Switch Frame Forwarding Question

rob42

With regard to the W. Odom Book: ICND1 100-101.

I've paraphrased a little, just to make this more readable.

Q5. Page 145
We have a switch with the following devices connected...
PC1, MAC Address 1111.1111.1111 connected to fa0/1
PC2, MAC Address 2222.2222.2222 connected to fa0/2
PC3, MAC Address 3333.3333.3333 connected to fa0/3

The switch has an empty CAM Table.

PC1 sends a frame DEST=2222.2222.2222 to the switch.

The next frame that the switch gets is from PC3, DEST=2222.2222.2222

What does the switch do with these frames?

Answers:
The switch forwards the frame out of fa0/1
The switch forwards the frame out of fa0/2

How can this be?

Surly, the switch would not send the 1st frame out of fa0/1, as it will have updated its CAM Table with MAC Address 1111.1111.1111 Port fa0/1, but would then flood the frame to fa0/2 and fa0/3.

I can see that the next frame would indeed be sent out of fa0/2 only, as the switch will now have updated its CAM Table with MAC Address 3333.3333.3333 Port fa0/3

For the 1st answer to be correct, would the switch would not have to flood the frame before updating its CAM Table?

What am I not getting here please?

clarson

for the information you provided, both those answers are wrong.

cam table empty
first frame from 1111.1111.1111 on fa0/1 goes into the cam table and the frame flooded out all other ports but fa0/1 because 2222.2222.2222 isn't in the cam table
next/second frame from 3333.3333.3333 on fa0/3 goes into the cam table and the frame is flooded out all other ports but fa0/3 because 2222.2222.2222 isn't in the cam table

clarson

ok looked the question up. and the answer is right.

as above the the switch floods the frame from 3333.3333.3333 to all other ports but fa0/3

and ports fa0/1 and fa0/2 are included in "all other ports but fa0/3"

so the frame is forwarded out fa0/1 and fa0/2. but also fa0/4, fa0/5 and etc.

rob42

Thanks for your response, clarson.

I'm still not getting it.

The scenario is (so far as I understand) PC1 [MAC 1111.1111.1111] sends a Frame to PC2 [MAC 2222.2222.2222]

SW1 receives the Frame from PC1 on its fa0/1 Interface.

SW1 updates its CAM Table with MAC Address 1111.1111.1111 Port fa0/1

SW1 Floods the Frame {so, sends it to fa0/2 & fa0/3, the only other active connections.}

PC3 [MAC 3333.3333.3333] sends a Frame to PC2 [MAC 2222.2222.2222]

The next Frame the Switch gets is from PC3 on its fa0/3 Interface (and so has not yet received a response from PC2).

SW1 receives the Frame from PC3 on its fa0/3 Interface.

SW1 updates its CAM Table with MAC Address 3333.3333.3333 Port fa0/3

SW1 Floods the Frame {so, sends it to fa0/2, the only other active connection. SW1 has both fa0/1 and fa0/3 in its CAM Table}

I just can't see why SW1 would flood either Frame to fa0/1??

clarson

because it is sending the frame to 2222.2222.2222 and that address isn't in the cam table.

1111.1111.1111 might not be the only mac address associated with fa0/1. it could have a hub/switch connected to that port and therefore could have several mac addresses coming in on that port. So, the switch floods the 2222.2222.2222 frame to all ports not fa0/3 (and of course on the same vlan). And, that includes fa/01.

rob42

Again, I thank you for your reply and I respect your explanation.

I can see that you are technically correct, but that is not what Q5 on Page 145 says. There's no mention of any other hubs/switches or vlans.

I'm also still confused as to why the first Frame (PC1 to PC2) is not flooded out of all active ports (that is fa0/2 & fa0/3), accept the one on which it was received (fa0/1).

I'm not being deliberately obtuse here, I just don't see how fa0/3 is not apart of the flooding when SW1 receives the first Frame from PC1, which is destined for an unknown MAC Address.

I can see why the second Frame could be flooded to fa0/1 & fa0/2, but not to fa0/3 because that was the port on which it was received, but I can't see how the first Frame is not subject to the same rules: flooded out of all active ports (that is fa0/2 & fa0/3), accept the one on which it was received (fa/01).

This is the only DIKTA question on page 145 that I'm hung-up on; the other eight I answered, 100%. I've even re-read Chapter 6, but I still can't get past this.

clarson

I can see that you are technically correct, but that is not what Q5 on Page 145 says. There's no mention of any other hubs/switches or vlans.

That is right. But, sw1's cam table doesn't have an entry for 2222.2222.2222. Which means it could be on any port but the port it was received on. which is why it floods the frame to all port that aren't fa0/3. which are fa0/1 and fa0/2.

The same applies for the first frame. no cam table entry for 2222.2222.2222. so the switch floods that frame out all ports but the one it was received on. it was received on fa0/1. So, it gets flooded out fa0/2 and fa0/3. Just like you thought it would be.

rob42

The same applies for the first frame. no cam table entry for 2222.2222.2222. so the switch floods that frame out all ports but the one it was received on. it was received on fa0/1. So, it gets flooded out fa0/2 and fa0/3. Just like you thought it would be.

Cool, thanks; that's good.

But, the two answers given are...

a. The switch forwards the frame out of fa0/1
b. The switch forwards the frame out of fa0/2

... no mention of fa0/3.

If the there were three answers, a. b. c. (c. being: The switch forwards the frame out of fa0/3) then I'd be happy, but no, just a. and b.

Do you see my problem?

clarson

the question asks what happens to the next (or second) frame received. and it is flooded out all ports not fa0/3. which is fa0/1, fa0/2
what you need to know for this question is that when the frame from 1111.1111.1111 is received on port fa0/1. It is the 1111.1111.1111 mac address and port that is put into the cam table. and not 2222.2222.2222 which is the destination port.

and actually c) is fa0/3 and d) is the frame is dropped. and they are not correct.
and c is incorrect because the frame flooded out all port not fa0/3. and d is incorrect because the frame is flooded not dropped.

I don't know why you think the frame is being flooded out fa0/3. It doesn't because the frame came in on port fa0/3.

yes the first frame was flooded out of all ports but fa0/1 which includes fa0/3. But, the question doesn't ask what happens to the first frame. It only asks what happens with the second frame.

rob42

O.M.G! I can't believe that I missed something so obvious

You're right.

Thank you for sticking with me on this and I'm sorry it's taken up your time.

Cheers bud.

clarson

good prep for the exam. read the questions well. going too fast can lead to mistakes. Don't want that on the exam.