STP Confusion

mahmoodmkl · ‎04-27-2008

Hi Guys,

I have tried very hard to understand STP but i strunk at one point.Please see the diagram below,this is reference to the STP diagram in cisco BCMSN.I think i am missing a little thing.

suppose switch A and B are connected to each other with ports 1 and 2 on both the sides and both the ports are configured as trunk links.

On Switch A i have a node suppose node 1 connected on port 3 and on switch B i have a node suppose node 2 conected on port 3.

If node 1 want to communicate with node 2 the request will be forwarded to switch B from ports 1 and 2 and switch B will learn that it can reach node 1 from ports 1 and 2 node 1's mac-address will be learned on port 3 now when node 2 replies back the packet is going to come from ports 1 and 2 from switch B to ports 1 and 2 on switch A here i dnot understand how the loop is forming.Please exlpian.

Thanks

Mahmood

Joseph W. Doherty · ‎04-27-2008

When node 1 initially sends to node 2, node 1's switch doesn't know what port to use so it floods the frame to all ports; which includes your two trunk ports, 1 and 2 (assuming the trunks pass VLAN traffic for node 1).

When node 2's switch sees the inbound frame, if it knows node 2 is on port 3, it will forward both trunks frame to port 3. However, if it doesn't yet know where node 2 is connected, it too will flood both inbound frames to all ports; i.e. it will reflect the inbound frames back to ports 1 and 2. (I.e. it knows the source MAC is on ports 1 and 2 but doesn't know where the destination MAC is.)

When node 1's switch sees the reflected frames, it will flood them to all ports since it still doesn't know where the destination MAC is. Now the initial frame will just loop because of unknown destination on both switches.

As above, if node 2's switch did know where node 2 was connected, it wouldn't need to flood the initial frame, but also consider broadcast frames such as node 1 ARPing for node 2. The broadcast frame would be flooded back and forth between the two switches.

PS:

Did you intend to have an attachment on your post?

Istvan_Rabai · ‎04-28-2008

To make a little addition to Joseph's post:

Actually, there will be frames looping in 2 counter-rotating directions, because the unknown unicast frame from node 1 will be flooded on both port 1 and port 2.

Cheers:

Istvan