STP selecting root ports

rakem

this definition is taken from 'CCNA: Cisco Certified Network Associate Study Guide, 5th Edition (640-801) - Todd Lammle

"Root port - Always the linke directly connected to the root bridge, or the shortest path to the root bridge. If more than one link connects to the root bride, then a port cost is determined by checking the bandwidth of each link. The lowest cost port becomes the root port"

My question is, on switches most of the ports have the same bandwidh, say you have a switch which has only 100Mbps ports, how will the switch elect the root port if all the ports have the same bandwidth?

A 100Mbps port has a cost of 19. So i assume that you would need to manually change the port cost to make it lower if all the switch ports have the same cost. Is this correct?

thanks.

marlon23

It will select the port with lowest port number as a root port. (example : fa0/5 < fa0/11)

rakem

really? i havent read that anywhere, all i have found is that is chooses the port with the lowest cost.

borumas

My understanding is the root port is the port connected either to the root bridge or to a device that is closest to the root bridge. With that said say you have 8 switches with 48 fast ethernet ports and 2 gigabit portsconnected with stacking gbics and the root bridge is in another location on the lan but has a direct fiber link to gi0/1 on the top switch, well to that switch gi0/1 would be the root port. In that same scenario say switch 2 in the stack is connected to switch 1 on gi0/2, then that port would be the root port on that switch. I could be wrong but that's my understanding of that concept.

rakem

yes but what im saying is, if your switches are all fastethernet ports then STP would have to use cost to determine the root port. And since the cost would be the same for every port then you would have to configure the cost manually using the spanning-tree [cost] interface command.

EdTheLad

The root port is the port that has the lowest cost to the root,the port that is directly connected to the root bridge is not necessarily the root port as there may be an indirect path to the root bridge with a lower cumulative path cost.
Selecting the root port takes the following into consideration:
1) cumulative path cost to root
2) mac address of neighboring switch
3)port cost

If a switch has multiple ports connected to the root bridge,lowest port cost is used, if the cost is the same on all ports, the port with the lowest port number will be the root port.So port fa0/1 has preference to be root over the rest and port fa0/12 has least preference to be root on a 12 port switch.

rakem

what happens if you manually configure port 12 to have a lower cost than port 1?

darkuser

http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/sw_ntman/cwsimain/cwsi2/cwsiug2/vlan2/stpapp.htm


http://www.cisco.com/en/US/tech/tk389/tk621/technologies_configuration_example09186a008009467c.shtml