CCNA STP Interview Questions and Answer

CCNA STP Interview Questions and Answer.

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STP stands for Spanning Tree Protocol. Spanning-tree protocol (STP) is used to prevent a loop in the Layer 2 switching network by logically blocking redundant links.

What is the layer 2 loop?

The process of forwarding a single frame around and around among switches is known as a layer 2 loop.

Why does a frame stuck in a layer 2 loop forever?

Because in layer 2 header there is no field that can prevent loop like TTL field in a layer 3 header. As a result, when layer frames stuck in a loop it goes for ever.

How does STP remove the loop?

STP uses Spanning Tree Algorithm, and finds redundant links in Switch Network and put into a blocking state.

BPDU or Bridge Protocol Data Units a type of STP message which switches use to exchange information of each other. BPDU contains information that help the switch to determine the topology. BPDU is sent in every 2 Sec.

What are the different types of BPDU?

Configuration BPDU or Hello BPDU, used for spanning tree topology or instance calculation.

Topology Change Notification (TCN) BPDU, used to announce the information about the changes in a network topology.

For all switches in a network to agree on a loop-free topology, there must be a switch that can be used as a guide.

BPDU contains a field. Each Switch has a unique identifier which is called Bridge ID or Switch ID, an 8-byte value (2 byte priority field and 6-byte system ID, system ID is the universal unique MAC address of each switch)

Switch with the best (numerically lower) Switch ID or Bridge ID in a layer 2 topology is called the root switch.

Priority comes first, if it ties then MAC address is used to select root switch.

“Interview Questions and Answers on stp”

Each (non-Root) the switch has exactly one Root Port, which represents the best path (lowest path cost) to the Root switch.

The designated port (DP) on a link in the port, from which the link gets the best (least cost) Hello/BPDU onto a link during STP convergence.

All ports of root switch are designated port.

Port is in always forwarding state.

The ports those are neither root port nor designated port.

These ports are blocked by STP, known as blocked port.

What is the method of selecting a root port?

Total Path Cost to Root Bridge — lowest is best.

In case of a tie, Connected Bridge ID — lowest is best.

Connected switch’s Port ID (Port Priority.Port Number) — lowest is best.

Local Port number — lowest is best.

What are the different states of ports in STP?

The time interval between Configuration BPDUs sent by the Root Bridge. The default Hello,Time value is of 2 seconds.

The time interval that a switch port spends in both the Listening and Learning states. The default value is 15 seconds.

What is the Max-Age Timer?

The time interval that a switch stores the best BPDU it has received before discarding it.

Near about 50 to 52 seconds theoretically.

Practically 32 to 35 sec.

A common instance of Spanning Tree, it is basically use a single instance calculation for all VLAN.

Per VLAN Spanning Tree, it is Cisco Proprietary and calculates instance for each and every VLAN.

Cisco made the PVST as per IEEE standard (802.1d) and given the name PVST+ (802.1d).

“STP Interview Questions and Answers”

Redundant links between switches will be blocked with no capability for load balancing. Conditions also can occur that would cause CST to mistakenly enable forwarding on a link that does not carry a specific VLAN, while other links would be blocked.

A topology change occurs when a switch port either moves from blocking state to Forwarding state or moves from Forwarding state to the Blocking state.

Port Fast immediately brings an interface configured as an access or trunk port to the forwarding state from a blocking state, bypassing the listening and learning states.

In case of change of state of a port fast enabled port no TCN BPDU is sent by the switch.

BPDU a guard is access port security features. If BPDU received by BPDU guard enabled port then the port will go to error disable state.

- Yes we can if the port is connected to Server.

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