Spanning Tree Protocol (STP) Flashcards

An Aruba CX switch has been added to a network and must be prevented from ever becoming the root bridge. Which of the following spanning-tree commands will best achieve this goal?

Answer: spanning-tree priority 61440

The root bridge election is based on the Bridge ID (BID), which is a combination of a 2-byte priority and the switch's MAC address. The switch with the lowest BID becomes the root. To prevent a switch from becoming the root, its priority should be set to the highest possible value. The priority value is configured in increments of 4096, and the highest standard value is 61440.

A network administrator is configuring Rapid Spanning Tree Protocol (RSTP) on Aruba CX switches. Which three port states are valid in an RSTP environment?

Answer: Discarding, Learning, Forwarding

Rapid Spanning Tree Protocol (RSTP, 802.1w) improves on the original 802.1D STP by reducing convergence time. It achieves this in part by simplifying the port states. RSTP uses only three port states: Discarding (which combines the 802.1D states of Disabled, Blocking, and Listening), Learning, and Forwarding.

On an Aruba CX switch, which spanning-tree feature should be configured on an edge port connected to an end-user device to immediately place the port into a forwarding state and shut it down if it receives a BPDU?

Answer: BPDU Guard

BPDU Guard is a security feature designed for host-facing ports (edge ports) that should never be connected to another switch. If a port with BPDU Guard enabled receives a BPDU packet, it immediately transitions to an err-disabled state, effectively shutting down the port to prevent potential loops. While Admin-Edge (or PortFast) places a port immediately into forwarding state, it does not provide the protective shutdown action upon BPDU receipt; BPDU Guard adds this crucial security layer.

A network engineer is analyzing the STP topology on a network of Aruba CX switches. A non-root switch has two redundant uplinks to two different distribution switches. One uplink port is in the Root Port role. According to RSTP (802.1w), what is the most likely role of the second uplink port that is currently blocked?

Answer: Alternate Port

In RSTP, an Alternate Port is a port that provides a redundant path toward the root bridge but is currently in a discarding state. It receives more useful BPDUs from another switch but is not the Root Port. If the current Root Port fails, the Alternate Port can quickly transition to the forwarding state, providing fast convergence. A Backup Port provides a redundant link to the same segment and is less common.

Which Spanning Tree Protocol standard, supported on Aruba CX switches, allows for VLAN load balancing by creating multiple spanning tree instances and mapping VLANs to these instances?

Answer: MSTP (802.1s)

Multiple Spanning Tree Protocol (MSTP, IEEE 802.1s) is designed to improve network scalability and efficiency. It allows administrators to group multiple VLANs into a single spanning tree instance. This enables traffic for different sets of VLANs to follow different paths, allowing for load balancing across redundant links that would otherwise be blocked by STP or RSTP. Aruba CX switches use MSTP by default.

Two Aruba CX switches, SW1 and SW2, are connected. Both are configured with the default STP priority. The MAC address of SW1 is 00:00:00:00:00:AA and the MAC address of SW2 is 00:00:00:00:00:BB. Which switch will be elected as the root bridge, and why?

Answer: SW1, because it has a lower MAC address.

The STP root bridge election process selects the switch with the lowest Bridge ID (BID). The BID is composed of the bridge priority (default is 32768) and the switch's MAC address. When the priorities are the same, the switch with the lowest MAC address will have the lower BID and will be elected as the root bridge. In this scenario, SW1 has the lower MAC address.