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Webcast-Catalyst9k
New Member

STP Root Port/Designated Port

I'm preparing to retake my CCNP Switching test right now and I ran across something in my reading that has me stumped.

It has to do with the designated port.

Lets say there are two switches, switch A and switch B. Switch A is the root bridge. Lets also say that the connection between switch A and Switch B has a hub on it. (Not a great design, I know)

If a packet comes in on the hub it will get transmitted out to both switch A and switch B. Since switch A is the root bridge, it also means that it is the designated port for that segment. So it should be the one to pick up the packet for forwarding, right?

Well, what if the packet destination is to a device on Switch B???

Switch B isn't blocking, because it's port it a root port. So do both switches pick it up and try to forward it?

I know I'm missing something basic here, but I just can't seem to find an answer.

Thanks!

3 REPLIES
Silver

Re: STP Root Port/Designated Port

Remember that both root ports and designated ports are in a forwarding state. Once STP has converged into a loop free topology all forwarding ports will forward traffic so in your case both switches will try to forward it but only the one with the end device will actually deliver the packet.

New Member

Re: STP Root Port/Designated Port

Hi,

Switch B isn't blocking, because it's port it a root port. So do both switches pick it up and try to forward it?

Response: Yes, but Switch A would NOT forward the packet where it is come from. And Switch B will forward the packet to the designated host.

New Member

Re: STP Root Port/Designated Port

Ok, that makes sense then. I was thinking that switch B wouldn't answer anything on that segment since it wasn't the designated port, and that didn't seem right at all.

The thought that both switches would receive the packet didn't seem right either until you pointed out the obvious that Switch A would see that the packet's destination belongs on to the segment that it just came from, so it would just drop it at that point.

Thanks!

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