I don't understand the answer on a question about a switch forwarding decision

RichardNieland · March 2016

Dear Colleagues,

I am preparing for the ICND1 exam and use the Cisco Official Certification Guide. The first question in Chapter 6 Review is a question about how a switch decides to forward a frame destined for a known unicast MAC address.

a. It compares the unicast destination address to the bridging, or MAC address, table.
b. It compares the unicast source address to the bridging, or MAC address, table.
c. It forwards the frame out all interfaces in the same VLAN except for the incoming
interface.
d. It compares the destination IP address to the destination MAC address.
e. It compares the frame’s incoming interface to the source MAC entry in the MAC
address table.

I picked answer A, but it should be B according to the book. Can somebody please explain me why my answer was wrong?

ITsubced · March 2016

Hello RichardNieland,
Please consider that I am a ICND1 student, and not certified yet.

I think you misread the book :
I have the same book, so i answer the question 1 on 144 with answer A like you did.
I check the answers wrote in p 146 (page down) : 1) A.

So, according to the book, it is the answer A.
So you are right, the answer is A.

To go a little further, you need to know that before sending a frame, the switch compare the unicast source address with the unicast destination address.
If they are the same, the frame is not forwarded.

bhcs2014 · March 2016

Hello Richard. Can you post the specific question? Looks like just a matter of question wording.

Switches do populate their CAM tables by looking at source MAC addresses of incoming frames. This could explain B being the answer. A could also be correct because the switch looks at the CAM table to decide which port to forward the frame out of. When a frame is received it will compare both the source/dest. MAC to the CAM table..

RichardNieland · March 2016

Dear ITsubced and bhcs2014

Thanks for your replies! I use the following book to prepare for the ICND1 exam:

Cisco CCENT/CCNA ICND1 100-101
Official Cert Guide, Academic Edition
Wendell Odom, CCIE No. 1624
Copyright © 2013 Pearson Education, Inc.

And indeed it's the question on page 144

1. Which of the following statements describes part of the process of how a switch decides
to forward a frame destined for a known unicast MAC address?
a. It compares the unicast destination address to the bridging, or MAC address, table.
b. It compares the unicast source address to the bridging, or MAC address, table.
c. It forwards the frame out all interfaces in the same VLAN except for the incoming
interface.
d. It compares the destination IP address to the destination MAC address.
e. It compares the frame’s incoming interface to the source MAC entry in the MAC
address table.

However on page 147, the answer is 1 B in my book.

Answers to Review Questions:
1 B 2 C 3 A 4 B 5 A and B 6 A 7 A, B, and C 8 B and D 9 D

Anyway, I understand that switches look inside their MAC/CAM tables to match the destination MAC address of an incoming frame and forward it to the outgoing port. It's also true that the switch fills it's table by examining the source MAC address of incoming frames, but that's not the really question - it says to forward a frame destined for a known unicast MAC address. what happens is that the timer for this entry is only set to 0.

I guess my book is outdated

HAMP · March 2016

RichardNieland wrote: »

I guess my book is outdated

Its not outdated, it just has errors in it(as do most books).

Goto the website and download the Errata for your book, it has most of the needed changes people have found and submitted.

BTW,

Good catch with knowing the right answer!

RichardNieland · March 2016

Hi HAMP,

Thanks. It's certainly not outdated and I enjoy very much reading the book. However, the wrong answer was not mentioned in the errata document. There is another question in the same switching review that I don't understand.

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.

There are 3 PCs connected to a switch that has an empty MAC table. I guess the first thing it does is checking its table (notices that it has no entries) and flooding the frames out of all its interfaces (except the one on which it has received the frame). The source MAC addresses from the frames that return from the PCs are then entered to build the MAC table with the corresponding incoming interfaces. Subsequently answer A en B apply. Is my thinking above correct?

loki2043 · March 2016

To my knowledge the answer would be B and nothing else. The switch already has both PC1 and PC2 assigned in the CAM table thus it would just update the CAM table with PC3 info and then forward the packet out fa0/2. unless the switch has been idle quite some time and it cleared out the whole CAM table before it received the frame from PC3.

RichardNieland · March 2016

Dear loki2043,

According to the book, the correct answers are A and B but not explained. It starts with no dynamically learned MAC addresses, so in my opinion it first starts to broadcast to use the source MACs of the replying, attached PCs to fill its table. Then answer A and B is applied. Of course my picture could be wrong...

Best regards,

Richard

RichardNieland · March 2016

The switch begins with an empty MAC table (no dynamically learned MAC addresses):

PC1 is sending to PC2; the switch learns source MAC 1111.1111.1111 is on Fa0/1, however it doesn’t know from its table at which port PC2 is attached, so it copies the frame out of Fa0/2 and Fa0/3 (flooding) and learns from the reply of PC2 that it is on Fa0/2 (source MAC 2222.2222.2222 is on Fa0/2 is written in its table) - answer B applies

Now PC3 is sending to PC2; because the switch now has an entry in the MAC table, knowing that PC2 is on interface Fa0/2, the frame is immediately sent - matching answer B again.

It seems that only answer B is correct.

loki2043 · March 2016

You need a different book.... lol

theodoxa · March 2016

loki2043 wrote: »

To my knowledge the answer would be B and nothing else. The switch already has both PC1 and PC2 assigned in the CAM table thus it would just update the CAM table with PC3 info and then forward the packet out fa0/2. unless the switch has been idle quite some time and it cleared out the whole CAM table before it received the frame from PC3.

Read the wording carefully. It is a tricky question. PC1 sends a frame to PC2. When the frame hits F0/1 on the switch, the switch adds PC1's MAC address to the CAM table and floods the frame out all interfaces except F0/1. Now, normally PC2 would send a reply and its MAC would be added to the CAM table. BUT, the text says that the next frame received is from PC3 destined for PC2. So, what it is saying is that before PC2 was able to reply, PC3 also sent it a frame. Since, PC2 has yet to send a response, the switch does not yet know which port it is on, so it has to flood the frame received from PC3 out F0/1 and F0/2.

RichardNieland · March 2016

Thanks Theodora,

Your explanation makes sense! But when PC1 is causing the switch flooding the frame out all interfaces except F0/1, it goes out of Fa0/2 and Fa0/3, matching answers A and C. Only when PC3 finally induces the switch to flood searching for PC2 - in the second part of the question - the correct answers A and B apply.

loki2043 · March 2016

theodoxa wrote: »

Read the wording carefully. It is a tricky question. PC1 sends a frame to PC2. When the frame hits F0/1 on the switch, the switch adds PC1's MAC address to the CAM table and floods the frame out all interfaces except F0/1. Now, normally PC2 would send a reply and its MAC would be added to the CAM table. BUT, the text says that the next frame received is from PC3 destined for PC2. So, what it is saying is that before PC2 was able to reply, PC3 also sent it a frame. Since, PC2 has yet to send a response, the switch does not yet know which port it is on, so it has to flood the frame received from PC3 out F0/1 and F0/2.

D'oh! Okay that makes sense.

I don't understand the answer on a question about a switch forwarding decision

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