Cisco CCNA mock exam questions sample test – Question 160

.The exhibit displays the MAC address table of a switch in your network, along with the location of each device connected to the switch.
Which of the following frames will cause the switch to add a new MAC address to its table and forward the frame to all ports when the frame is received?

A. source MAC: 12-34-56-78-9A-BC, destination MAC: ff-ff-ff-ff-ff-ff
B. source MAC: ff-ff-ff-ff-ff, destination MAC: 12-34-56-78-9A-BC
C. source MAC: 12-34-56-78-9A-BF, destination MAC: 12-34-56-78-9A-BC
D. source MAC: 12-34-56-78-9A-BC, destination MAC: 12-34-56-78-9A-BF


Correct Answer: A

Explanation:
The only frame that will be handled in the specified way is the one with a source MAC of 12-34-56-78-9A-BC and a destination MAC of ff-ff-ff-ff-ff-ff. Since the source address 12-34-
56-78-9A-BC is not already in the MAC table, the switch will add it. It will forward the frame to all ports because the destination is the broadcast MAC address of ff-ff-ff-ff-ff-ff.
A frame with a source MAC of ff-ff-ff-ff-ff-ff and a destination MAC of 12-34-56-78-9A-BC is an impossible combination. That would mean that the frame is coming from all devices,
which is not possible.
The frame with a source MAC of 12-34-56-78-9A-BF and a destination MAC of 12-34-56-78-9A-BC would be sent to all ports because the destination MAC address is not in the MAC
address table. However, the switch would not add a new MAC address to the table because the source address is already in the table.
The frame with a source MAC of 12-34-56-78-9A-BC and a destination MAC of 12-34-56-78-9A-BF would not be forwarded to all ports because the destination MAC address is in the
table. The switch would add a new MAC address to the table because the source MAC address is not currently in the MAC address table.
Objective:
LAN Switching Fundamentals
Sub-Objective:
Interpret Ethernet frame format
References:
Cisco Press > Articles > Cisco Certification > CCNA Routing and Switching > Basic Data Transmission in Networks: MAC Tables and ARP Tables
How do Switches Work?