From the course: CCNP Switching (300-115) Cert Prep: 1 Layer 2 Technologies

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STP Config: Path and timers

STP Config: Path and timers

- [Instructor] Influencing which ports become root ports allows an administrator to control the flow of traffic. Recall that the root port cost is calculated as a BPDU is received by a switch. The cost of the link the BPDU was received on is added to the cumulative total. It is for this reason that if I want to manipulate traffic, I'll adjust the cost on the receiving root port. If I want a port not to be used, I'll artificially set its cost high. Alternately I could set a preferred port's cost low. This is done with the interface command, spanning-tree cost. The cost adjustment is for all VLANs on this interface unless the VLAN keyword is used. Then individual VLANs can have their cost adjusted. Another way to affect STP decisions is to adjust the port ID. Where an interface cost adjusts how a switch determines the root port itself, manipulating port ID will affect how downstream switches determine their root port. This value is only significant between two neighbors. To view the…

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