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Chapter 14
Configuring STP
Forwarding State
A Layer 2 interface in the forwarding state forwards frames. The interface enters the forwarding state
from the learning state.
An interface in the forwarding state performs these functions:
•
•
•
•
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:
•
•
•
•
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
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 14-2
DP
RP = Root Port
DP = Designated Port
78-17058-01
Receives and forwards frames received on the interface
Forwards frames switched from another interface
Learns addresses
Receives BPDUs
Discards frames received on the interface
Discards frames switched from another interface for forwarding
Does not learn addresses
Does not receive BPDUs
Spanning-Tree Topology
DP
DP
A
RP
DP
RP
DP
RP
B
C
Figure
14-2, Switch A is elected as the root switch because
D
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
Understanding Spanning-Tree Features
14-7
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Troubleshooting
