CCNP Network Assurance Practice Question

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✓

  UplinkFast

  Why this is correct

  This is a Cisco proprietary STP enhancement that accelerates convergence after a direct link failure on a switch port that was in a blocking state. When a root port fails, UplinkFast immediately transitions a designated alternate port to forwarding state without waiting for the normal MAX Age + Forward Delay timers (typically 30-50 seconds). It works by caching a multicast frame from the root to verify the alternate path, but the primary effect is fast failover to a redundant uplink, often within 1-5 seconds. This directly improves convergence time.

• ✓

  BackboneFast

  Why this is correct

  This is another Cisco enhancement that reduces convergence time after an indirect link failure, such as a link going down in the core backbone that causes a switch's root port to go into a listening state. Normally, a switch would wait for MAX Age (20 seconds) to expire before recomputing root, but BackboneFast uses a Root Link Query (RLQ) protocol to quickly determine if the current root is still alive. If the root is reachable via an alternate path, the switch can bypass the MAX Age delay and immediately transition to the forwarding state, cutting convergence time significantly.

• ✗

  Root Guard

  Why it's wrong here

  This is a security feature that enforces the placement of the root bridge in the network by preventing a designated port from becoming a root port. If a switch configured with root guard receives a superior BPDU (one that would cause it to become root), it puts the port into a root-inconsistent state (blocking), rather than allowing the new switch to take over. It does not accelerate convergence; it actually prevents topology changes caused by unauthorized devices, but it has no effect on the STP timers or the speed of recalculations.

• ✗

  BPDU Guard

  Why it's wrong here

  This is a security feature used on access ports to prevent rogue switches from injecting BPDUs into the network. When BPDU guard is enabled, if a port receives any BPDU, it is immediately shut down (err-disabled), protecting the spanning-tree domain from unauthorized switches. This is a defensive mechanism, not a convergence accelerator; it doesn't alter STP timers or forwarding decisions during a legitimate failure, so it has no bearing on how quickly the network recovers after a topology change.

• ✗

  PortFast

  Why it's wrong here

  This feature configures a port that connects to a single host or endpoint (like a PC or server) to transition directly from blocking to forwarding state, bypassing the listening and learning states. It is intended for access ports where no switch is connected, to avoid the 30-second delay when a host comes up. However, PortFast only speeds up the initial link-up on an edge port, not the convergence of spanning-tree topology after a link failure or change elsewhere in the network; it doesn't help a switch recover from a lost link in the core.