STP Concepts: Redundancy in Layer 2 Switched Networks

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Questions and Answers

What is the purpose of the BID in a BPDU frame?

To configure the desired root bridge switch

To elect the spanning tree instance

To determine the root path cost

To identify the sending switch and the root bridge (correct)

What happens when two or more switches have the same priority?

The switch with the highest MAC address becomes the root bridge

The switch with the lowest MAC address becomes the root bridge (correct)

A new spanning tree instance is created

The default BID is changed to a lower value

What is the primary purpose of STP?

To increase link saturation in switched networks

To prevent Layer 2 loops in Ethernet networks (correct)

To reduce IPv4 packet retransmission

To enhance IPv6 packet processing

What is the default BID value?

32768 Signup and view all the answers

What happens to a Layer 3 packet when the TTL or Hop Limit field reaches zero?

It is dropped by the router Signup and view all the answers

What is the purpose of the Root ID in a BPDU frame?

To identify the root bridge Signup and view all the answers

What is the result of a Layer 2 loop in an Ethernet network?

MAC address table instability and high CPU utilization Signup and view all the answers

Why is it important to configure the desired root bridge with a lower priority?

To prevent two or more switches from having the same priority Signup and view all the answers

What happens after the root bridge is elected?

The STA determines the root path cost Signup and view all the answers

What is the purpose of the TTL or Hop Limit field in IPv4 and IPv6 packets?

To limit the number of times a packet can be retransmitted Signup and view all the answers

What is the value of the extended system ID associated with each switch?

1 Signup and view all the answers

What type of frame is forwarded out all ports, except the ingress port, by a switch?

Unknown unicast frame Signup and view all the answers

What is the purpose of the STA?

To determine the best paths to the root bridge Signup and view all the answers

What is the result of a switch's MAC address table constantly changing?

High CPU utilization Signup and view all the answers

What happens when a Layer 2 loop occurs in an Ethernet network without STP?

Broadcast, multicast, and unknown unicast frames loop endlessly Signup and view all the answers

Why is path redundancy important in Ethernet networks?

To eliminate the possibility of a single point of failure Signup and view all the answers

What is the correct term to refer to the original implementation of spanning tree?

STP Signup and view all the answers

What is the version of spanning tree specified in IEEE-802.1D-2004?

RSTP Signup and view all the answers

What is the default spanning tree protocol used in Cisco switches running IOS 15.0 or later?

PVST+ Signup and view all the answers

What is the term used to describe the assumption made by STP?

One spanning tree instance Signup and view all the answers

What is the main difference between STP and RSTP?

STP is the original implementation, RSTP is the newer version Signup and view all the answers

What must be done to run the rapid spanning tree protocol on a switch?

Explicitly configure the switch for rapid spanning tree mode Signup and view all the answers

What is the name given to the original IEEE 802.1D version of spanning tree?

CST Signup and view all the answers

What is the purpose of spanning tree protocols?

To create a loop-free topology Signup and view all the answers

What determines the preferred path in STP operations?

The path with the lowest overall cost to the root bridge Signup and view all the answers

What happens to all other redundant paths in STP operations?

They are blocked Signup and view all the answers

What is the role of a designated port in STP operations?

To have the best path to receive traffic leading to the root bridge Signup and view all the answers

What happens to a port that is not a root port or a designated port?

It becomes an alternate or blocked port Signup and view all the answers

What is the purpose of alternate ports in STP operations?

To prevent loops in the network Signup and view all the answers

Which ports on the root bridge are designated ports?

All ports Signup and view all the answers

What is the state of an alternate port?

Discarding or blocking state Signup and view all the answers

What is the result of having a loop in the network?

The network becomes unstable and may crash Signup and view all the answers

What is the primary purpose of Spanning Tree Protocol (STP) in a Layer 2 network?

To create a loop-free Layer 2 topology Signup and view all the answers

What is the result of redundant paths in a switched Ethernet network?

The creation of physical and logical Layer 2 loops Signup and view all the answers

What happens when a loop occurs in an Ethernet LAN?

Ethernet frames continue to propagate until a link is disrupted Signup and view all the answers

What is the purpose of redundant networks in a hierarchical design?

To prevent disruption of network services to users Signup and view all the answers

How does STP respond to a failure in the network?

By recalculating and opening up previously blocked ports Signup and view all the answers

What is the benefit of having alternate physical paths for data to traverse the network?

It makes it possible for users to access network resources despite path disruption Signup and view all the answers

What is the role of logical redundancy in a hierarchical design?

It is part of the design to eliminate single points of failure Signup and view all the answers

What is required in an Ethernet LAN?

A loop-free topology with a single path between devices Signup and view all the answers

Study Notes

Purpose of STP

Redundancy in Layer 2 networks is essential for eliminating single points of failure and preventing disruption of network services to users.
Physical redundancy requires the addition of physical paths, and logical redundancy must also be part of the design.
Redundant paths in a switched Ethernet network can cause both physical and logical Layer 2 loops.
Ethernet LANs require a loop-free topology with a single path between any two devices.

Spanning Tree Protocol

Spanning Tree Protocol (STP) is a loop-prevention network protocol that allows for redundancy while creating a loop-free Layer 2 topology.
STP logically blocks physical loops in a Layer 2 network, preventing frames from circling the network forever.

STP Recalculation

STP compensates for a failure in the network by recalculating and opening up previously blocked ports.

Issues with Redundant Switch Links

Path redundancy provides multiple network services by eliminating the possibility of a single point of failure.
A Layer 2 loop can result in MAC address table instability, link saturation, and high CPU utilization on switches and end-devices, making the network unusable.

Layer 2 Loops

Without STP enabled, Layer 2 loops can form, causing broadcast, multicast, and unknown unicast frames to loop endlessly.
This can bring down a network quickly.
When a loop occurs, the MAC address table on a switch will constantly change with the updates from the broadcast frames, resulting in MAC database instability.

STP Operations

STP was developed specifically as a loop prevention mechanism for Layer 2 Ethernet.
The switch with the lowest BID will become the root bridge.
The administrator should configure the desired root bridge switch with a lower priority.
The switch with the lowest hexadecimal MAC address value is the preferred root bridge.
Paths with the lowest cost become preferred, and all other redundant paths are blocked.
The designated port is a port on the segment that has the internal root path cost to the root bridge.
If a port is not a root port or a designated port, then it becomes an alternate (or backup) port.

Evolution of STP

STP has been superseded by the Rapid Spanning Tree Protocol (RSTP).
The IEEE uses "STP" to refer to the original implementation of spanning tree and "RSTP" to describe the version of spanning tree specified in IEEE-802.1D-2004.
Cisco switches running IOS 15.0 or later, run PVST+ by default, which incorporates many of the specifications of IEEE 802.1D-2004.
Switches must be explicitly configured for rapid spanning tree mode in order to run the rapid spanning tree protocol.

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Description

Learn about the causes of loops in a Layer 2 network and how Spanning Tree Protocol (STP) works to eliminate single point of failures in hierarchical design.