Ethernet-Chapter 9 – Flashcards
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Who originally published the Ethernet standard and when was that?
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Published in 1980 by a consortium of Digital Equipment Corporation, Intel, and Xerox (DIX).
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Who and when published LAN standards?
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1985-Institute of Electrical and Electronics Engineers (IEEE)
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What is the number for the Ethernet LAN standard?
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802.3
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What layers of the OSI model do Ethernet standards operate at?
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1 and 2
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What does Ethernet Layer 1 involve?
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-signals -bit streams that travel on the media -physical components that put signals on media -various topologies
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What are the Ethernet Layer 2 functions that overcome the limitations of Ethernet Layer 1?
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-Connects to the upper layers via the LLC -Uses addressing schemes to identify devices -Uses frames to organize bits into groups -Uses MAC to identify transmission sources
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What are the two sublayers of the Data Link Layer?
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The Logical Link Control (LLC) sublayer and the Media Access Control (MAC) sublayer.
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What are the 4 functions of the LLC?
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1-Makes connection with the upper layers 2-Frames the network layer packet 3-Identifies the network layer protocol 4-Remains relatively independent of the physical layer
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What type of environment is LLC implemented in?
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Software
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Where is the MAC sublayer implemented?
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NIC
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What are the 2 primary functions of the MAC sublayer?
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Data Encapsulation Media Access Control
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What are the 3 primary functions of data encapsulation?
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Frame delimiting Addressing Error detection
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What are the two main functions of the media access control?
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Control of frame placement on and off the media Media recovery
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What is the underlying topology of Ethernet?
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Mulit-Access Bus
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What is the media access control method for Ethernet?
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CSMA/CD Carrier Sense Multiple Access with Collision Detection
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What are the four factors that have made Ethernet a success?
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1-Simplicity and ease of maintenance 2-Ability to incorporate new technologies 3-Reliability 4-Low cost of installation and upgrade
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What part of the Ethernet standard remains consistent across all of its physical implementations?
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Frame structure
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What was the historic network that Ethernet was based on called? ***digital radio network designed to transmit information over a shared radio frequency between the Hawaiian Islands.******
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Alohanet
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Describe the method of transmission on a hub?
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A frame received in one port is transmitted out all other ports
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What is legacy Ethernet?
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10Base-T network w/ a hub at its center
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What is the current standard for Ethernet?
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100Base-T network w/ switches
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What is full duplex?
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A connection that can carry both transmitted and received signals at the same time
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What is the difference when forwarding a frame from a SWITCH vs. HUB?
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Hub always send the signal out all ports and a Switch sends directly to device if it's address is known
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What two new uses are requiring the move to Gigabit Ethernet?
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Voice over IP (VoIP) and multimedia services
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Define a MAN?
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Ethernet connection across an entire city
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What is the most significant difference between the IEEE 802.3 (original)/ & the Revised IEEE 802.3?
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The addition of a Start Frame Delimiter (SFD) and a small change to the Type field to include the Length
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What were the minimum and maximum sizes of Ethernet frames originally?
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The minimum frame size as 64 bytes and the maximum as 1518 bytes
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What is the new maximum size for a frame?
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1522 bytes
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What happens if the size of a transmitted frame is less than the minimum or greater than the maximum?
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It is dropped
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What are the six portions of an Ethernet frame discussed?
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Preamble, start frame delimiter, destination address, source address, length/type, 802.2 header and data, frame check sequence
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What is the Frame Check Sequence (FCS)?
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(4 bytes) used to detect errors in a frame
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What is the algorithm used in determining the value of the FCS?
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Cyclical redundancy check
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What happens if the FCS values do not match?
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The frame is dropped
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What was created to assist in determining the source and destination address?
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Media Access Control (MAC) address
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How is an Ethernet MAC address represented?
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A 48-bit binary value expressed as 12 hexadecimal digits
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Define the two parts of an Ethernet MAC address?
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All MAC addresses assigned to a NIC or other Ethernet device must use that vendor's assigned OUI as the first 3 bytes. All MAC addresses with the same OUI must be assigned a unique value (vendor code or serial number) in the last 3 bytes.
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What is another name for the MAC address?
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Burned-in address
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As an Ethernet frame passes through a network what happens if that devices MAC address matches the destination MAC address?
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The NIC passes the frame up the OSI layers, where the decapsulation process take place
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As an Ethernet frame passes through a network what happens if that devices MAC address does not match the destination MAC address?
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The frame is discarded
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Where can you find your computer's MAC address?
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Ipconfig/all or ipconfig
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What is the problem with physical addressing?
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No meaning outside the local network
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What is the difference between physical (Network Layer) and logical (Data Link Layer) addressing?
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The Network layer address enables the packet to be forwarded toward its destination. The Data Link layer address enables the packet to be carried by the local media across each segment.
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When a unicast transmission used?
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When a frame is sent from a single transmitting device to single destination device
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When is a broadcast transmission used?
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All hosts on that local network (broadcast domain) will receive and process the packet
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How is a broadcast IP address represented?
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All 1's in the host portion of the IP address
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How is a broadcast MAC address represented?
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FF-FF-FF-FF-FF-FF
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When is a multicast transmission used?
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When a frame is sent to a group of computers
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How can a multicast packet only be used?
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As the destination of a packet
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What type of address does the source have?
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Unicast
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What is the multicast MAC address?
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Begins with 01-00-5E The value ends by converting the lower 23 bits of the IP multicast group address into the remaining 6 hexadecimal characters of the Ethernet address. The remaining bit in the MAC address is always a "0".
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How does a shared media environment operate?
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All devices have guaranteed access to the medium, but they have no prioritized claim on it. If more than one device transmits simultaneously, the physical signals collide and the network must recover in order for communication to continue.
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When does a device on a shared environment transmit?
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When it detects that no other computer is sending a frame, or carrier signal
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Define latency.
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The time delay between when a process is started and that same process is detected
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Define collision.
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When two stations transmit at the same time
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How does collision detection occur?
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When a device is in listening mode it can detect an increase in amplitude of the signal above the normal level
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What happens when all devices on a shared medium detect a collision?
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They send out a jamming signal
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What is invoked after a device receives a jamming signal?
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The backoff algorithm
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How does a backoff algorithm work?
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All devices stop transmitting for a random amount of time
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What mode does a device return to after the backoff algorithm has expired?
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Listening
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What are the three reasons given that the growth of the Internet has resulted in more collisions?
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More devices are being connected to the network. Devices access the network media more frequently. Distances between devices are increasing.
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Define a collision domain?
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The media within a network where collsions can occur
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What does the connection of hubs and repeaters do to collision domains?
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They increase the collision domain size
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What happens when the number of collisions increases significantly?
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Reduces the network's efficiency and effectiveness until the collisions become a nuisance to the user
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Which devices extend the latency in the medium?
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Hubs and repeaters
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Why would an increased latency cause more collisions?
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The other device may not hear the signal before it begins to transmit
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What is sent before an entire Ethernet frame is sent?
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The preamble
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What does asynchronous communication mean?
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Each receiving device will use the 8 bytes of timing information to synchronize the receive circuit to the incoming data and then discard the 8 bytes
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What does synchronous communication mean?
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Timing information is not required
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What is a bit time?
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Period of time is required for a bit to be placed and sensed on the media
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What is the slot time?
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The time it takes for an electronic pulse to travel the length of the maximum theoretical distance between two nodes.
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What is interframe spacing?
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Time measured from the last bit of the FCS field of one frame to the first bit of the Preamble of the next frame
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What happens to interframe spacing as the network speed increases?
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The time reduces
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What happens when the MAC layer is unable to send a frame after 16 attempts?
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It gives up and generates an error to the Network layer
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What are the four data rates are currently defined for operation over optical fiber and twisted-pair cables?
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10 Mbps - 10Base-T Ethernet 100 Mbps - Fast Ethernet 1000 Mbps - Gigabit Ethernet 10 Gbps - 10 Gigabit Ethernet
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What are the three principal 10 Mbps implementations of Ethernet?
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10BASE5 using Thicknet coaxial cable 10BASE2 using Thinnet coaxial cable 10BASE-T using Cat3/Cat5 unshielded twisted-pair cable
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What are the most popular implementations of 100 Mbps Ethernet?
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100BASE-TX using Cat5 or later UTP 100BASE-FX using fiber-optic cable
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What is Gigabit Ethernet more susceptible to because of its speed?
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Noise
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What type of transmission does 1000Base-T allow for?
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The transmission and reception of data in both directions - on the same wire and at the same time
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What are the two main causes of noise on the line?
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Cable and termination problems
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What are the ways that 10Gbps can be compared to other varieties of Ethernet?
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Frame format is the same, allowing interoperability between all varieties of legacy, fast, gigabit, and 10 gigabit Ethernet, with no reframing or protocol conversions necessary. Bit time is now 0.1 nS. All other time variables scale accordingly. Because only full-duplex fiber connections are used, there is no media contention and CSMA/CD is not necessary. The IEEE 802.3 sublayers within OSI Layers 1 and 2 are mostly preserved, with a few additions to accommodate 40 km fiber links and interoperability with other fiber technologies.
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What is the problem associate with the classic Ethernet implementation using hubs?
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High levels of collisions on the LAN
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List the four problems related to the implementation of classic Ethernet?
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Scalability Latency Network Failure Collisions
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How do switches segment a LAN?
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Separate collision domains
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What does the separation of collision domains mean for the bandwidth to the device?
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Full media bandwidth to each device
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What are the three reasons that bandwidth increases dramatically when each device is connected to a switch port?
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Dedicated bandwidth to each port Collision-free environment Full-duplex operation
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What are the three reasons that hubs are still being used?
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Availability, Economics, and Requirements
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Describe selective forwarding?
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Establishing a momentary point-to-point connection between the transmitting and receiving nodes
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What is the process called when the switch holds the frame until the receiving node is available called?
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Store-and-forward
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Describe the store-and-forward operation mode of a switch?
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The switch receives the entire frame and checks the FCS before forwarding the frame
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How does a switch use a MAC table?
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The incoming frame destination MAC address is compared to the MAC table to decide which port to forward the frame to
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What are the five basic operations of a switch?
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Learning Aging Flooding Selective Forwarding Filtering
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How does a switch learn a MAC address?
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The source address of the incoming frame
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What happens when an addresses countdown reaches 0?
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The address is removed from the MAC table
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What happens when a switch does not know the destination address?
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The frame is flooded out all but the incoming port
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What type of frames will not be forwarded?
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A frame with a destination of the same port, a corrupt frame, and security reasons
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What are the two basic functions of ARP?
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1-Resolving IPv4 addresses to MAC addresses 2-Maintaining a cache of mappings
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What are the names of the tables that store the list of MAC addresses with the network address?
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ARP table or ARP Cache
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What are the two ways a computer can gather MAC addresses?
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From an incoming frame or using an ARP
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What are the 2 destination addresses in an ARP Request?
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Broadcast MAC address and the destination MAC address
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If the destination is on the local network segment, what MAC address is used?
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The device MAC address
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If the destination is on a remote network, what MAC address is used?
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The gateway MAC address
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If the MAC address of the gateway is unknown, what process is used?
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ARP Request
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What is the most common reason for using a proxy ARP?
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When the device can not determine if the host is on the same network or not
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How long are addresses held in the ARP table?
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2 minutes unless used again in those 2 minutes and then 10 minutes
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How could ARP requests impact performance on a network?
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Too many broadcasts at once that each host receives and processes therefore slowing down the network
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What is another problem that may be associated with ARP?
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Security- ARP spoofing which causes errors on the network
