I have one question here.Have tried to set Port-priority for one VLAN (VlAN 20) to ensure that the interface g1/1 will be root for VLAN20 instead of g2/1.
To my surprise, g2/1 still is the root for VLAN20. I have put it lower than 128.. which tried at 80.
But when I modified the root port path to 1, I managed to control the root to the other interface.
What is the reason for this? Any factors that causing this?
thanks in advanced.
Port priority wouldn't work in your scenario. It would only work both ports (g1/1 & 2/1) connect to the same upstream switch. Since they are connected to two different switches the proper way to influence root port election is to change the port cost on the 3750 switch. Lower the port cost of g1/1 to something lower than gig2/1 on the 3750 switch to make g1/1 become the root port.
If 2 switches are connected this way:
Switch1 (g1/1) Root FWD -----TRUNK---(DESG Fwd g1/1) Switch 2
Switch1 (g1/2) ALT BLK ------TRUNK---(DESG FWD) g1/2)Switch2.
Now in this case, if you want to make Switch1's g 1/2 as the root, you can do it in 2 ways.
1) Lower the STP cost for port g1/2 On SWITCH1
2) Lower the priority for port g1/2 on SWITCH2.