Transcript No Slide Title

Cisco Systems CCNA Version 3 Semester 1
Module 8
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 1
Module 7 CCNA1 Form B exam
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 2
Overview
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 3
This module explores the effects of collisions and broadcasts on
network traffic and then describes how bridges and routers are
used to segment networks for improved performance.
Students completing this module should be able to:
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Define bridging and switching.
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Define and describe the content-addressable memory (CAM) table.
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Define latency.
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Describe store-and forward and cut-through switching modes.
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Explain Spanning-Tree Protocol (STP).
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Define collisions, broadcasts, collision domains, and broadcast
domains.
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Identify the Layer 1, 2, and 3 devices used to create collision
domains and broadcast domains.
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Discuss data flow and problems with broadcasts.
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Explain network segmentation and list the devices used to create
segments.
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 4
Module Overview
8.1 Ethernet Switching
8.1.1 Layer 2 bridging
8.1.2 Layer 2 switching
8.1.3 Switch operation
8.1.4 Latency
8.1.5 Switch modes
8.1.6 Spanning-Tree Protocol
8.2 Collision Domains and Broadcast Domains
8.2.1 Shared media environments
8.2.2 Collision domains
8.2.3 Segmentation
8.2.4 Layer 2 broadcasts
8.2.5 Broadcast domains
8.2.6 Introduction to data flow
8.2.7 What is a network segment?
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 5
8.1.1 Layer 2 bridging
Bridges & Switches learn MAC
addresses from the source host.
(The source address field.)
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 6
8.1.2 Layer 2 switching
Replacing hubs with bridges or switches to increase the
number of collision domains is called segmentation
Wireless Bridge
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 7
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A bridge has only two ports and divides a collision
domain into two parts.
All decisions made by a bridge are based on MAC
(physical) or Layer 2 addressing and do not affect
the logical or Layer 3 addressing.
(A router use the destination IP address to make a
forwarding decisions.)
Thus, a bridge will divide a collision domain but has no
effect on a logical or broadcast domain.
No matter how many bridges are in a network, unless
there is a device such as a router that works on
Layer 3 addressing, the entire network will share the
same logical broadcast address space.
A bridge will create more collision domains but will
not add (or create) additional broadcast domains.
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 8
8.1.3 Switch operation
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A switch is simply a bridge with many ports.
When only one node is connected to a switch
port, the collision domain on the shared media
contains only two nodes.
The two nodes in this small segment, or
collision domain, consist of the switch port
and the host connected to it.
These small physical segments are called
microsegments.
A bridge or switch increase the
number of collision domains but
have no impact on broadcast
domains
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 9
8.1.3 Switch operation
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Most switches and most network interface cards (NICs) are capable of
supporting full duplex.
In full duplex mode, there is no contention for the media.
Thus, with switched full-duplex transmission there are no collision domains .
You have collision-free transmission.
Theoretically, the bandwidth is doubled when using full duplex.
A switch uses full-duplex mode
to provide full bandwidth
between two nodes on a network.
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 10
8.1.3 Switch operation
Content-addressable memory (CAM)
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Entering data into the memory will return the associated address.
Application-specific integrated circuit (ASIC)
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Undedicated logic gates that can be programmed to perform functions at logic speeds.
CAMs are often used in
caches and memory
management units. A
CAM can also operate
as a data parallel
processor.
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 11
8.1.4 Latency
Sources of Latency
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Media delays caused by the finite speed that signals can travel through the
physical media.
Circuit delays caused by the electronics that process the signal along the path.
Software delays caused by the decisions that software must make to
implement switching and protocols.
Delays caused by the content of the frame and where in the frame switching
decisions can be made.
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 12
8.1.5 Switch modes
Store and Forward Reads
the entire Frame
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The switch receives the entire frame before sending it out the destination port.
Verifies the Frame Check Sum (FCS) and Length before sending it to the destination.
If the frame is invalid, it is discarded at the switch rather than the ultimate destination.
This form of switching has the highest latency and will depend on the length of the frame.
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 13
Fragment Free
8.1.5 Switch modes
Fragment Free Reads 64 bytes confirming
that it is not a fragment (runt)
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Fragment-free reads the first 64 bytes, which includes the frame header, and switching begins
before the entire data field and checksum are read.
Therefore the latency time is always fixed.
This mode verifies the addressing (MAC) and Logical Link Control (LLC) protocol data.
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 14
8.1.5 Switch modes
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Both the source port and destination port must be
operating at the same bit rate to keep the frame intact.
This is called synchronous switching.
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100Mb to server
Cut-Through Reads up to the
Destination MAC address
100Mb to clients
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A switch can start to transfer the frame as soon as the destination MAC address is read.
Therefore the latency time is always fixed.
The FCS is not read before switching begins, therefore there is no error checking.
Switching at this point is called cut-through switching and results in the lowest latency.
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 15
8.1.5 Switch modes
1000Mb to server
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100Mb to clients
If the bit rates are not the same, the frame must be stored at one bit rate before it is sent out at
the other bit rate.
This is known as asynchronous switching.
Store-and-forward mode must be used for asynchronous switching.
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 16
8.1.6 Spanning-Tree Protocol
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When multiple switches are arranged in a simple hierarchical
tree, switching loops are unlikely to occur.
However, switched networks are often designed with
redundant paths to provide for reliability and fault
tolerance.
Switching Loop
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Switching loops can occur by design or by accident.
Can lead to broadcast storms that will overwhelm a network.
The Spanning-Tree Protocol (STP) counteracts switching
loops.
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 17
8.1.66.1.2
Spanning-Tree
Servers Protocol
The concentration of network
resources makes the data easier
to back up and maintain.
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 18
8.1.6 Spanning-Tree Protocol
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Each switch in a LAN using STP sends special messages called Bridge
Protocol Data Units (BPDUs)
This tells other switches that it exists, and is used to elect a root bridge for
the network.
The switches then use the Spanning-Tree Algorithm (STA) to resolve and
shut down redundant paths.
STP creates a logical hierarchical tree with no loops.
However, the alternate paths are still available should they be needed.
You must know the five STP states and their purpose.
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 19
8.1.6 Spanning-Tree Protocol
STP
Forwarding
STA
Forwarding
Blocking
Designated port
BPDUs
Root Bridge
Forwarding
Blocking
Root port
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 20
Module Overview
8.1 Ethernet Switching
8.1.1 Layer 2 bridging
8.1.2 Layer 2 switching
8.1.3 Switch operation
8.1.4 Latency
8.1.5 Switch modes
8.1.6 Spanning-Tree Protocol
8.2 Collision Domains and Broadcast Domains
8.2.1 Shared media environments
8.2.2 Collision domains
8.2.3 Segmentation
8.2.4 Layer 2 broadcasts
8.2.5 Broadcast domains
8.2.6 Introduction to data flow
8.2.7 What is a network segment?
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 21
8.2.1 Shared media environments
Layer 1 media and topologies are reviewed here.
Shared Environments - Direct
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 22
8.2.1 Shared media environments
Layer 1 media and topologies are reviewed here.
Shared Environments - Indirect
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Point-to-point network environment – Is widely used in dialup network connections.
It is a shared networking environment in which one device is connected to only one other device
eg. connecting a computer to an Internet service provider by modem and a phone line.
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 23
8.2.2 Collision domains
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 24
8.2.2 Collision domains
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reduce the size of the collision domains
add extra managed switches
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 25
8.2.2 Collision domains
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 26
8.2.2 Collision domains
Extended shared media. Multiple hosts
have access to the same medium.
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 27
8.2.2 Collision domains
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 28
8.2.2 Collision domains
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 29
8.2.2 Collision domains
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 30
8.2.3 Segmentation
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 31
8.2.3 Segmentation
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 32
8.2.3 Segmentation
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 33
8.2.3 Segmentation
Bridge
•Breaking up a LAN into multiple collision domains increases the opportunity for each host in the
network to gain access to the media.
•This effectively reduces the chance of collisions and increases available bandwidth for every host.
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 34
8.2.4 Layer 2 broadcasts
Bridge
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Layer 2 devices forward broadcast and if excessive, can reduce the efficiency of the entire LAN.
Broadcasts must be controlled at Layer 3, as Layer 2 and Layer 1 devices cannot.
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 35
8.2.4 Layer 2 broadcasts
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Generally, IP workstations cache 10 to 100 addresses in their ARP tables for about two hours.
The ARP rate for a typical workstation might be about 50 addresses every two hours or 0.007
ARPs per second.
Thus, 2000 IP end stations produce about 14 ARPs per second.
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 36
8.2.4 Layer 2 broadcasts
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 37
1.
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Layer 1 device always forwards the frame.
Layer 2 device wants to forward the frame.
Layer 3 device will not forward the frame unless it has to.
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 38
8.2.5 Broadcast domains
A router (layer 3 device) solves the
problem of too much broadcast traffic.
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Router
Routers actually work at Layers 1, 2, and 3.
Like all Layer 1 devices they have a physical connection to, and transmit data onto, the media.
They have a Layer 2 encapsulation on all interfaces and perform just like any other Layer 2 device.
It is Layer 3 that allows the router to segment broadcast domains.
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 39
8.2.6 Introduction to data flow
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In order for a packet to be forwarded through a router it must have already been processed by a
Layer 2 device and the frame information stripped off.
Layer 3 forwarding is based on the destination IP address and not the MAC address.
For a packet to be forwarded it must
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contain an IP address that is outside of the range of addresses assigned to the LAN and
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the router must have a destination to send the specific packet to in its routing table.
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 40
8.2.7 What is a network segment?
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 41
Summary
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 42
FIN
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 43
Nov-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod8 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 44