Spanning Tree Part 1

Here is the basic topology.

R1 and R3 are connected using port FastEthernet0/0
R1 and R2 are connected using port FastEthernet0/6
R2 and R3 are connected using port FastEthernet0/12



Lets create a new VLAN 100 on each ‘switch’



So now lets run through all the switches setting up each connected port as a trunk. We don’t need to set the encapsulation since these only support 802.1q but we’ll do it anyway...



When all the ports are up lets wait for spanning-tree to converge.



On R3 the port FastEthernet0/12 is ‘blocking’.



So let’s move the blocked port to R2’s FastEthernet0/12 interface. To do that we need to make the cost of ‘getting’ to R3 easier than getting to R2. So lets lower the port cost on the interface closest to the root switch.




The default port cost for FastEthernet is 19 so a cost of 1 is preferred. Now looking at R2 we’ve got the result we wanted.