ICND1-4-Understanding LAN Network Topologies-36c.txt

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ICND1-4-Understanding LAN Network Topologies-36c.txt
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2013-07-25 17:23:20
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CISCO ICND1 LAN
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ICND1-4-Understanding LAN Network
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  1. Describe the difference between physical network topology and logical network topology?
    Physical topologies refer to the physical layout of devices and network media. Logical topologies refer to the logical paths in which data accesses the media and transmits packets across it.
  2. What are the five types of physical topologies implemented in today's networks?
    • The five types of physical topologies implemented in today's networks are as follows:
    • - Bus
    • - Ring
    • - Star
    • - Extended star
    • - Mesh
  3. What physical network topology connects all devices to one cable?
    Bus topology. A bus topology connects all devices to a single cable. This cable connects one computer to another. In a logical bus topology, only one packet can be transmitted at a time.
  4. Describe a star and extended star physical topology?
    • Star and extended star physical topologies are made of a central connection point, such as a hub or switch, where all cable segments connect. A star topology resembles spokes in a bicycle wheel and is the network topology of choice in Ethernet networks.
    • When a star network is expanded to include additional network devices that connect to a main center network device, it is called an extended star topology.
  5. Describe a ring topology?
    • In a ring topology, all hosts and devices are connected in the form of a ring or circle. The following two types of ring networks exist:
    • - Single-ring: In a single-ring network, all devices share a single cable and data travels in one direction. Each device waits its turn to send data over the network.
    • - Dual-ring: A dual-ring network has a second ring to add redundancy and allows data to be sent in both directions.
  6. What physical network topology connects all devices to each other?
    A mesh network topology connects all devices to each other for fault tolerance and redundancy.
  7. What is the difference between a mesh and a partial-mesh topology?
    A full-mesh topology connects all nodes to one another for full redundancy. In a partial-mesh, at least one node maintains multiple connections to all other devices.
  8. What is the logical topology of Ethernet?
    Bus. Ethernet uses a logical bus topology and either a physical bus or star topology.
  9. What do wireless networks use to communicate to end devices?
    Radio frequency (RF) or infrared waves.
  10. In wireless LANs (WLANs), what distributes the wireless signal?
    • Access points (AP) or a wireless hub.
    • APs or wireless hubs distribute the wireless signal, and nodes receive the wireless signal through a wireless adapter card.
    • What are two types of Layer 1 network devices?
    • Two types of Layer 1 network devices are as follows:
    • - Repeaters: Regenerate and retime network signals, amplifying them to allow the signal to travel a longer distance on a network media.
    • - Hubs: Known as a multiple-port repeaters, hubs also regenerate and retime network signals. The main difference between a hub and a repeater is the number of cables that connect to the device. A repeater typically has 2 ports, whereas a hub has from 4 to 48 ports.
  11. What are some network devices that operate at the data link layer (Layer 2)?
    • Bridges and switches are network devices that operate at the data link layer. Both devices make decisions about what traffic to forward or drop (filter) by MAC addresses, and logical network addresses are not used at this layer. Data link layer devices assume a flat address space.
    • Typically, a bridge is designed to create two or more LAN segments and is software implemented.
    • A switch is a hardware version of a bridge, but that has many more ports than a bridge, and is designed to replace a hub while providing the filtering benefits of a bridge.
  12. What is a LAN segment?
    • A LAN segment is a network connection made by a single unbroken network cable.
    • Segments are limited by physical distance because, after a certain distance, the data transmission becomes degraded because of line noise and the reduction of signal strength.
  13. What are the Ethernet segment distance limitations for the following?
    • - 100BASE-TX
    • - 100BASE-FX
    • - 1000BASE-T
    • The Ethernet segment distance limitations are as follows:
    • - 100BASE-TX: 100 meters
    • - 100BASE-FX: 400 meters
    • - 1000BASE-T: 100 meters
    • - 1000BASE-LX: 550 meters for multimode fiber, 10 km for single-mode fiber
    • - 1000BASE-SX: 250 meters for multimode fiber, 550 meters for single-mode fiber
    • - 1000BASE-CX: 25 meters
  14. What devices can you use to extend a LAN segment?
    • To extend a LAN segment, you can use the following devices:
    • - Hubs
    • - Repeaters
    • - Bridges
    • - Switches
  15. How do collisions occur in Ethernet?
    Collisions occur on a shared LAN segment when two devices try to communicate at the same time. In a shared Ethernet segment, only one device can transmit on the cable at a time. When two devices try to transmit at the same time, a collision occurs.
  16. What are collision domains?
    • A collision domain defines a group of devices connected to the same physical medium.
    • A collision occurs when two packets are sent at the same time and collide with each other. When a collision occurs, a jam signal is sent from a workstation. A collision affects all the machines on the segment, not just the two that collided; when the jam signal is on the wire, no workstations can transmit data. The more collisions that occur in a network, the slower it will be, because the devices must resend the packets that collided.
  17. What happens when you segment the network with hubs/repeaters?
    • Because hubs and repeaters operate at the physical layer of the OSI model, segmenting a network with these devices appears as an extension to the physical cable.
    • Hubs and repeaters are transparent to devices; they are unintelligent devices. All devices that connect to a hub/repeater share the same bandwidth. Hubs/repeaters create a single broadcast and collision domain.
  18. What is the advantage of segmenting a network with bridges/switches?
    Bridges/switches operate at Layer 2 of the OSI model and filter by MAC address. Each port on a bridge/switch provides fully dedicated bandwidth and creates a single collision domain. Because bridges/switches operate at Layer 2 of the OSI model, they cannot filter broadcasts, and they create a single broadcast domain.
  19. What devices are used to break up collision domains?
    Switches, bridges, and routers are used to break up collision domains. They create more collision domains and fewer collisions. Each port on a bridge, switch, and router creates one collision domain. For example, if you have a switch with 24 ports, you have 24 separate collision domains.
  20. In an attempt to extend your Ethernet segment, you add a 24-port hub. How many collision domains will you have in the segment with the addition of the hub?
    1. A hub only extends the network segment, and all devices share the same segment bandwidth. As a result, a hub does not create more collision domains.
  21. In an attempt to extend your Ethernet segment you add a 24-port switch. How many collision domains and broadcast domains will you have in the segment with the addition of a switch?
    • 24 collision domains and 1 broadcast domain.
    • Because switches operate at Layer 2 of the OSI model, they can divide the network into different segments, thus creating more collision domains. Each port on a switch creates one collision domain. Also, because a switch operates at Layer 2 of the OSI model, it cannot filter broadcasts. As such, a switched network will have one broadcast domain.
  22. What are broadcast domains?
    A broadcast domain defines a group of devices that receive each other's broadcast messages. As with collisions, the more broadcasts that occur on the network, the slower the network will be. This is because every device that receives a broadcast must process it to see whether the broadcast is intended for that device.
  23. You install a 6-port router on your network. How many collision domains and broadcast domains will be created on the network with the addition of the 6-port router?
    • Six collision domains and six broadcast domains.
    • Each interface on a router creates a collision domain and a broadcast domain.
  24. List four advantages that Layer 2 switches have over bridges?
    • Four advantages that Layer 2 switches have over bridges are as follows:
    • - A high-speed backplane that enables multiple simultaneous conversations to occur.
    • - Data-buffering capabilities that store and forward packets to the correct ports or port.
    • - Higher port densities versus bridges.
    • - Lower latency than bridges. Layer 2 switches are implemented in hardware, allowing millions of bits per second to be transmitted at the same time.
  25. What is a broadcast storm?
    roadcast storms occur when many broadcasts are sent simultaneously across all network segments. They are usually caused by Layer 2 loops because of spanning tree failures, a bad network interface card (NIC), a faulty network device, or a virus.
  26. What three major functions do Layer 2 switches provide?
    • - Address learning
    • - Packet forwarding/filtering
    • - Loop avoidance with the Spanning Tree Protocol
  27. What are the three switching methods (frame transmission modes) in Cisco Catalyst switches?
    • The three frame operating modes to handle frame switching are as follows:
    • - Store-and-forward
    • - Cut-through
    • - Fragment-free
  28. What is the Cisco Catalyst store-and-forward switching method?
    In the store-and-forward switching method, the switch's incoming interface receives the entire frame before it forwards it. The switch computes the cyclic redundancy check (CRC) to make sure that the frame is not bad. If the frame is good, the switch forwards it. If the CRC is bad, the switch drops it. If the frame is a runt (less than 64 bytes, including the CRC) or a giant (more than 1518 bytes, including the CRC), the switch discards it. Because the switch stores the frame before forwarding it, latency is introduced in the switch. Latency through the switch varies with the size of the frame.
  29. What is the Cisco Catalyst cut-through switching method?
    In cut-through switching mode, the switch only checks the frame's destination address and immediately begins forwarding the frame out the appropriate port. Because the switch checks the destination address in only the header and not the entire frame, the switch forwards a collision frame or a frame that has a bad CRC.
  30. What is the Cisco Catalyst fragment-free switching method?
    Also known as modified cut-through, fragment-free switching checks the first 64 bytes before forwarding the frame. If the frame is less than 64 bytes, the switch discards the frame. Ethernet specifications state that collisions should be detected during the first 64 bytes of the frame. By reading the first 64 bytes of the frame, the switch can filter most collisions, although late collisions are still possible.
  31. What is an example of Layer 2 addresses?
    • MAC addresses.
    • What is an example of Layer 3 addresses?
    • IP addresses.
  32. If a sending device does not know the MAC address of the receiving device, what tool is used to find the MAC address?
    • Address Resolution Protocol (ARP).
    • ARP is a local broadcast sent to all devices on the local segment to find the MAC address of a host.
  33. Host A wants to send data to host B. Host B is on a different segment from host A. The two segments are connected to each other through a router. What happens to the MAC address of host A during data transit to host B?
    • Because host B is on a different segment that is separated by a router, the MAC address of host A will change. Anytime a frame passed through a router, a router rewrites the MAC address to the MAC address of the router and then sends the frame to the local host.
    • In this case, the router will change the MAC address of the frame sent from host A. Host B will see that the frame came from the MAC address of the router with the IP address of host A.
  34. What is the function of CSMA/CD algorithm in Ethernet technologies?
    Carrier sense multiple access collision detect (CSMA/CD) defines how the Ethernet media is accessed.

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