flooding in switch cisco

[adrianoap_rj1]

dear colleagues

i am with a doubt, about a question of the book CCENT/CCNA ICND1 of the author Wendell Odom third edition as depict:

5. 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? (Choose two answers.)

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.

The answer of the book is A and B, the author say that have two answer, but in my understand the PC1 send a frame to switch with address to PC2, so the switch receive the MAC of the PC1 and after the switch send a flooding for all PCs (PC2,PC3),The PC2 receive the request of PC1 and reply with your MAC that also is record in MAC table, in second situation, the PC3 send a frame with destination to PC2, as the switch already have the MAC in your table, thus the switch send directly to PC2, the answer will be only letter B, the would be wrong according the author.

I am right?

Thanks for your help.

[Cisco Freak]

Hi Adrian,

The switch will send the from PC to both F0/1 and Fa0/2.

Note that the switch will write a MAC address to its table based on the source MAC.

In the stated situation, the switch has learnt MAC of both PC1 and PC3 as they have sourced frames towards switch. PC2's MAC is still not in the MAC address table as it has not sourced packets till now. So the switch will flood the packet through all the switchport belonging to the same VLAN and over the trunk ports also.

CF

[adrianoap_rj1]

Hi, 

this issue really confused me, I need to read more carefully not to miss and do more such questions, really appreciate the help forum.

thanks.

[Valeriehsh]

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@Cisco Freak wrote:

Hi Adrian,

The switch will send the from PC to both F0/1 and Fa0/2.

Note that the switch will write a MAC address to its table based on the source MAC.

In the stated situation, the switch has learnt MAC of both PC1 and PC3 as they have sourced frames towards switch. PC2's MAC is still not in the MAC address table as it has not sourced packets till now. So the switch will flood the packet through all the switchport belonging to the same VLAN and over the trunk ports also.

CF

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I'm new to Networking and I just came across this question in the book too. I'm wondering why not B & C then?

When you mentioned flooding, I guess you meant flooding from fa0/3?

But since the switch received from fa0/1 first, fa0/1 would have done the flooding first isn't it?

Thanks!

[Richard Burts]

First let us review some basics about the layer 2 switch behavior. This question is fundamentally about the switch behavior when it is attempting to forward a frame for which it does not know where the destination mac address is located. In this case the switch will forward the frame to every port in the vlan except for the port on which the frame was received. Since the frame was received on fa0/3 then the switch will not forward the frame to fa0/3 and C is not a correct choice.

You ask about the frame from fa0/1. And you are correct that this frame would have been flooded also. In that case it would have been forwarded to fa0/2 and fa0/3. But the original question  was not asking about that frame. I believe that the point of having the question mention the frame from fa0/1 is to make the point that the switch already learned the mac address associated with fa0/1 but floods the frame from fa0/3 to fa0/1 even though the switch already knows the mac address associated with fa0/1.

HTH

Rick

[Valeriehsh]

I see! Got it! Thanks!

[Richard Burts]

You are welcome. I am glad that my explanation was helpful. This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.

HTH

Rick

[Jon Marshall]

The book is right.

From the question -

If the next frame to reach the switch is a frame sent by PC3

You have assumed that PC2 has sent a response to PC1. And if it had done then you would be right and the answer would be B only.

But the question makes no mention of a response from PC2.

The question says PC1 sent a frame and then the very next frame came from PC3. The switch has not received a frame from PC2 so it doesn't know on which port the mac address for PC2 is.

Jon