Which command produced the following output?
A. switch# show spanning-tree vlan 100
B. switch# show vlan 100
C. switch# show spanning-tree summary
D. switch# show interface vlan 100
E. switch# show spanning-tree inconsistent ports
Correct Answer: A
Explanation/Reference:
Answer:
The command show spanning-tree vlan 100 was used to provide the output in the exhibit. This output helps to identify the state of each port on the switch that is a member of VLAN 100. It is also used to identify the root bridge in the spanning tree.
The command show vlan 100 will provide basic information about VLAN 100, such as what ports are assigned to it, but will not display the STP information about the VLAN as the exhibit shows.
The command show spanning-tree summary can be used to verify the enabling of the extended system ID. This command is not used to provide the output in the exhibit.
The command show interface vlan 100 displays the same kind of information as would be displayed for any other interface, including the IP address configuration and whether the interface is up. It does not provide STP information about the switch as displayed in the exhibit.
The command show spanning-tree inconsistent port is used to identify inconsistent ports on a switch. This can occur as a result of implementing the Root Guard feature on a switch. Root Guard can be implemented on a port to prevent the reception of superior BPDUs from causing a new root bridge from being elected. This can sometimes occur when a new switch is introduced with an unknown bridge ID. When a port is configured with Root Guard and it receives a superior BPDU, it will block the port, discard the BPDU, and assign a state of inconsistent to the port.
Below is an example of the partial output of the show spanning-tree inconsistent ports command:
The output shows that devices connected to ports Fa0/1 and Fa0/2 are sending superior BPDUs (perhaps from a new switch). Because of this, no traffic will be forwarded across the ports. Once these superior BPDUs are stopped by changing the priority of the new switch, the interfaces will recover and resume normal operation.
Objective:
Layer 2 Technologies Sub-Objective:
Configure and verify spanning tree
References:
Cisco > Cisco IOS LAN Switching Command Reference > set port flowcontrol through show udld > show spanning-tree
Explanation/Reference:
Answer:
The command show spanning-tree vlan 100 was used to provide the output in the exhibit. This output helps to identify the state of each port on the switch that is a member of VLAN 100. It is also used to identify the root bridge in the spanning tree.
The command show vlan 100 will provide basic information about VLAN 100, such as what ports are assigned to it, but will not display the STP information about the VLAN as the exhibit shows.
The command show spanning-tree summary can be used to verify the enabling of the extended system ID. This command is not used to provide the output in the exhibit.
The command show interface vlan 100 displays the same kind of information as would be displayed for any other interface, including the IP address configuration and whether the interface is up. It does not provide STP information about the switch as displayed in the exhibit.
The command show spanning-tree inconsistent port is used to identify inconsistent ports on a switch. This can occur as a result of implementing the Root Guard feature on a switch. Root Guard can be implemented on a port to prevent the reception of superior BPDUs from causing a new root bridge from being elected. This can sometimes occur when a new switch is introduced with an unknown bridge ID. When a port is configured with Root Guard and it receives a superior BPDU, it will block the port, discard the BPDU, and assign a state of inconsistent to the port.
Below is an example of the partial output of the show spanning-tree inconsistent ports command:
The output shows that devices connected to ports Fa0/1 and Fa0/2 are sending superior BPDUs (perhaps from a new switch). Because of this, no traffic will be forwarded across the ports. Once these superior BPDUs are stopped by changing the priority of the new switch, the interfaces will recover and resume normal operation.
Objective:
Layer 2 Technologies Sub-Objective:
Configure and verify spanning tree
References:
Cisco > Cisco IOS LAN Switching Command Reference > set port flowcontrol through show udld > show spanning-tree
