Module 5 Flashcards
What is the purpose of STP?
STP is a loop-prevention network protocol that allows for redundancy while creating a loop-free Layer 2 topology.
Who invented the Spanning Tree Algorithm?
Radia Perlman.
What does the Spanning Tree Algorithm (STA) do?
Creates a loop-free topology by selecting a single root bridge where all other switches determine a single least-cost path.
What is the function of BPDU guard in Cisco switches?
Puts the switch port in an error-disabled state upon receipt of any BPDU to protect against potential loops.
What are the three timers required for STP convergence?
- Hello timer
- Forward delay timer
- Max age timer
What are the port states in STP?
- Blocking
- Listening
- Learning
- Forwarding
- Disabled
What is the difference between RSTP and STP?
RSTP provides faster convergence than STP.
What does PVST+ stand for?
Per-VLAN Spanning Tree Plus.
When is PortFast used?
On access ports to allow immediate network access.
What is a broadcast storm?
An abnormally high number of broadcasts overwhelming the network.
What happens when a Layer 2 loop occurs?
It causes MAC address table instability, link saturation, and high CPU utilization.
What is the purpose of the Spanning Tree Protocol?
To prevent endlessly circulating frames in a Layer 2 network.
What does RSTP stand for?
Rapid Spanning Tree Protocol.
What are the three port states in RSTP?
- Learning
- Forwarding
- Discarding
What is an unknown unicast frame?
A frame for which the switch does not have the destination MAC address in its MAC address table.
What is the role of the root bridge in STP?
It is the central point of the spanning tree topology from which all paths are calculated.
Fill in the blank: STP prevents loops by configuring a loop-free path through the network using strategically placed _______.
[blocking-state ports]
What is the internal root path cost?
The sum of all the individual port costs along the path from the switch to the root bridge.
How does STP handle network failures?
By dynamically unblocking previously blocked ports.
True or False: RSTP is backward compatible with STP.
True.
What is the main benefit of using Layer 3 routing over Layer 2?
It allows for redundant paths and loops in the topology without blocking ports.
What is the function of a designated port in STP?
It is the port on a segment that has the internal root path cost to the root bridge.
What does the acronym BPDU stand for?
Bridge Protocol Data Unit.
What is the role of the alternate port in STP?
It serves as a backup path to the root bridge.
What does the acronym MSTP stand for?
Multiple Spanning Tree Protocol.
What is Rapid PVST+?
Cisco’s enhancement of RSTP that provides a separate instance of RSTP per VLAN.
What is the purpose of the hello timer in STP?
To determine the frequency of sending BPDUs.
Fill in the blank: Without STP enabled, ________ loops can form, causing broadcast, multicast and unknown unicast frames to loop endlessly.
[Layer 2]
What is the main purpose of the Spanning Tree Algorithm (STA)?
To create a loop-free topology in a network
The STA achieves this by selecting a root bridge, blocking redundant paths, and recalculating paths in case of link failure.
What does PVST+ stand for?
Per-VLAN Spanning Tree Plus
PVST+ is a Cisco enhancement of STP that provides a separate 802.1D spanning tree instance for each VLAN in the network.
What is RSTP?
Rapid Spanning Tree Protocol
RSTP is an evolution of STP that provides faster convergence.
What does MSTP stand for?
Multiple Spanning Tree Protocol
MSTP is an IEEE standard that maps multiple VLANs into the same spanning tree instance.
What is the default bridge priority for Cisco switches?
32768
The bridge priority can be configured in increments of 4096, with a lower value being preferable.
What happens if two or more switches have the same priority in STP?
The switch with the lowest MAC address becomes the root bridge
This is because the MAC address is used as a tie-breaker when priorities are the same.
What is the role of designated ports in STP?
Designated ports are the ports that have the best path to the root bridge for each segment
All ports on the root bridge are designated ports.
Fill in the blank: The internal root path cost is equal to the sum of all the _______ along the path to the root bridge.
port costs
This cost determines the preferred path to the root bridge.
What are BPDUs used for in STP?
To share information about switches and their connections
BPDUs are used to elect the root bridge, root ports, designated ports, and alternate ports.
What is the purpose of the Hello Timer in STP?
It defines the interval between BPDUs
The default Hello Timer is 2 seconds but can be modified.
What does the Forward Delay Timer control in STP?
The time spent in the listening and learning states
The default is 15 seconds but can be adjusted.
True or False: All ports attached to end devices are designated ports.
True
This is part of the STP port role assignments.
What is the significance of the Max Age Timer in STP?
It is the maximum time a switch waits before attempting to change the STP topology
The default is 20 seconds but can be modified.
What occurs during the first step of STP operations?
Elect the root bridge
The root bridge serves as a reference point for all path calculations.
Fill in the blank: If a port is not a root port or a designated port, it becomes an _______ port.
alternate
Alternate ports are in a blocking state to prevent loops.
What is the primary focus of the latest IEEE documentation on spanning tree?
RSTP has superseded STP
The IEEE uses ‘STP’ to refer to the original implementation and ‘RSTP’ for the updated version.
How does STP handle multiple equal-cost paths to the root bridge?
It uses a tie-breaking process based on sender BID, port priority, and port ID
This ensures a consistent selection of root ports in complex topologies.
What is the significance of the extended system ID in STP?
It is added to the bridge priority to identify the VLAN
This helps in distinguishing between multiple VLANs in BPDU frames.
What does MST provide in terms of RSTP instances?
Up to 16 instances of RSTP
MST allows combining many VLANs into a common RSTP instance.
