Question 1,583 of 1,420
mediumMultiple ChoiceObjective-mapped
How to Interpret `show spanning-tree vlan` Output for STP Analysis
A network engineer runs the following command on Switch SW2:
SW2# show spanning-tree vlan 20
VLAN0020 Spanning tree enabled protocol ieee Root ID Priority 24596 Address aabb.cc00.0200 Cost 4 Port 1 (GigabitEthernet0/1) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32778 (priority 32768 sys-id-ext 20) Address aabb.cc00.0300 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- ------------------------------ Gi0/1 Root FWD 4 128.1 P2p Gi0/2 Desg FWD 4 128.2 P2p Gi0/3 Altn BLK 4 128.3 P2p
Based on this output, what can be concluded?
Quick Answer
The correct conclusion is that GigabitEthernet0/3 is in the Blocking state to prevent a loop, as indicated by the "Altn BLK" role and status in the output. This is because SW2 is not the root bridge—its bridge priority of 32778 is higher than the root priority of 24596—so it must have one root port (Gi0/1) to reach the root bridge, one designated port (Gi0/2) on its segment, and any remaining redundant ports become alternate ports that are placed in the blocking state to break potential loops. On the ENCOR 350-401 exam, this type of show spanning-tree output interpretation tests your ability to quickly identify root bridge status, port roles, and STP states, with a common trap being to assume a switch with a lower bridge ID is always the root—remember that the root is determined by the lowest bridge priority, not the local switch’s ID. A useful memory tip: "Alternate = Blocking, Designated = Forwarding, Root = Forwarding" helps you map roles to states at a glance.
⚠ Common exam trap
Cisco often tests the distinction between the root bridge's priority and a non-root switch's priority, including the sys-id-ext addition, so candidates mistakenly think the bridge priority is the base priority without adding the VLAN ID.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
GigabitEthernet0/3 is in the Blocking state to prevent a loop.
The output shows that SW2's Gi0/3 is in the Alternate (Altn) role and Blocking (BLK) state. In Rapid PVST+ (IEEE 802.1w), an alternate port provides a backup path to the root bridge and is placed in a blocking state to prevent Layer 2 loops. This confirms that Gi0/3 is blocking to maintain a loop-free topology.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
SW2 is the root bridge for VLAN 20.
Why it's wrong here
Incorrect. The root ID shows a different bridge (aabb.cc00.0200) with lower priority.
- ✓
GigabitEthernet0/3 is in the Blocking state to prevent a loop.
Why this is correct
Correct. Gi0/3 is shown as Altn BLK (Alternate, Blocking), which prevents a loop.
- ✗
The root port cost is 8.
Why it's wrong here
Incorrect. The root cost is 4, as shown in the Root ID section.
- ✗
SW2's bridge priority is 32768.
Why it's wrong here
Incorrect. The bridge priority is 32778 (32768 + 20 for VLAN 20).
Visual reference
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Same concept, more angles
5 more ways this is tested on 350-401
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A network engineer runs the following command on Switch SW5: SW5# show spanning-tree vlan 50 VLAN0050 Spanning tree enabled protocol ieee Root ID Priority 24626 Address aabb.cc00.0800 Cost 4 Port 1 (GigabitEthernet0/1) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32768 (priority 32768 sys-id-ext 50) Address aabb.cc00.0900 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 sec Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- ------------------------------ Gi0/1 Root FWD 4 128.1 P2p Gi0/2 Desg FWD 4 128.2 P2p Gi0/3 Desg FWD 4 128.3 P2p Gi0/4 Altn BLK 4 128.4 P2p Based on this output, how many ports are in the Forwarding state?
easy- A.1
- B.2
- ✓ C.3
- D.4
Why C: The output shows four interfaces: Gi0/1 (Root FWD), Gi0/2 (Desg FWD), Gi0/3 (Desg FWD), and Gi0/4 (Altn BLK). Three ports (Gi0/1, Gi0/2, Gi0/3) are in the Forwarding state, while Gi0/4 is in the Blocking state. Therefore, the correct answer is 3.
Variation 2. A network engineer runs the following command on Switch SW3: SW3# show spanning-tree vlan 30 VLAN0030 Spanning tree enabled protocol ieee Root ID Priority 24606 Address aabb.cc00.0400 Cost 12 Port 2 (GigabitEthernet0/2) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32798 (priority 32768 sys-id-ext 30) Address aabb.cc00.0500 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 sec Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- ------------------------------ Gi0/1 Desg FWD 4 128.1 P2p Gi0/2 Root FWD 12 128.2 P2p Gi0/3 Desg FWD 4 128.3 P2p Based on this output, what is the root path cost from SW3 to the root bridge for VLAN 30?
hard- A.4
- ✓ B.12
- C.16
- D.20
Why B: The root path cost is the cumulative cost from the local switch to the root bridge. In the output, the Root ID section shows a cost of 12, and the Gi0/2 interface is the root port with a cost of 12. This cost represents the total path cost from SW3 to the root bridge for VLAN 30, making option B correct.
Variation 3. A network engineer runs the following command on Switch SW6: SW6# show spanning-tree vlan 60 VLAN0060 Spanning tree enabled protocol ieee Root ID Priority 24636 Address aabb.cc00.0a00 Cost 8 Port 1 (GigabitEthernet0/1) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32768 (priority 32768 sys-id-ext 60) Address aabb.cc00.0b00 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 sec Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- ------------------------------ Gi0/1 Root FWD 8 128.1 P2p Gi0/2 Desg FWD 4 128.2 P2p Gi0/3 Altn BLK 4 128.3 P2p Gi0/4 Desg FWD 4 128.4 P2p Based on this output, what is the bridge priority of the root bridge for VLAN 60?
medium- A.24576
- ✓ B.24636
- C.32768
- D.32828
Why B: The root bridge's priority is shown in the 'Root ID' section as 'Priority 24636'. This value includes the system ID extension (VLAN 60), so the actual bridge priority is 24636 - 60 = 24576. However, the question asks for the bridge priority as displayed in the output, which is 24636. Option B is correct because the output explicitly lists the root bridge priority as 24636.
Variation 4. A network engineer runs the following command on Switch SW4: SW4# show spanning-tree vlan 40 VLAN0040 Spanning tree enabled protocol ieee Root ID Priority 24616 Address aabb.cc00.0600 Cost 8 Port 1 (GigabitEthernet0/1) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32768 (priority 32768 sys-id-ext 40) Address aabb.cc00.0700 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 sec Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- ------------------------------ Gi0/1 Root FWD 8 128.1 P2p Gi0/2 Desg FWD 4 128.2 P2p Gi0/3 Desg FWD 4 128.3 P2p Based on this output, which port is the root port?
easy- ✓ A.GigabitEthernet0/1
- B.GigabitEthernet0/2
- C.GigabitEthernet0/3
- D.There is no root port because SW4 is the root bridge.
Why A: The root port is the port on a non-root bridge that provides the lowest-cost path to the root bridge. In the output, SW4 is not the root bridge because its Bridge ID (priority 32768 + VLAN 40) is higher than the Root ID priority 24616, and the Root Cost is 8 via Gi0/1. The 'Role' column shows Gi0/1 as 'Root' and its 'Sts' is 'FWD', confirming it is the root port.
Variation 5. A network engineer runs the following command on Switch SW7: SW7# show spanning-tree vlan 70 VLAN0070 Spanning tree enabled protocol ieee Root ID Priority 24646 Address aabb.cc00.0c00 Cost 4 Port 1 (GigabitEthernet0/1) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32768 (priority 32768 sys-id-ext 70) Address aabb.cc00.0d00 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 sec Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- ------------------------------ Gi0/1 Root FWD 4 128.1 P2p Gi0/2 Desg FWD 4 128.2 P2p Gi0/3 Altn BLK 4 128.3 P2p Based on this output, which port is the alternate port?
easy- A.GigabitEthernet0/1
- B.GigabitEthernet0/2
- ✓ C.GigabitEthernet0/3
- D.There is no alternate port.
Why C: The alternate port is GigabitEthernet0/3 because it is in the 'Altn' role with a 'BLK' (blocking) state. In Rapid Spanning Tree Protocol (RSTP) and Per-VLAN Spanning Tree Plus (PVST+), an alternate port provides an alternative path to the root bridge and remains blocked unless the current root port fails. The output clearly shows Gi0/3 as 'Altn BLK', making it the alternate port.
Last reviewed: Jul 4, 2026
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