Ccna certification Practice Tests

280

 

Appendix 

■

  Answers to Practice Test Questions

 175.  D.  When the Individual/Group (I/G) high order bit is set to 1, the frame is a broadcast or 

a multicast transmission. The OUI assigned by the IEEE is only partially responsible for 

MAC uniqueness. The vendor is responsible for the last 24 bits of a MAC address.

 176.  B.  When you’re diagnosing frame forwarding on a switch, the MAC address table needs 

to be inspected to see if the switch has learned the destination MAC address. You can 

use the command 

show mac address-table to inspect the MAC address table. The 

command 

show route is incorrect; it only displays layer 3 route decision information.  

The command 

show mac table is incorrect. The command show interface is incorrect; 

it will not display information on how the switch will make forward/filter decisions.

 177.  B.  The mechanism that switches use for loop avoidance is STP, or Spanning Tree Protocol. 

Port channels are used to aggregate bandwidth between two switches. Ether channels is 

a Cisco-centric term for port channels. Trunk is a switchport mode that allows multiple 

VLANs to traverse over a single link by tagging each frame with the respective VLAN.

 178.  D.  When loop avoidance such as STP is not employed and loops exist, you will get 

duplicate unicast frames and broadcast storms. This will inevitably thrash the MAC 

address table and degrade bandwidth to nothing.

 179.  C.  Store-and-forward mode is the default mode for mode edge switching equipment. 

Store-and-forward receives the frame, calculates the CRC, and then makes a forwarding 

decision. Cut-through mode allows the switch to make a forward/filter decision 

immediately after the destination MAC address is received. Frag-free mode inspects 

the first 64 bytes of an incoming frame, before a forward/filter decision is made. Fast 

switching is a method in which a caching table is created for MAC addresses received so 

that switching can be made faster.

 180.  B.  Fragment-free mode reads the first 64 bytes and deems the frame intact and 

forwardable. This is because most collisions that would create frame fragments happen 

within the first 64 bytes of a frame. This method of switching is often found on SOHO 

switching equipment. Store-and-forward mode is the default mode for mode edge 

switching equipment. Store-and-forward receives the frame, calculates the CRC, and then 

makes a forwarding decision. Cut-through mode allows the switch to make a forward/

filter decision immediately after the destination MAC address is received. Fast switching is 

a method in which a caching table is created for MAC addresses received so that switching 

can be made faster.

 181.  D.  The interface shows a high number of collisions. In a full-duplex network connection, 

there should be no collisions. This would suggest that the port on the switch or the NIC 

in the computer is set to half-duplex. Half-duplex causes collisions, which can degrade 

bandwidth by 40% to 60%. Upgrading the computer to 100 Mb/s will not solve the 

high collision rate; it will just allow more collisions to happen more quickly. You cannot 

conclude there is a wiring issue from the details of the interface. Although it is always 

possible a NIC is going bad, the NIC is still passing traffic, and there are not enough 

details in the output of the interface.

---

Chapter 1: Network Fundamentals (Domain 1)