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320

Configuring STP

Information About Configuring STP

 Forwards frames switched from another interface

 Learns addresses

 Receives BPDUs

Disabled State

A Layer 2 interface in the disabled state does not participate in frame forwarding or in the spanning tree. An interface in

the disabled state is nonoperational.

A disabled interface performs these functions:

 Discards frames received on the interface

 Discards frames switched from another interface for forwarding

 Does not learn addresses

 Does not receive BPDUs

How a Switch or Port Becomes the Root Switch or Root Port

If all switches in a network are enabled with default spanning-tree settings, the switch with the lowest MAC address

becomes the root switch. In Figure 37 on page 320, Switch A is elected as the root switch because the switch priority

of all the switches is set to the default (32768) and Switch A has the lowest MAC address. However, because of traffic

patterns, number of forwarding interfaces, or link types, Switch A might not be the ideal root switch. By increasing the

priority (lowering the numerical value) of the ideal switch so that it becomes the root switch, you force a spanning-tree

recalculation to form a new topology with the ideal switch as the root.

Figure 37 Spanning-Tree Topology

When the spanning-tree topology is calculated based on default parameters, the path between source and destination

end stations in a switched network might not be ideal. For instance, connecting higher-speed links to an interface that

has a higher number than the root port can cause a root-port change. The goal is to make the fastest link the root port.

For example, assume that one port on Switch B is a Gigabit Ethernet link and that another port on Switch B (a 10/100

link) is the root port. Network traffic might be more efficient over the Gigabit Ethernet link. By changing the spanning-tree

port priority on the Gigabit Ethernet port to a higher priority (lower numerical value) than the root port, the Gigabit Ethernet

port becomes the new root port.

86475

RP = Root Port

DP = Designated Port

Cisco IE-5000 User Manual

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