Need help with a Q in the ODOM book?

Beany

Morning,

need this clearing up,

the following question in the book:

Q) 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, connects to 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?


a. The switch forwards the frame out interface Fa0/1.
b. The switch forwards the frame out interface Fa0/2.
c. The switch forwards the frame out interface Fa0/3.
d. The switch discards (filters) the frame.

Answer = a+ b

my answer is B. The reason why i picked B is because when the frame was sent from PC1 to PC2, the switch learnt the mac-address AND interface for PC1 and placed it in his table. So when PC3 sends a frame to PC2, the switch should send it to PC2 port because it knows PC2 isnt on the PC1 interface and it doesnt send a frame back to the interface that sent the frame. This was my thinking but the actual answer is A+B, why is this?

can someone please explain?

many thanks

palit

Here the mac address table has only one entry when PC3 sends a frame to PC2.The entry of MAC 1111.1111.1111.corresponding to interface Fa0/1.When the next frame sent by PC3 reaches the switch the mac address table doesn't know what to do with the frame as it has got no entry in the mac table (entries are learned from the source MAc address)so it forwards the frame to all interfaces other than the one from which it is received.The switch received the frame from interface fa0/3.

iamme4eva

A switch only adds MAC addresses learnt from the SOURCE MAC address within the frame.

If a switch doesn't have an entry for a MAC address in it's table, then it sends the frame out of every interface except the one it received it on.

Beany

palit wrote: »

Here the mac address table has only one entry when PC3 sends a frame to PC2.The entry of MAC 1111.1111.1111.corresponding to interface Fa0/1.When the next frame sent by PC3 reaches the switch the mac address table doesn't know what to do with the frame as it has got no entry in the mac table (entries are learned from the source MAc address)so it forwards the frame to all interfaces other than the one from which it is received.The switch received the frame from interface fa0/3.

oh okay. quite strange i thought it might be clever enough to realise not to send it out interface Fa0/1 knowing it's a different MAC address

fadhil

it is true that the answer is a+b
because when pc1 send a frame to pc2,a switch check on mac table if here is for mac address of a sender. if not the switch forward a frame to all ports except for the port which has received the frame and copy a mac address of a source place into mac table with its interface, hence in a table there is only one mac address.
when pc3 send frame the switch receive and look at its table, no entry for mac address of pc2 hence forward the the frame to all ports except for the port which has received the frame

Zartanasaurus

Beany wrote: »

oh okay. quite strange i thought it might be clever enough to realise not to send it out interface Fa0/1 knowing it's a different MAC address

Why should it assume there's only 1 MAC address associated with a port?