STP Ports on 2 switches?

wallpaper_01

If say I have 2 switches that are equal priority connected via 2 equal speed connections, how does the non root bridge switch decide which port is blocking and which is the root port?

Because I know it will have the higher mac, but how does it decide on the ports between just the two?

From testing im inclined to say it just chooses the higher port number as the blocking port but don't want to presume this...

steveyeung

the lower port no. becomes the root port.

wallpaper_01

Thanks, Id gone through all my notes and couldnt find this info but had a feeling this was the case, thanks!

AwesomeGarrett

Switches will select the best path to the root based on the following:


1. Lowest root bridge ID - once the root bridge has been elected, the root bridge will send BDPU's out its designated port and the switch will receive the same BPDU on both ports.

2. Lowest root path cost to root bridge - after receiving the BPDU's its going add the port cost to the BPDU from the port it came in on. If Fast Ethernet, both ports will add the cost of 19.

3. Lowest sender bridge ID - then is going to examine the Bridge ID of the switch that sent the BPDU. Since it received them from the root bridge this will be a tie.

4. Lowest sender port ID - finally, if all of the above tie, its will come down to the lowest sending port.


You can influence this decision by changing the port priority on the sending switch port. Using the spanning-tree vlan (vlan id) port-priority interface subcommand, changing it on the sending switch will force the receiving switch to choose the lowest sender port id. The port ID comes from the port priority value (by default its 128 ) plus the actual port number (Fa0/1, Fa0/2 etc). Remember the vlan keyword in the command will change the port priority just the vlans listed. You can use the spanning-tree port-priority interface subcommand and it will change it on the entire port affecting all vlan that traverse the port.


EDIT: hope that made sense, one of my short term goals is to be able to explain the technology to others. Please feel free to jump in if I got any of that wrong as its not about being right or wrong but about becoming a better network engineer.