In a redundant switched topology when we have redundant links the spanning-tree will block some of the interfaces and create a loop free layer 2 switched network.
In this post we will see another method to deal with the redundant switched networks with the help of the Flexlinks. Flexlinks will have one port in the active state and the other one in the backup state.
In the topology below in ideal case the port Fa0/1 on SW3 will be put in blocking state if the STP is running. But in our topology we will run flexlink, so one port will be the active link while the other one will be put in the backup state. The backup link will be active only when the active link fails.
The sw3 will have Fa0/0 in active state and the port Fa0/1 will be in backup state
When we configure interfaces as flexlinks then ports will not send BPDU. There is no way to detect loops in this case as we don’t run spanning-tree. Whenever our active interface fails the backup interface will take over.
We first make Fa0/1 as backup interface for Fa0/0.
Spanning-tree is being disabled for these interfaces.
Next we will shut down the port Fa0/0 so the port Fa0/1 which was the backup will now become active.