Spanning Tree Protocol (STP) Fundamentals
What is the primary purpose of the Spanning Tree Protocol (STP) in a switched network?
To prevent switching loops in a network with redundant switched paths.
Define the role of the Root Port in STP and explain how it is determined.
The Root Port is the port with the lowest path cost to the root bridge; it is selected based on the bandwidth of available links.
What are the main differences between IEEE 802.1d and IEEE 802.1w (RSTP)?
IEEE 802.1w (RSTP) provides faster convergence than 802.1d, introduces new port roles (Alternate and Backup), and eliminates delay timers.
What is the consequence of a failed STP configuration where blocked ports incorrectly transition to forwarding state?
Network loops occur, leading to excessive traffic, CPU overload, and potential network failure.
What is the primary advantage of using EtherChannel in a network?
It aggregates multiple physical links into a single logical link, increasing bandwidth and redundancy while preventing STP from blocking redundant links.
How does STP determine which links to disable in a redundant network topology?
By using the Spanning-Tree Algorithm (STA) to track all links and disable redundant ones.
What is the difference between a Designated Port and a Non-Designated Port in STP?
A Designated Port has the lowest cost path to a segment and forwards traffic, while a Non-Designated Port has a higher cost and remains in a blocking state.
What is the purpose of PVST+ (Per-VLAN Spanning Tree Plus), and how does it differ from standard STP?
PVST+ creates a separate STP instance for each VLAN, allowing more efficient use of links and load balancing.
How does BPDU Guard protect against unauthorized switches in a network?
It disables a port if a BPDU is received on a PortFast-enabled port, preventing unauthorized switches from participating in STP.
What are the key requirements for configuring EtherChannel between two switches?
The ports must have matching speed, duplex settings, and VLAN configurations.
What is a BPDU, and what role does it play in STP?
A Bridge Protocol Data Unit (BPDU) is a network message exchanged between switches to determine the STP topology and elect the root bridge.
Explain the five port states in STP and their functions.
What is the main advantage of Rapid PVST+ compared to PVST+?
It incorporates Rapid Spanning Tree Protocol (RSTP) for faster convergence while still maintaining per-VLAN STP instances.
What is PortFast, and why is it used on access ports?
PortFast allows ports to immediately transition to the forwarding state, reducing delays when connecting end devices.
Differentiate between PAgP and LACP in EtherChannel configuration.
PAgP is Cisco’s proprietary protocol for link aggregation, while LACP is the IEEE standard (802.3ad) that works in multi-vendor environments.
How is the root bridge selected in an STP network?
The switch with the lowest Bridge ID (combination of bridge priority and MAC address) is elected as the root bridge.
What is meant by ‘convergence’ in STP, and why is it important?
Convergence occurs when all switches have transitioned to either forwarding or blocking states, ensuring a stable, loop-free network.
Why might an organization choose to use 802.1s (MSTP) instead of other spanning-tree protocols?
MSTP reduces the number of STP instances by mapping multiple VLANs to a single spanning-tree instance, improving scalability.
Describe a situation where an STP failure could cause a broadcast storm.
If a blocked port is mistakenly enabled, broadcast frames can circulate indefinitely, consuming all available bandwidth and CPU resources.
How does EtherChannel impact STP and routing protocols in a network?
STP treats EtherChannel as a single link, preventing unnecessary blocking, while routing protocols see it as one path, reducing overhead.
What happens in an STP-enabled network if the root bridge fails?
The remaining switches will perform a re-election process, and a new root bridge will be selected based on the lowest Bridge ID.
How long does STP (802.1d) take to transition from a blocking state to a forwarding state by default?
50 seconds (15 seconds each for listening and learning states, plus additional delay timers).
How does Rapid Spanning Tree Protocol (RSTP) achieve faster convergence compared to traditional STP?
By eliminating delay timers and introducing new roles (Alternate and Backup) that allow faster recovery from topology changes.
What command can be used on a Cisco switch to verify the current STP topology and root bridge election?
show spanning-tree
What happens if one of the physical links in an EtherChannel bundle fails?
The remaining links continue operating without disruption, as traffic is redistributed across the active links.