Question

Unanswered Question
Mar 31st, 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, 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.

The correct answer according to book author is A and B

I selected only option B, because once switch has sent a frame of PC1 to PC2, now PC2 should be in its MAC table,

PLEASE ADVISE

Regards,

Junaid

I have this problem too.
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Hi Junaid

In my opinion none of the answers listed above are correct.

The switch starts with an empty CAM table (MAC Address Table, so it had no idea whcih ports any of the 3 PC's are found off. Then the switch receives a frame from PC1 destined for PC2.

The switch will make an entry in it's CAM table noting that the MAC Address 1111.1111.1111 is located out of port F0/1. The switch will then FLOOD the frame from PC1 out of all ports except the port the frame arrived on (F0/1).

The next frame the switch receives is from PC3 again looking for PC2. As (according to the question) the switch has not yet received a reply from PC2 to the frame from PC1, the switch has no idea which port PC2 is located off, so again the switch will make an extry in its CAM table noting that the MAC Address 3333.3333.3333 is located off of F0/3 and will FLOOD this frame out of all ports except the port the frame arrived on (F0/3)

Best Regards,

Michael

junshah22 Mon, 03/31/2008 - 22:08

Thanks Michael,

You are right, but in the question only two options are valid, like,

when switch received a frame from f0/3 destined for f0/2 it will broadcast the frame out all interfaces,, right,,

If switch will broadcast the frame out all interfaces then port f0/1 and f0/2 will receive this broadcast, and the answer is showing this two options

a and b

Regards,

Junaid

Hi Junaid

In the question in you first port it is stated that the switch starts with an empty CAM table.

The switch then receives a frame from PC1 with a destination MAC of 2222.2222.2222.

The switch builds its CAM table based on frames received, so the switch will enter into its cam table that the MAC address 1111.1111.1111 can be found off of port F0/1. The frame is then FLOODED out ot every port except the port the frame arrived on (F0/1)

The question then states that the NEXT FRAME the switch receives is a frame from PC3 again destined for PC2.

There is no mention of the return from from PC2 to PC1, so the switch has no idea what port PC2 is located off.

The switch will make an entry in is CAM table that the MAC address 3333.3333.3333 can be found off of port F0/3 and will FLOOD the frame out of every port except the port the frame arrived on (F0/3).

In my opinion this question is at best ambigious and at worst misleading.

IMHO the statements

a. The switch forwards the frame out interface Fa0/1.

and

b. The switch forwards the frame out interface Fa0/2.

suggest unicast frames where this is not the case.

It is my understanding that when a switch FORWARDS a frame it is unicast. When a switch FLOODS a frame, it is broadcast out of every port except the port the frame arrived on. And when a switch FILTERS a frame, that frame is dropped by the switch.

So I would therefore disbute that answers A & B are correct as incorrect terminology is used.

Just my 2 cents worth.

Best Regards,

Michael

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