Confusion about spanning-tree

[dominique.rossignol]

hi everybody,

thanks for your support

(I'm french)

I open this discussion to have some more information about how spanning-tree send the BPDUs between switches: i never find this information (?)

SPT is a L2 protocol, but do the BPDU need another L2 protocol like ethernet to send the BPDUs ?  I read that ethernet LLC and/or SNAP frame have SNAP IDs to carry BPDUs (ex:0x010B)

If you look at fields in BPDU packet, you don't find any MAC addresses fields

So, here is what i don't understand very well: if the BPDU has to be carried in the payload of another L2 frame (like ethernet), well, it is not a L2 protocol (it looks like L3, no ?)

Or, are BPDUs sent from port to port ?

There is confusion in my mind about that

Thank you

Regards

[Dennis Mink]

Dominique,

A switch sends BPDUs using  the MAC address of the port it is sending the frame from and a multicast MAC address as the destination:  01:80:C2:00:00:00

It is definitely not Layer 3, because it does live at the Data Layer (L2), and STP is used to prevent L2 loops.

So because really ethernet is point to point physically, even though the BPDU uses multicast it is always only gonna be picked up on one physical port and in that context it is really sent from port to port.

Please rate if useful

*great question BTW

[dominique.rossignol]

hi Dennis,

thanks for your reply (once again,  i'm french in new caledonia, so my english is not as good as it should be)

I did not know that ports have their own MAC address (in the control plane is not it ? it will be source of new questions in other discussions)

I read this topic about port MAC

https://learningnetwork.cisco.com/thread/15380

Ok for your answer, but

1) Look at this other topic

https://learningnetwork.cisco.com/docs/DOC-26145

you will see a packet capture of an IEEE 802.3 frame showing a BPDU frame of 802.1D; that is why i open the discussion; why is there a bpdu in an ethernet frame ?

2) in BPDU L2 frames there is no MAC addresses fields....i wonder how the switch (the port ?) knows it is a BPDU. Everywhere, I found logical explanations of how STP work to prevent loops (cost, root bridge, root port, alternate port, etc...), but i didnot found anywhere the L2 explanation on what happens exactly between ports switches to achieve this result. Does a port receive any reply using its unicast port MAC address from another port in the STP process ?

So it is yet a bit mysterious ....do you have docs about L2 mechanism of STP ?

Regards

[Dennis Mink · Accepted solution]

Dominique,

first of all., nothing wrong with your english mate.

I think I am starting to understand your question. so in the document there is a packet capture of a L2 ethernet frame received on the multicast MAC, In it is a STP BPDU which is pretty much the payload of the ethernet frame. 

So in that same document it discusses SNAP, an I think the trick to understand the answer to your question is to understand SNAP, so snap pretty much announces which protocol follows int the frame's payload e.g VTP, STP, CDP etc.  sure IPv4 has its own protocol ID as well, but that is L3.

If that makes sense,

If not, hit me again