Unknown device mac address listed in switch table on start out

[arturochari]

Hi all!

After playing a little bit with a topology randomly created in packet tracer, I figured out that the switch (Switch0) stores a unknown device mac address on start out, wich I think that it belongs to a indirectly connected PT-Bridge interface. (See topology)
I dont know if I am right. But the Switch0 also stores the mac address interface from the other switch (Switch1) on start out. I can check it by seeing this mac address in the Switch1 interface (Fa0/1).
I have not chance to verify the bridge mac addesss to argue it, and I am not sure that bridge interfaces has got mac addresses assigned.
Any comment that will clear this issue will be welcome!
Thanks in advance!

Arturo.

[omz]

Hi

switch learn the mac-address based on the source of incoming frames.

in your example - 

switch0 is showing 2 mac addresses it learned dynamically. 

its learns these mac-addresses from spanning-tree frames. Switch1 and Bridge0 are the only other devices that would generate spanning-tree frames. And hub will just forward it out of all interfaces.

mac-address learned on switch0 -  Fa3/1 would be the mac-address of the interface on switch1

and mac-address on Fa0/1 would be the mac-adress of the bridge 

if you switch to simulation mode and open the packets you should be able to see the mac-addresses 

hope this helps 

[omz]

[Martin L]

that appears to be DTP frame sent by Bridge 0 which should have MAC assigned to each of his interfaces. Bridge is like a switch just with fewer ports. DTP role is to figure out port type (access, trunk, dynamic, auto,etc). You should see DTP sent by switches as well.

about 10 seconds after you see DTP sent by Bridge 0, you will see Switch1 and then Switch 0 sending its own DTPs.

DTP, CDP, STP, VTP are so called Control management frames; they are automatic -unless u turn they off-and helping networking devices to stay afloat.

Regards, ML
**Please Rate All Helpful Responses **

[omz · Accepted solution]

sure you can see the mac address in any L2 frame.. I don't think this where switch0 learned the mac address.

the switch0 doesn't bootup until around 10s

then the ports go through listening ..  learning  .. until around 30s nothing get populated in the switch0 mac address table.

dtp frame at switch0 shows the destination mac-address 0030.f238.99d6 so the switch will not learn .. switch learns on source address in the incoming frame.

 at around 30s switch0 sees a STP frame with the source of bridge0 

This is where the switch ports are in the learning state and learned the mac address of bridge0

[Martin L]

I see, so you saying that Bridge MAC should not be in MAC address-table on any switches. possible bug in PT

"dtp frame at switch0 shows the destination mac-address 0030.f238.99d6 " <<Wrong, it is source MAC of Bridge0. Destination is all Cs, 0100.0ccc.cccc

Only Blocking port does not send BPDUs, but listens to BPDUs. During Listening and Learning BPDUs are flowing both ways. 

[arturochari]

Thanks a lot for the very explained reply!

I also tried to trace the flow packets at simulation mode and I went to inspect at the STP packets. So I can argue that the two bridge interfaces has got mac addresses. Am I right? 

Eth0/1: 0030.f238.99d6

Eth1/1: 0060.3E80.04C0

If I am right, so would be useful that next versions of PT will add this information at Bridge interfaces.
What do you think about it?

Warm regards!

Arturo.