Interview questions on STP

What is the Spanning Tree Protocol used for

Spanning tree protocol is used to prevent looping of frames. Unlike IP, which has a TTL value field in the packet which is decremented every time the packet passes through a router and is dropped once the value reaches 0, the frame does not have any similar field. So if a scenario arises where a frame is looping in a switched environment, the frame would not automatically time out. The spanning tree protocol is used for preventing the looping.

Can STP be configured on a router

STP can be configured only on switches and not on routers. STP is used to prevent frame looping

Is STP required to be configured when there is only one switch involved.

It is not required. STP is used to shutdown redundant links between switches to prevent loops.

Is a generic ethernet frame modified when STP is configured on the network

STP is a protocol. It has it’s own frame when configured. So it would not affect a generic ethernet frame on the network.

How many designated ports can be available on a root bridge

All ports on a root bridge is designated ports.

How many root bridges can be available on a STP configured network


If the priority value of the two switches are same, which switch would be elected as the root bridge

The switch with the lowest mac-address value would be elected as the root bridge

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