Ccna certification Practice Tests

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Appendix 

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  Answers to Practice Test Questions

 190.  B.  When a MAC address is unknown by the switch, the switch will forward the frame to 

all ports; this is also called flooding the frame. When the destination system acts upon it, 

the switch learns its MAC address through source MAC address learning. The uplink port 

will receive a copy of the flooded frame, but it will receive this copy along with all other 

active ports. Switches never drop frames unless the CRC for the frame is incorrect. The 

frame will never be forwarded to a broadcast MAC address of ffff.ffff.ffff unless that is 

the intended destination.

 191.  A.  Since there is nothing in the current MAC address tables or either switch, the incoming 

frame on Switch A will be flooded to all ports. This will include the port connecting 

Switch B. Switch B in turn will flood the frame to all ports. However, nothing will respond 

on Switch B since the host is on Switch A. Although this is wasted traffic, it is minimal 

since it is only the initial communications. Switch A solely flooding traffic would happen 

under two specific conditions: the interface of Fa0/3 on Switch A was down or Switch 

B has an entry for the destination host. Neither of these conditions exists in the exhibit; 

therefore, both switches will flood the frame out all active ports. Switch A cannot switch 

the frame to Fa0/2 (Computer C) because the MAC address table is empty for that port.

 192.  A.  Since there is nothing in the current MAC address tables or either switch, the incoming 

frame on Switch A will be flooded to all ports. This will include the port connecting 

Switch B. Switch B in turn will flood the frame to all ports. Switch A solely flooding traffic 

would happen under two specific conditions: the interface of Fa0/3 on Switch A was down 

or Switch B has an entry for the destination host. Neither of these conditions exists in the 

exhibit; therefore, both switches will flood the frame out all active ports. Both switches 

will flood the frame because both of their MAC address tables are empty. Switch B cannot 

switch the frame to Fa0/3 (Computer F) because the MAC address table is empty for that 

port.

 193.  C.  The only time a frame is dropped is when the cyclic redundancy check (CRC) 

calculated against the frame’s payload deems the frame corrupt. If the destination MAC 

address is not known, it will be flooded to all active ports on the switch, but it will not be 

dropped. If the source MAC address of the frame is unknown in the MAC address table, 

the source MAC address will be learned on the incoming port. If the destination MAC 

address exists in another switch’s MAC address table, then it will either be forwarded 

to that switch or flooded to that switch if the current switch has an empty MAC address 

table for the entry.

 194.  A.  The forward/filter function of a switch is used to look up the destination MAC 

address in a MAC address table and decide the egress interface for the frame. If the MAC 

address is not in the table, the frame is forwarded out all of the interfaces. When the client 

responds, its source MAC address will be recorded in the MAC address table for future 

lookup. Address learning is performed when frames enter into the interface; the source 

address is written to the port it is observed on. Loop avoidance employs Spanning Tree 

Protocol (STP) to block redundant links to prevent loops. Frame flooding is a default 

action of the forward/filter process when a destination frame is unknown.

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Chapter 1: Network Fundamentals (Domain 1)