STP Quiz: Spanning Tree Concepts and Operation

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Questions and Answers

What does the BID field of a BPDU frame consist of?

Port priority

Root bridge MAC address

System ID Extension

Bridge priority, Extended system ID, MAC address (correct)

Which spanning tree protocol uses perVLAN RSTP?

Rapid PVST+ (correct)

PVST

PVST+

STP

What is the primary purpose of Extended System ID in PVST+?

Determines the root bridge

Unique BIDs for each VLAN (correct)

Assigns port states

Identifies the VLAN participating in STP

How many port states are used in PVST+ operation?

5 Signup and view all the answers

In root-bridge election manipulation, what action can lead to a switch becoming the root bridge?

Assigning a lower priority Signup and view all the answers

What type of BPDUs does RSTP use?

Type 2, version 2 Signup and view all the answers

What issue at Layer 1 of the network can occur due to MAC database instability?

Continued propagation of frames Signup and view all the answers

In the context of Spanning Tree Algorithm, what is the purpose of blocking redundant paths?

To prevent loop formation Signup and view all the answers

Which type of port in the Spanning Tree Algorithm is closest to the root bridge?

Root port Signup and view all the answers

What is the purpose of the root bridge in STP?

To serve as a reference point for STP calculations Signup and view all the answers

What happens when unknown unicast frames are sent onto a network with loops?

Duplicate frames arrive at the destination device Signup and view all the answers

How does STP ensure only one logical path between all destinations on the network?

By intentionally blocking redundant paths Signup and view all the answers

What determines the internal root path cost from a switch to the root bridge?

The sum of individual port costs along the path to the root bridge Signup and view all the answers

What information does each switch maintain locally regarding STP operation?

The BID, root ID, and root path cost Signup and view all the answers

What is the default frequency of sending BPDU frames in STP?

Every two seconds Signup and view all the answers

In STP, what role is automatically configured for all switch ports on the root bridge?

Designated port Signup and view all the answers

What role do alternate ports play in STP operation?

Does not send or receive traffic on that segment Signup and view all the answers

Study Notes

Spanning Tree Concepts

The switch with the lowest BID (Bridge ID) becomes the root bridge.
Spanning Tree Algorithm determines the root bridge based on the root path cost.

STP Operation

Root Path Cost: internal root path cost is determined by summing up individual port costs along the path from the switch to the root bridge.
Custom cost can be applied using the spanning-tree cost value interface configuration command.
The show spanning-tree command verifies the port and internal root path cost to the root bridge.

Port Role Decisions for RSTP

Root bridge automatically configures all its switch ports in the designated role.
Designated ports are configured for all LAN segments.
The switch with the lower cost path to the root bridge has its port selected as the designated port.
Alternate ports do not send or receive traffic on that segment.

802.1D BPDU Propagation and Process

BPDU frames are sent every two seconds by default.
Each switch maintains local information about its own BID, the root ID, and the root path cost.

Issues with Layer 1 Redundancy

MAC Database Instability: no mechanism to block continued propagation of frames on a switched network.
Broadcast Storms: a broadcast storm occurs when many broadcast frames consume all available bandwidth.
Duplicate Unicast Frames: unknown unicast frames sent onto a looped network can result in duplicate frames arriving at the destination device.

Spanning Tree Algorithm

Ensures only one logical path between all destinations on the network by intentionally blocking redundant paths.
Port Roles:
- Root ports: ports closest to the root bridge.
- Designated ports: non-root ports permitted to forward traffic.
- Alternate and backup ports: blocking state to prevent loops.
- Disabled ports: a switch port that is shut down.

Root Bridge

Serves as a reference point for all STP calculations.
BID field of a BPDU frame consists of three separate fields:
- Bridge priority (default 32768).
- Extended system ID (identifies the VLAN participating in STP).
- MAC address (deciding factor when bridge priorities are equal).

Varieties of Spanning Tree Protocols

Overview of PVST+: Cisco developed PVST+ to run an independent instance of IEEE 802.1D for each VLAN in the network.
Port States and PVST+ Operation:
- Five port states: Blocking, Listening, Learning, Forwarding, and Disabled.
- Extended system ID ensures switches have unique BIDs for each VLAN.
Rapid PVST+: the Cisco implementation of perVLAN RSTP, achieving faster convergence.
Edge Ports: a switch port that is never intended to be connected to another switch.

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Description

Test your knowledge of Spanning Tree Protocol (STP) concepts and operations, including root bridge selection, path cost calculation, and custom port costs. Explore how to configure and verify STP settings on network switches.