Home » Cisco » 300-115 » What is the easiest way to force a specific switch to become the spanning-tree root bridge for a VLAN?
What is the easiest way to force a specific switch to become the spanning-tree root bridge for a VLAN?
A. Raise the spanning-tree priority value on the switch.
B. Lower the spanning-tree priority value on the switch.
C. Raise the port-cost value of an interface on the switch.
D. Lower the port-cost value of an interface on the switch.
Correct Answer: B
Explanation/Reference:
Answer:
The spanning-tree root bridge is the bridge with the lowest bridge ID. The bridge ID is a value calculated from the bridge priority and the bridge MAC address. Therefore, lowering the bridge-priority value lowers the bridge ID, which can force the switch to become the root bridge.
The easiest way to force a specific switch to become the spanning-tree root bridge for a VLAN is to lower its priority using the spanning-tree vlan vlan_id priority priority command. For example, the following command will configure the switch as the root bridge for VLAN 10:
switch(config)# spanning-tree vlan 10 priority 4096
The priority value of 4096 is used by convention. It could be set to any value as long as it is lower than any other switch in the VLAN. The priority value 4096 is typically used when forcing the placement of the root bridge, and 8192 is used to force placement of the secondary root bridge. These values work because the default priority value for switches is 32768.
Lowering the port cost of an interface is an effective way to force spanning tree to put the interface into a forwarding state. However, it does not affect the placement of the root bridge.
Objective:
Layer 2 Technologies Sub-Objective:
Configure and verify spanning tree
References:
Cisco IOS LAN Switching Configuration Guide, Release 12.4 > EtherSwitch Network Module > Configuring Spanning Tree on a VLAN > VLAN Root Bridge and VLAN Bridge Priority
