port priorirty or port cost?

[Marvin Tapessur_1]

Can't seem to figure out what the difference is. I'm not talking about the RSTP global priority but on port level.

Can anybody here explain this to me?

[Kell van Daal · Accepted solution]

Hi Marvin,

The difference is as following:

The port(s) with the better priority will allways be used for forwarding.
With cost, this doesn't need to be the case. There will still be a calculation to determine the best path to the root bridge, and the adjusted cost will be part of it.

When to use the one or the other?

Use priority if you allways want one (or more) particular port to forward the traffic (if the root bridge is reachable from that link offcourse). Use cost if you want more flexibility, eg: Don't use a particular port, unless somewhere else in the network a particular link breaks, then use that particular port.

In most cases both don't need to be set, if you have your root bridge in a sensible spot.
If you do have to use one of them, I would recommend priority, because changing default path costs will make your network more complex, and will make troubleshooting more difficult.

[Marvin Tapessur_1]

Thanks Kell, exactly what I needed.