stp

ntmanjunath · ‎02-18-2008

I connected one layer 3 switch to Hub as shown in the attached fig.I want to know which port will be come designated port, non designated port ,root bridge and root port.

fred.mancen · ‎02-18-2008

Hi.

As the hub doesn't have STP running, the switch itself will block one of the ports ramdomly, since the ports have the same hardware (speed, media type, cost) and settings. But if you prefer, you can set one of the ports to be the root port, just changing the cost on the port settings, using the command "spanning-tree cost ", where n is the cost you want to assign to the port.

Regards.

Francois Tallet · ‎02-18-2008

First, I'd like to make sure that your ports f0/0 and f0/1 are really switchports, else STP is not relevant in your setup.

Else, as Fred mentioned, the hub will not run STP and just treats BPDUs as data traffic. So the switch will have to block one of its port based on its own information.

Basically, the port that sends the "Best" BPDU will end up designated forwarding, while the other will be alternate (blocking). This is done in a deterministic way, by comparing the content of the BPDU. The fields of a BPDU sent by the switch are: Root ID, cost to the Root, Sender Bridge ID and the Sender Port ID. Of course, the three first parameters are identical, which means that the tie breaker will be the Sender Port ID. Basically, the port with the numerically lowest priority will be designated, while the other will be alternate. So you can influence the location of the blocked port by tuning the port priority.

Regards,

Francois