Switching query from Odoms Book

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Hi All,

A question and answer from Odoms book - I'm a bit lost with it. I would have initially thought the frame came in on a different interface from the one in mac address-table output, so it would update this? The answer is B btw according to the book. Can someone explain this? Does the VLAN play a part in this?

A frame just arrived on SW2's Interface Fa0/3, with source MAC 1111 and destination MAC 2222. Interface Fa0/3 is assigned to VLAN 2. Considering the output from SW2 below, which of the following answers must be true regarding what the switch does with this frame.
SW2# show mac address-table
VLAN
1 1111 Dynamic Gi0/2
1 2222 Dynamic Fa0/13

A. The switch will discard the frame
B. The switch will forward the frame out port Fa0/13
C. The switch will change the existing entry for 1111 as seen in the command output, only changing interface to fa0/3
D. The switch will forward the frame out both Gi0/2 and Fa0/13
E. The switch will add a new entry to the MAC table for 1111.

Aman_saini

In my point of view option C is correct because both ports are in different vlans forwarding to Fa0/13 port will not be considered as far as there is no inter valn routing. There will be only change in Mac-add table for 1111 device. If i am wrong correct me please.

NetworkVeteran

Odom is wrong. Errors happen in big books.

Since there are no entries for VLAN2, an entry will be created for 1111 / fa03 / vlan2 and the frame will be forwarded out all other ports on vlan2.

A-D are totally wrong.

E is the best answer.

Carl_S_901

NetworkVeteran wrote: »

Odom is wrong. Errors happen in big books.

Since there are no entries for VLAN2, an entry will be created for 1111 / fa03 / vlan2 and the frame will be forwarded out all other ports on vlan2.

A-D are totally wrong.

E is the best answer.

I've been scratching my head for the last 10 minutes on this one too. Odom's full answer in Appendix A is:

"B. The command output lists only two entries, and both of those are for VLAN 1. The command, show mac address-table, lists dynamic MAC table entries for all VLANs, so you can deduce that there are no VLAN 2 MAC table entries. As a result, the switch will flood the frame in all ports in VLAN 2, except the port in which the frame arrived. As for learning, because Fa0/3 is in VLAN 2, the switch, not having a MAC table entry for 0200.1111.1111 in VLAN 2, will add an entry listing VLAN 2, 0200.1111.1111, interface Fa0/3."

which supports what you said and what I thought.

E does seem like the best answer.

If it said choose TWO answers I would say A as well as wouldn't the frame be dropped with no other ports in VLAN 2 to flood it out?

NetworkVeteran

Amani wrote:

C. The switch will change the existing entry for 1111 as seen in the command output, only changing interface to fa0/3.

In my point of view option C is correct because both ports are in different vlans forwarding to Fa0/13 port will not be considered as far as there is no inter valn routing. There will be only change in Mac-add table for 1111 device. If i am wrong correct me please.

The mac-address-table key on Cisco switches is <vlan>+<mac>, in other words it has a per-vlan forwarding database, so receiving new information about vlan2+1111 doesn't affect an entry for vlan1+1111.

Carl_S_901 wrote: »

If it said choose TWO answers I would say A as well as wouldn't the frame be dropped with no other ports in VLAN 2 to flood it out?

It's possible fa0/3 is the only port on VLAN 2, however we cannot know that for certain from the available output. All we can be sure of is that no MAC addresses on VLAN 2 have been learned upto this point. For example, 1111 is on VLAN 2 and isn't in the table, but is certainly there. If you had to pick two, this would be the backup. If it were choose as many as are correct, then E would be the only provably correct answer.

Carl_S_901

There is a thread on the Cisco Learning Network about this question which I just replied to as well. You can find it here:

https://learningnetwork.cisco.com/thread/39445

Carl_S_901

NetworkVeteran wrote: »

It's possible fa0/3 is the only port on VLAN 2, however we cannot know that for certain from the available output. All we can be sure of is that no MAC addresses on VLAN 2 have been learned upto this point. For example, 1111 is on VLAN 2 and isn't in the table, but is certainly there. But if you had to pick two, this would be the backup.

OK. I was thinking that the output from the show mac address-table was the complete list but it very well could be a subset.

NetworkVeteran

Carl_S_901 wrote: »

OK. I was thinking that the output from the show mac address-table was the complete list but it very well could be a subset.

The output of show mac-address table lists only what MAC addresses have been learned. It is not a complete list of all connected devices, nor a complete list of all ports assigned to a VLAN.

Carl_S_901

NetworkVeteran wrote: »

The output of show mac-address table lists only what MAC addresses have been learned. It is not a complete list of all connected devices, nor a complete list of all ports assigned to a VLAN.

Doh.

Yes, you are obviously correct. Thanks for the follow up.