45 Questions
What is the primary advantage of RSTP networks?
They offer high availability
What happens to a port that has been disabled when it is enabled again?
It enters the blocking state
What is the purpose of STP?
To balance the states across ports and avoid bridge looping
How does RSTP prevent network loops?
By blocking redundant paths on a network
What is the first step in the RSTP algorithm?
Determine the root bridge
What determines the root bridge in RSTP?
The switch with the lowest bridge priority
What happens to all interfaces on the root bridge in RSTP?
They are placed in a forwarding state
What is the purpose of the root port in RSTP?
To provide the best path to the root bridge
How does the port cost affect the root port selection in RSTP?
The lower the port speed, the higher the cost
What type of data units do bridges exchange to carry out STP operations?
Bridge protocol data units (BPDUs)
What is the initial state of a port when STP is enabled on a network bridge?
Disabled
What happens to frames received from the attached network segment in the blocking state?
They are discarded
What happens to the address table in the learning state?
It is updated with learned information
What is the function of a port in the forwarding state?
It forwards frames across the network segments
What is the purpose of BPDUs in STP operations?
To provide network information to bridges
What happens to frames forwarded from another port in the listening state?
They are discarded
What is the transition from the blocking state to the listening state?
The port starts receiving and processing BPDUs
What is the final state of a port after STP operations?
Forwarding
What is the main purpose of Spanning Tree Protocol in a switched network?
To monitor network links and prevent looping
Who invented the original Spanning Tree Protocol and algorithm?
Radia Perlman
What is the IEEE standard for Spanning Tree Protocol?
IEEE 802.1D
What is the primary function of an algorithm in Spanning Tree Protocol?
To search for redundant links in the LAN
What happens to redundant links in Spanning Tree Protocol?
They are put in blocking state
What is the main reason for using Spanning Tree Protocol in a network with redundant paths?
To prevent looping in the network
What does Spanning Tree Protocol use to control the flow of traffic?
Algorithm
What is the role of bridges in Spanning Tree Protocol?
To continually understand the topology of the network
What is a characteristic of Spanning Tree Protocol?
It monitors network links and prevents looping
What is the main criteria for selecting the root port in a non-root switch?
Low port cost
What is the role of a designated port in RSTP?
To provide the lowest root path cost in a LAN segment
What happens to an alternate port when the root port fails?
It becomes the new designated port
What is the role of a backup port in RSTP?
To act as a backup for the designated port
What is the name of the messages exchanged between switches on the network?
Bridge Protocol Data Units (BPDUs)
How many root ports are there per non-root switch in RSTP?
One
What is the state of a port that is not passing data to other switches on the network?
Discarding
What is the primary difference between RSTP and STP?
RSTP has fewer port states than STP
What is the purpose of a designated port in a LAN segment?
To provide the lowest root path cost
What is a major advantage of RSTP over STP?
Faster convergence time
What is the function of the Alternate port in RSTP?
To directly enter the forwarding state
What is the primary purpose of MSTP?
To provide multiple forwarding paths for data traffic
What is the Listening state used for in STP?
Sending and receiving BPDUs
What is the benefit of MSTP in terms of bridge computations?
Bridges have less computations to perform
What is the standardized protocol for MSTP?
IEEE 802.1Q-2014
What is the function of the Blocking state in STP?
No forwarding, processing, or learning of MAC addresses
What is the advantage of MSTP in terms of VLANs?
MSTP permits numerous VLANs to share STP instances
What is the main difference between RSTP and STP?
RSTP has a faster convergence time
Study Notes
Spanning Tree Protocols (STP)
- Spanning tree protocols are used in switched networks to monitor and control traffic flow, preventing data loops and ensuring network performance.
- The original STP algorithm was invented by Radia Perlman in 1985, and later standardized by IEEE as IEEE 802.1D.
- STP uses an algorithm to search for redundant links in the LAN and select the best paths, putting links in either forwarding or blocking states.
STP Port States
- There are five STP port states:
- Disabled: The port is totally disabled and does not participate in frame forwarding or STP operations.
- Blocking: The port does not participate in frame forwarding and discards frames received from the attached network segment, but continues to listen for and process BPDUs.
- Listening: The port discards frames from the attached network segment or forwarded from another port, but receives BPDUs and redirects them to the switch module for processing.
- Learning: The port listens for and processes BPDUs, but discards frames from the attached network segment or forwarded from another port, and starts updating the address table with learned information.
- Forwarding: The port moves from the learning state to the forwarding state, starts forwarding frames across network segments, and continues to receive and process BPDUs and update the address table.
Rapid Spanning Tree Protocol (RSTP)
- RSTP is an improvement over STP, providing high availability of network and loop-free topology within Ethernet networks.
- RSTP prevents network loops by blocking redundant paths on a network, and establishes a root bridge and blocking specific ports to prevent network loops.
- RSTP algorithm steps:
- Determine the root bridge: The switch with the lowest bridge priority will be the root bridge, and if there is a tie, the switch with the lowest MAC address is selected.
- All interfaces on the root bridge are placed in a Forwarding state.
- All other switches on the network (non-root) select a root port: The root port is the best path to the root bridge based on its port cost.
- Designated ports are selected: These ports are permitted to forward traffic and are also selected based on port cost.
- All other ports are in a discarding (blocking) state.
RSTP Port Roles
- Root Port: The port that receives BPDUs on the root bridge.
- Designated Port: The port selected in each LAN segment that provides the lowest root path cost.
- Alternate Port: A backup for the root port in case of failure, and is blocked during typical operation of the root port.
- Backup Port: A backup for the designated port, and is normally blocked when the designated port is operating correctly.
RSTP States
- RSTP has three port states: Discarding, Learning, and Forwarding.
- The Disabled, Blocking, and Listening states described by STP have been combined into the Discarding state in RSTP.
- RSTP allows the Alternate port and Backup port to directly enter the forwarding state, reducing convergence time.
Multiple Spanning Tree Protocol (MSTP)
- MSTP creates multiple spanning trees (instances) for each VLAN on a single physical network.
- MSTP allows each VLAN to have a configured root bridge and forwarding topology.
- MSTP uses RSTP to provide rapid convergence, enables VLANs to be grouped into a spanning-tree instance, provides multiple forwarding paths for data traffic, and enables load balancing.
- MSTP is standardized by the IEEE within the 802.1Q-2014 report.
Learn about Spanning Tree Protocols and their role in switched networks, including the algorithm and port states.
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