MAC Learning

network489 · ‎06-18-2008

PC1, with MAC address 1111.1111.1111, is connected to Switch SW1's Fa0/1 interface. PC2, with MAC Address 2222.2222.2222, is connected to SW1's Fa0/2 interface. PC3, with MAC address 3333.3333.3333, is connected to Switch SW1's Fa0/3 interface. The Switch Begins with no dynamically learned MAC addresses, followed by PC1 sending a frame with a destination address of 2222.2222.2222. If the next frame to reach the switch is a frame sent by PC3, destined for PC2's MAC address of 2222.2222.2222. which of the following are true?

I selected answer "B" that stated, "The switch will forward the frame out interface Fa0/2. However, the book also stated that "A" was also correct because the switch will forward the frame out interface Fa0/1 as well.

I did not understand that because it incenuates that the switch had to broadcast to find where PC2 was located. Did not the switch learn PC2's MAC Addresses from ARP Responses as well?

ohassairi · ‎06-18-2008

you said PC1 sent a frame to PC2. sw does not know where is PC2 so it will broadcast the frame.

PC3 sent a frame to PC2, sw still does not know where is PC2. so it will broadcast it too. it only can know where is PC2 if PC2 answers PC1.

you said : Did not the switch learn PC2's MAC Addresses from ARP Responses as well?

the question suppose no ARP before communication: PC1 and PC3 know the MAC @ of PC2 (may be from cache)

crow930us · ‎06-19-2008

There is no listing for PC2 in the switches CAM table. When PC1 & PC3 send a request to the switch, the switch makes an entry in it's CAM table that associates port Fa0/1 with PC1 and port Fa0/3 with PC3. Because there is no entry in the CAM table for PC2, the switch will send an ARP broadcast out every port except the port it received the request on. When PC2 replies to the ARP request, at that point will the switch make an entry in it's CAM table that associates port Fa0/2 with PC2 and start unicasting frames to PC2.