N10-009 Question 310
Single answerSwitching issues: STP (Network loops, Root bridge selection, Port roles, Port states), Incorrect VLAN assignment, ACLsA technician adds a new access switch to an office network and connects it to two existing distribution switches for redundancy. Minutes later, users report intermittent connectivity, MAC address table instability, and very high broadcast traffic. On the new switch, one uplink port is forwarding and the other is blocking. The technician also notices that the new switch has become the STP root bridge, even though the network design specifies that one of the distribution switches should be the root. Which action should the administrator take FIRST to restore the intended Layer 2 design and reduce the likelihood of loops?
- A
Lower the STP bridge priority on the intended distribution switch so it becomes the root bridge
- B
Change both uplink ports on the new switch to access mode so STP no longer blocks traffic
- C
Move client devices on the new switch to a different VLAN to reduce broadcast traffic
- D
Apply an ACL on the new switch to block broadcast frames between the uplinks
Show answer and explanation
Correct answer: A
Explanation
This scenario describes a classic STP design problem: a newly introduced switch unexpectedly became the root bridge, causing the topology to reconverge in a way that does not match the intended design. Symptoms such as excessive broadcast traffic, intermittent connectivity, and MAC address table instability commonly appear during Layer 2 loops or unstable STP convergence. The fact that one uplink is forwarding and the other is blocking is normal STP behavior; STP intentionally blocks one redundant path to prevent loops. The real issue is root bridge selection.
In STP, the root bridge is elected based on the lowest bridge ID, which includes configurable bridge priority and the switch MAC address. Best practice is to explicitly configure the intended distribution switch with a lower bridge priority so it consistently wins the election. This gives administrators predictable control over root path selection and port roles such as root ports, designated ports, and alternate/blocked ports.
The distractors target common switching misconceptions. VLAN changes do not fix root election issues, although incorrect VLAN assignment is another valid cause of user connectivity problems in other scenarios. Likewise, ACLs are not a tool for controlling Layer 2 STP loop behavior. Vendor documentation from Cisco and general IEEE 802.1D/802.1w guidance emphasize deliberate root bridge placement, redundant links with STP, and avoiding accidental root election by access-layer switches.
- A. Correct.
Correct. STP elects the switch with the lowest bridge ID as the root bridge, and bridge priority is the primary administrative value used to influence that election. If the newly added access switch became root unexpectedly, the proper fix is to lower the bridge priority on the intended distribution switch so it regains root status. This aligns with best practice: place the root bridge at the distribution/core layer where path control is intentional and predictable. Once the correct root is restored, STP can recalculate port roles appropriately and maintain a loop-free topology.
- B. Incorrect.
Incorrect. Converting uplinks to access mode would not solve the root bridge selection problem and would likely break VLAN trunking between switches if multiple VLANs are in use. STP operates on switched links regardless of whether they are access or trunk ports. The blocked port is a normal STP loop-prevention behavior, not something to disable by changing port mode.
- C. Incorrect.
Incorrect. An incorrect VLAN assignment can cause users to lose connectivity to the proper subnet or services, but it would not explain the unexpected root bridge election or the MAC table instability caused by a Layer 2 loop condition. Changing client VLANs may reduce traffic within one broadcast domain, but it does not address the primary switching issue described.
- D. Incorrect.
Incorrect. Standard ACLs on switches/routers are used to filter Layer 3 or Layer 4 traffic, not to stop Layer 2 broadcast frames in order to replace STP loop prevention. Broadcast storms from switching loops are handled by STP and proper switching design, not by ACLs. This option reflects a common misunderstanding of what ACLs can control.