Identifying Root Ports spanning tree protocol root switch [continue]

Identifying Root Ports (continued)

Spanning tree protocols

SwitchB has two paths to the Root Bridge:

·        A direct connection to SwitchA, with a path cost of 4.

·       

Another path through SwitchD, with a path cost of 16.

 

 stp protocol

The lowest cumulative path cost is considered superior, thus the port directly connecting to SwitchA will become the root port. A BPDU advertising a higher path cost is often referred to as an inferior BPDU.

SwitchD also has two paths to the Root Bridge:

·        A path through SwitchB, with a path cost of 8.

·        A path through SwitchE, with a path cost of 12.

·        The port to SwitchB is preferred, and will become the root port.

Recall that the Root Bridge will advertise BPDU’s with a path cost of 0. As the downstream switches receive these BPDU’s, they will add the path cost of the receiving port, and then advertise the cumulative cost to neighbors.

For example, SwitchC will receive a BPDU with a path cost of 0 from SwitchA, which is the Root Bridge. SwitchC will add the path cost of its receiving port, and thus SwitchC’s cumulative path cost will be 4.

SwitchC will advertise a path cost of 4 to SwitchE, which will add the path cost of its receiving port. SwitchE’s cumulative path cost will thus be 8.

Path cost can be artificially adjusted on a per-port basis:

SwitchD(config)# int g2/22

SwitchD(config-if)# spanning-tree vlan 101 cost 42