question about Spanning Tree Protocol, root bridge / designated port election

[Alan_4]

Hello everyone,

I have added an image to this thread, and i was wondering if the root, designated and blocking ports are correct.

Switch A connected to the root bridge switch C has a higher path cost, that is why I made it look like it is in a blocking state, but to be honest I think that I have never seen a forwarding root bridge port being blocked on the otherside.

Is this correct, or does the priority go before the root path cost, what could make the root port of switch A to switch D a designated port and the port from switch A to switch C a root port I guess?

Any help is appreciated, also I do not own this picture and I do not own any rights for this picture it is from the internet, I just added the root bridge, dp, rp, fw, blocking state texts.

[Alan_4]

bump, anyone?

[nixpengu1n]

Hello,

Priority you configure on a switch affects only root bridge election. STP tree is calculated using path cost which is affected by speed & duplex of each port participating in STP process + interface number.

More information could be found here:

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/5234-5.html

[Kevin SAS · Accepted solution]

Hi,

Switch A connected to the root bridge switch C has a higher path cost, that is why I made it look like it is in a blocking state, but to be honest I think that I have never seen a forwarding root bridge port being blocked on the otherside.

-> off course you can, if rpc is better with other switch like this picture.

Is this correct, or does the priority go before the root path cost, what could make the root port of switch A to switch D a designated port and the port from switch A to switch C a root port I guess?

Priority is used to elect root switch, you can change port cost on switch A toward C with a value less than 8 and port will become RP. (do not be confused with port priority which something different)

In any case, your scheme is correct on all point.

To sum up that, always remember how stp is working, it considers in following order these informations :

- root bridge ID (priority and MAC if priority are at default value), so in your case C is the root bridge

- root path cost (4 for Gb link and 19 for 100mb for classic stp pre 2004), you said something about switch A. Better RPC is through switch D, RPC = 8, so port on swA toward C is blocking

- sender bridge id (SBID)

- sender port ID (SPID)

- receiver port id (RPID)

Hope it helps you