Interconnecting the LANs (Questions/Answers)

Questions and Problems
Section 4-2

1. What are the seven layer s of the OSI model?
     i. 7-Application
     ii. 6-Presentation
     iii. 5-Session
     iv. 4-Transport
     v. 3-Network
     vi. 2-Data Link
     vii. 1-Physical

2. Which OSI layer is responsible for adding a header that includes routing information?
     i. Network Layer

3. Which OSI layer is considered the media access control layer?
     i. Data Link Layer

4. Which OSI layer combines messages and segments into packets?
     i. Transport Layer

5. Which OSI layer is responsible for the mechanical connection to the network?
     i. Physical Layer

6. The OSI layer responsible for data compression and encryption is which layer?
     i. Presentation Layer

7. TCP functions at what layer of the OSI model?
     i. Transport Layer

8. HTTP functions at what layer of the OSI model?
     i. Application Layer

9. IP and IPX are examples of protocols that operate in what layer of the OSI model?
     i. Network Layer

10. The network interface card operates at what layer of the OSI model?
     i. Physical Layer

11. Why are the layers of the OSI model important to the network administrator?
     i. To troubleshoot network problems by verifying functionality of each layer.
     ii. In many cases it requires the network administrator to isolate at what layer the network problem occurs.

Section 4-3

12. What is a bridge?
     i. A networking device that uses the MAC address to forward data and interconnect two LANs.

13. Define a segment.
     i. A section of a network separated by bridges, switches and routers.

14. What information is stored in a bridge table?
     i. MAC addresses and port locations for hosts connected to the bridge ports.

15. What is an association on a bridge and how is it used?
     i. Indicates that the destination MAC address for a host is connected to one of the ports on the bridge.

16. Excessive amounts of broadcasts on a network are called a ___________.
     i. Broadcast Storm

17. What command is used on a computer to view the contents of the ARP cache?
     i. arp -a

18. An empty ARP cache indicates what?
     i. The MAC addresses in the bridge table have all expired from no data traffic and have been deleted.

19. Why do entries into the bridging table have a limited lifetime?
     i. MAC addresses into the bridge table are temporary. The MAC address entry to the bridge table remains active as long as there is periodic data traffic activity from that host on its port. Therefore the MAC address entries have become inactive and therefore the expiration timer has expired and not renewed since no new data traffic has occurred.

20. Which of the following are advantages of using a bridge to interconnect LANs?
     b. Relatively inexpensive
     d. Easy to install
     e. Reduces collision domains

Section 4-4

21. The network switch operates at what layer of the OSI model?
     i. 2: Data Link Layer

22. Another name for a switch is
     b. Multiport bridge
23. How does a switch provide a link with minimal collisions?
     i. Simultaneous direct data connections for multiple pairs of hosts connected to the network.
     ii. Only two computers that established the link will be communicating over the channel.
     iii. Their link is isolated from any other data traffic.

24. The link for a switch connection is isolated from other data traffic except for what type of messages?
     i. Multicast Messages – Message are sent to a specific group of hosts on the network.
     ii. Broadcast Messages – Message is sent to all devices connected to the LAN.

25. Explain what data traffic is sent across a network when a computer pings another computer. A hub is used to interconnect the computers.
     i. And echo request is sent, the HUB broadcasts the request to all computers connected to the LAN, The computer that holds the IP address replies with its MAC address to the HUB, the HUB then sends the requests back to the computer that initiated the ping command and a direct line of communication is established between the two computers. The computer that initiated the ping command then receives the data packets from the destination computer.

26. Explain what data traffic is seen by computer 3 when computer 1 pings computer 2 in a LAN. A switch is used to interconnect the computers.
     i. Computer 3 only sees/receives an ARP request asking who has the specific IP address because after that a direct line of communication between computer 1 and 2 is established by the switch that prevents computer 3 from seeing the data traffic between computers 1 and 2.

27. Explain the concept of dynamic assignment on a switch.
     i. MAC addresses are assigned to a port when a host is connected.

28. Define aging time on a switch.
     i. The length of time a MAC address remains assigned to a port.

29. Explain how a switch learns MAC addresses, and where does a switch store the address.
     i. By extracting the MAC address information from the headers of Ethernet data packet header of transmitted data packets.
     ii. The switch then maps the extracted MAC address to the port where the data packet came in and stores it in the Content Addressable Memory (CAM).
     iii. Content Addressable Memory (CAM) – A table of MAC addresses and port mapping used by the switch to identify connected networking devices.

30. What happens if a MAC address is not stored in CAM on a switch?
     i. Flooding – Term used to describe what happens when a switch doesn’t have the destination MAC address stored in CAM.
     ii. The packet is transmitted out all switch ports except for the port where the packet is received.

31. What two modes are used by a switch to forward frames?
     i. Store- and-Forward
     ii. Cut-Through

32. What switch mode offers minimum latency?
     i. Cut-Through

33. What is error threshold, and what mode is it associated with?
     i. Error Threshold – The point where the number of errors in the data packets has reached a threshold and the switch changes from the cut-through to the store-and-forward mode.
     ii. Adaptive Cut-Through mode – Switches when the errors in the data packets has been exceeded

34. Explain the difference in store-and-forward and the cut-through mode on a switch.
     I. Store-and-Forward – The entire frame of data is received before any decision is made regarding forwarding the data packet to its destination. (The switch checks the data packet for errors before it is sent on to the destination.)
     II. Cut-Through – The data packet is forwarded to the destination as soon as the destination MAC address has been read.  (No error checking)
          i. Fast-Forward- Minimum switch latency
          ii. Fragment-Free – Fragment collision are filtered out by the switch.
               1. Fragments occur within the first 64 bytes of a data packet and if the packet is not 64 bytes it is discarded. Therefore no fragments.

35. How does a layer 3 switch differ from a layer 2 switch?
     i. They still work at layer 2 of the OSI model but additionally work at Layer 3 Network and use IP addressing for making decision s to route a data packet to the best direction.
     ii. They use ASICs (application specific integrated circuits) – Hardware to handle the packet switching.
     iii. They do Wire Speed Routing – Data packets are processed as fast as they are arriving.

36. What is mean by the term wire-speed routing?
     i. Data packets are processed as fast as they are arriving.

Section 4-5

37. A router uses the network address on a data packet for what purpose?
     i. To make routing decisions regarding forwarding data packets.

38. What is the logical address?
     i. Describes the IP address location of the network and the address location of the host in the network.

39. The physical connection where a router connects to the network is called the . . .
     d. Router interface

40. The connection to the router’s console input is typically which of the following?
     a. RS-232
41. AUI stand for . . .
     d. Attachment Unit Interface

42. The AUI port on a router connects to what networking protocol?
     d. Ethernet

Section 4-6

43. Define Enterprise Network.
     i. A term used to describe the network used by a large company.

44. The router interface most commonly used to interconnect LANs in a campus network is. . .
     c. Ethernet
45. Serial interfaces on a router are typically used to
     c. Connect to communication carriers
46. The designation E0 indicates. . .
     a. Ethernet port 0
47. Routing  tables on a router keep track of
     d. Routes to use for forwarding data to its destination

48. The convention used for naming of the serial port 0 n a router is . . .
     a. S0
49. Define the term gateway
     i. Describes the networking device that enables hosts in a LAN to connect to networks (and hosts) outside the LAN.

Section 4-7

50. What is the purpose of the fast link pulse?
     i. The auto negotiation protocol uses the (FLP) to carry the information between each end of a data link.
     ii. (FLP) Fast Link Pulse – Carries the configuration information between each end of a data link.

51. Define full duplex.
     i. The communication device can transmit between each end of a data link.

52. Define half duplex.
     i. The communication device can transmit and receive at the same time.

53. Which of the following is a disadvantage of the auto-negotiation protocol?
     b. A failed negotiation on a functioning link can cause a link failure.