร์
(Router)
และบอกถึง
ความแตกต่างจากอุปกรณ์อื่นๆ


สามารถบอกถึงสถานการณ์ต่างๆ ที่ต้องใช้เราเตอร์
(Router)
ได้

3

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Introduction

Many times it is necessary to connect a local area network to
another local area network or to a wide area network.

Local area network to local area network connections are often
performed with a bridge
-
like device.

Local area network to wide area network connections are usually
performed with a router.

A third device, the switch, can be used to interconnect segments
of a local area network.

Chapter Eight
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Local Area Networks: Internetworking


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Why Interconnect?

To separate / connect one corporate division with another.

To connect two LANs with different protocols.

To connect a LAN to the Internet.

To break a LAN into segments to relieve traffic congestion.

To provide a security wall between two different types of users.

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-

Local Area Networks: Internetworking


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Hubs

As seen earlier, a hub interconnects two or more workstations
into a local area network.

When a workstation transmits to a hub, the hub immediately
resends the data frame out all connecting links.

A hub can be managed or unmanaged. A managed hub possesses
enough processing power that it can be managed from a remote
location.

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Local Area Networks: Internetworking


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Bridges

A bridge (or bridge
-
like device) can be used to connect two
similar LANs, such as two CSMA/CD LANs.

A bridge can also be used to connect two closely similar LANs,
such as a CSMA/CD LAN and a token ring LAN.

The bridge examines the destination address in a frame and
either forwards this frame onto the next LAN or does not.

The bridge examines the source address in a frame and places
this address in a routing table, to be used for future routing
decisions.

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Local Area Networks: Internetworking


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Chapter Eight
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Local Area Networks: Internetworking


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Transparent Bridges

A transparent bridge does not need programming but observes all
traffic and builds routing tables from this observation.

This observation is called backward learning.

Each bridge has two connections (ports) and there is a routing
table associated with each port.

A bridge observes each frame that arrives at a port, extracts the
source address from the frame, and places that address in the
port’s routing table.

A transparent bridge is found with CSMA/CD LANs.

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-

Local Area Networks: Internetworking


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Chapter Eight
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Local Area Networks: Internetworking


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Chapter Eight
-

Local Area Networks: Internetworking


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Transparent Bridges

A transparent bridge can also convert one frame format to
another, but this does not happen too often anymore since most
networks are CSMA/CD.

Note that some people / manufacturers call a bridge such as this
a gateway or sometimes a router.

The bridge removes the headers and trailers from one frame
format and inserts (encapsulates) the headers and trailers for the
second frame format.

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Local Area Networks: Internetworking


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Remote Bridges

A remote bridge is capable of passing a data frame from one
local area network to another when the two LANs are separated
by a long distance and there is a wide area network connecting
the two LANs.

A remote bridge takes the frame before it leaves the first LAN
and encapsulates the WAN headers and trailers.

When the packet arrives at the destination remote bridge, that
bridge removes the WAN headers and trailers leaving the
original frame.

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-

Local Area Networks: Internetworking


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Chapter Eight
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Local Area Networks: Internetworking


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Spanning Tree Algorithm

What happens if you have many LANs interconnected with
multiple bridges, such as shown in the next slide?

Data that leaves one workstation could travel to a bridge, across
the next network, into the next bridge, and back onto the first
network.

A packet may continue to cycle like this forever!

Chapter Eight
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Local Area Networks: Internetworking


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17





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Spanning Tree Algorithm

How do we stop this from happening?

Disconnect one of the bridges? Maybe we want bridge
redundancy in case one bridge fails.

How about applying the
spanning tree algorithm.

How is the algorithm applied?

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Local Area Networks: Internetworking


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Spanning Tree Algorithm

Step 1: Designate a
Root Bridge

Step 2: Mark one port of each bridge as the
Root Port
. The root
port is the port with the least
-
cost path from that bridge to the
root bridge. The root ports are denoted with an asterisk in Figure
8
-
7b.

Step 3: The next step is to select a
designated bridge

for each
LAN. A designated bridge has the least
-
cost path between that
LAN and the root bridge. Mark the corresponding port that
connects that LAN to its designated bridge with two asterisks
(Figure 8
-
7b)

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-

Local Area Networks: Internetworking


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Spanning Tree Algorithm

Step 4: If a port has no asterisks, that port is redundant and can
be “removed”. Keep all ports with one or two asterisks. The
resulting configuration is shown in Figure 8
-
7c.

Note there is now only one way to get to any LAN or bridge
from any other LAN or bridge.

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Local Area Networks: Internetworking


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Switches

A switch is a combination of a hub and a bridge.

It can interconnect two or more workstations, but like a bridge, it
observes traffic flow and learns.

When a frame arrives at a switch, the switch examines the
destination address and forwards the frame out the one necessary
connection.

Workstations that connect to a hub are on a
shared segment
.

Workstations that connect to a switch are on a
switched segment
.

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Local Area Networks: Internetworking


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Switches

The backplane of a switch is fast enough to support multiple data
transfers at one time.

A switch that employs
cut
-
through architecture

is passing on the
frame before the entire frame has arrived at the switch.

Multiple workstations connected to a switch use
dedicated
segments
. This is a very efficient way to isolate heavy users
from the network.

A switch can allow simultaneous access to multiple servers, or
multiple simultaneous connections to a single server.

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Local Area Networks: Internetworking


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Chapter Eight
-

Local Area Networks: Internetworking


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Chapter Eight
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Local Area Networks: Internetworking


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Virtual LANs

A virtual LAN, or VLAN, is a logical subgroup within a local
area network that is created via switches and software rather than
by manually moving wiring from one network device to another

Even though the employees and their actual computer
workstations may be scattered throughout the building, LAN
switches and VLAN software can be used to create a “network
within a network.”

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Local Area Networks: Internetworking


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Virtual LANs

A relatively new standard, IEEE
802.1
Q, was designed to allow
multiple devices to intercommunicate and work together to
create a virtual LAN

Instead of sending a technician to a wiring closet to move a
workstation cable from one switch to another, an
802.1
Q
-
compliant switch can be remotely configured by a network
administrator

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Local Area Networks: Internetworking


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Isolating Traffic Patterns with Switches

Whether shared or dedicated segments are involved, the
primary
goal

of a switch is to isolate a particular pattern of traffic from
other patterns of traffic or from the remainder of the network

Switches, because of their backplane, can also allow multiple
paths of communications to simultaneously occur

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Local Area Networks: Internetworking


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Chapter Eight
-

Local Area Networks: Internetworking


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Chapter Eight
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Local Area Networks: Internetworking


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Isolating Traffic Patterns with Switches

Using a pair of routers, it is possible to interconnect to switched
segments, essentially creating one large local area network

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Local Area Networks: Internetworking


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Local Area Networks: Internetworking


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Full Duplex Switches

A full duplex switch allows for simultaneous transmission and
reception of data to and from a workstation.

This full duplex connection helps to eliminate collisions.

To support a full duplex connection to a switch, at least two pairs
of wires are necessary
-

one for the receive operation and one for
the transmit operation. Most people install four pairs today, so
wiring is not the problem.

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Local Area Networks: Internetworking


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Chapter Eight
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Local Area Networks: Internetworking


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Network Servers

Network servers provide the storage necessary for LAN
software.

They are usually the focal point for the network operating
system.

Increasingly, network servers are functioning as bridges,
switches, and routers. By adding the appropriate card, a server
can assume multiple functions.

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Local Area Networks: Internetworking


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Routers

The device that connects a LAN to a WAN or a WAN to a WAN.

A router accepts an outgoing packet, removes any LAN headers
and trailers, and encapsulates the necessary WAN headers and
trailers.

Because a router has to make wide area network routing
decisions, the router has to dig down into the network layer of
the packet to retrieve the network destination address.

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Routers

Thus, routers are often called “layer
3
devices”. They operate at
the third layer, or OSI network layer, of the packet.

Routers often incorporate firewall functions.

An example of a router’s operation is shown on the next slide.

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Local Area Networks: Internetworking


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Chapter Eight
-

Local Area Networks: Internetworking


39





LAN Internetworking In Action: A Small
Office Revisited

Recall the In Action example from Chapter Seven.

A small office with
20
workstations in one room and
15
workstations in another room were connected to a server via
100
BaseTX.

One hub was kept in a closet near the
20
workstations while a
second hub was near the server.

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Local Area Networks: Internetworking


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Chapter Eight
-

Local Area Networks: Internetworking


41





LAN Internetworking In Action: A Small
Office Revisited

Now Hannah wants to connect the LAN to the Internet.

She adds a router next to the server and connects it to the hub.

She connects the router to a high
-
speed telephone line such as a
T
-
1
service.

She will also have to program the router to perform IP
addressing and firewall functions.

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Local Area Networks: Internetworking


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Chapter Eight
-

Local Area Networks: Internetworking


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LAN Internetworking In Action: A Small
Office Revisited

Now network usage is so high that Hannah must consider
segmenting the network.

She decides to install a database server near the original server
and replace both hubs with switches.

Chapter Eight
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Local Area Networks: Internetworking


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Chapter Eight
-

Local Area Networks: Internetworking


Exercise


อะไรคือเหตุผลในการเชื่อมต่อเครือข่ายตั

งแต่สองเครือข่ายขึ

นไป


จงบอกหลักการท างานของอุปกรณ์ต่อไปนี


hub, bridge, transparent
bridge, switch
และ
router


จงบอกความแตกต่างของ
switch, hub
และ
bridge


Backward learning
และ
cut
-
through architecture
คืออะไร เกี่ยวข้อง
กันหรือไม่อย่างไร


หากท่านท างาน ณ บริษัทเล็กๆ แห่งหนึ่งที่มีจ านวนพนักงานจ านวน
12
คน มีเวิร์คสเตชั


5
เครื่องที่ต้องการเชื่อมต่อกับอินเตอร์เน็ต จง
ออกแบบเครือข่ายทั

งหมดที่เป็นไปได้ พร้อมบอกข้อดี ข้อเสียในแต่
ละแบบ พร้อมบอกถึงทางเลือกที่ดีที่สุด


45

46

Hands
-
on Projects


จงสร้างแผนที่ในมหาวิทยาลัยอีสเทิร์นเอเชีย และออกแบบเครือข่าย
ว่าฮับตั

งอยู่ที่ใดบ้าง
,
มีสวิตช์หรือไม่ ถ้ามีตั

งอยู่ที่ใดบ้าง
,
มีบริดจ์
หรือไม่ ถ้ามีตั

งอยู่ที่ใดบ้าง และ มีเราเตอร์หรือไม่ ถ้ามีตั

งอยู่ที่ใดบ้าง


และวาดรูปประกอบ