JNCIS-ENT · domain
Spanning Tree
Spanning Tree on JNCIS-ENT covers RSTP, MSTP, VSTP, and BPDU protection on Juniper EX Series switches. Questions test port roles and states, bridge priority defaults, MSTP instance 0 (CIST) behavior, edge port configuration, and loop-prevention reactions when BPDUs stop arriving.
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What to know about Spanning Tree
Be able to identify RSTP port roles and states, explain MSTP instance 0 (CIST) behavior, recall the default bridge priority on EX Series switches, and describe why edge ports are configured on access ports. The key is knowing how a port reacts when BPDUs stop arriving.
RSTP port roles and states, including designated, root, alternate, and discarding transitions
MSTP instance 0 (CIST) operation and how it maps VLANs across regions
Default bridge priority on Juniper EX Series switches when RSTP is enabled
Edge port configuration on access ports and its effect on convergence and topology changes
Why learners struggle
Why Spanning Tree questions are commonly missed
STP questions are missed when learners confuse bridge ID comparison direction, forget how ties are broken, or misidentify which port role a given switch port takes. The lowest bridge ID wins, but cost, port ID, and tiebreakers all play a role.
- ·Lowest bridge ID wins root election — lower priority wins, then lower MAC breaks ties
- ·Root port vs designated port — each non-root switch has exactly one root port
- ·Blocking ports are on non-designated segments — they receive BPDUs but do not forward
- ·PortFast bypasses STP states — only safe on access ports connected to end devices
- ·BPDU Guard shuts down PortFast ports that receive BPDUs — protecting against loops
- ·STP path cost uses link bandwidth — faster links have lower cost and are preferred
Watch out for
Common Spanning Tree exam traps
- ▸Confusing RSTP alternate and backup port roles, or assuming a port that stops receiving BPDUs always becomes root.
- ▸Forgetting that MSTP instance 0 is the CIST and carries all VLANs not explicitly mapped to another instance.
- ▸Assuming edge ports still participate in spanning tree; they do not generate topology changes but still guard against loops.
Question index
All Spanning Tree questions (38)
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You have an EX Series switch running RSTP. You want to configure a port connected to a printer to immediately transition to forwarding without going through the learning state. Which configuration statement should you use?
Medium2Which THREE parameters are included when calculating the MST region configuration digest? (Choose three.)
Medium3What is the primary function of the RSTP port role known as the 'Backup' port?
Easy4An operator configures RSTP across multiple EX Series switches. A rogue switch running legacy STP is connected to an RSTP switch. How does the RSTP switch handle interaction with the legacy STP switch?
Hard5You are configuring an EX Series switch and want to set the forward delay timer to 15 seconds. Which command is used?
Medium6An enterprise network runs MSTP. Instance 0 is always reserved for which specific purpose?
Hard7You are configuring Multiple Spanning Tree Protocol (MSTP) across multiple EX Series switches in your enterprise network. Which three parameters must be identical across all switches to ensure they belong to the same MST region?
Medium8Which THREE parameters must match across all switches for them to successfully form a single MST region? (Choose three.)
Medium9You need to ensure that a core switch retains its position as the root bridge even if a new switch with a manually lowered bridge priority is added to the network. Which feature should you configure on the core switch's ports facing the downstream switches?
Medium10Which TWO mechanisms are utilized by RSTP to achieve rapid convergence compared to legacy 802.1D STP? (Choose two.)
Hard11When a topology change occurs in an RSTP network, how are MAC address forwarding table entries affected across the switches?
Medium12What is the primary purpose of the Hello timer in RSTP?
Easy13Which TWO commands are valid Junos configuration hierarchy paths for setting up RSTP? (Choose two.)
Easy14What is the default bridge priority value assigned to Juniper EX Series switches when RSTP is enabled?
Easy15When configuring MSTP, how is the regional maximum hop count enforced across switches within the same MST region?
Hard16An organization runs MSTP with multiple instances. A packet belonging to a VLAN that is not explicitly mapped to any MST instance is processed in which manner?
Hard17An administrator configures root protection on a designated port. If that port subsequently receives a superior BPDU from another switch, what exact state does the port transition into?
Hard18Which TWO features help protect an RSTP topology from unauthorized switches or misconfigurations on access ports? (Choose two.)
Easy19Which TWO port roles are defined in Rapid Spanning Tree Protocol (RSTP) but did not exist in legacy 802.1D STP? (Choose two.)
Hard20You notice that an access switch running RSTP is continually dropping user traffic because a connected end-user device is sending inferior BPDUs that trigger topology changes. Which Juniper feature should you configure on the access ports to protect against this behavior?
Medium21Which TWO commands can be used to view spanning tree information on a Juniper EX Series switch? (Choose two.)
Easy22Which THREE statements describe the function of the Designated Port in RSTP? (Choose three.)
Medium23An EX Series switch running MSTP receives a BPDU from an upstream switch in a different MST region. How does the receiving switch identify the boundary between the two regions?
Hard24Which TWO parameters are included when a Junos switch calculates the MST configuration digest? (Choose two.)
Medium25You need to modify the bridge priority of an EX Series switch running RSTP to ensure it becomes the root bridge. Which configuration command correctly sets the bridge priority to 4096?
Medium26You want to disable spanning tree protocol entirely on a specific access interface (ge-0/0/10.0) of an EX Series switch while keeping RSTP active globally. Which configuration command achieves this?
Medium27An enterprise network uses MSTP. What is the function of the Internal Root in an MST region?
Hard28An administrator configures link aggregation (ae0) using LACP between two EX Series switches running RSTP. How does RSTP treat the aggregated interface ae0?
Hard29An administrator needs to configure MSTP on a group of EX Series switches. Which three parameters must be identical across all switches in the same MST region?
Hard30What is the primary benefit of enabling edge port configuration on switch access ports connected to end devices?
Easy31While examining an RSTP topology, you notice that a designated port has suddenly stopped receiving BPDUs from its upstream neighbor, causing the port to transition into a discarding state to prevent potential loops. Which feature is actively providing this protection?
Medium32What action does the 'set protocols rstp max-age <seconds>' command perform on a Juniper switch?
Easy33Which TWO statements are correct regarding the operation of MSTP instance 0 (CIST)? (Choose two.)
Hard34You have an EX Series switch running RSTP. You notice that a port connected to a server is flapping between forwarding and discarding states. The server NIC is configured for teaming but is not sending BPDUs. You have configured the port as an edge port using 'set protocols rstp interface ge-0/0/5 edge'. However, the port still flaps. What is the most likely cause?
Hard35You are troubleshooting an RSTP network. You notice that a port on an EX Series switch is flapping between Discarding and Forwarding states. You suspect a BPDU inconsistency. Which command would you use to check for BPDU inconsistencies on the switch?
Hard36What is the role of the Root Port in an RSTP topology?
Easy37When configuring MSTP, what purpose does the regional configuration digest serve?
Hard38Which THREE states are defined in Rapid Spanning Tree Protocol (RSTP)? (Choose three.)
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Frequently asked questions
- What does the Spanning Tree domain cover on the JNCIS-ENT exam?
- Be able to identify RSTP port roles and states, explain MSTP instance 0 (CIST) behavior, recall the default bridge priority on EX Series switches, and describe why edge ports are configured on access ports. The key is knowing how a port reacts when BPDUs stop arriving.
- How many questions are in this domain?
- This page lists all 38 Spanning Tree questions in the JNCIS-ENT question bank. The actual exam draws from this domain proportionally to its weighting in the official exam blueprint.
- What is the best way to practise this domain?
- Start with a short focused session (10 questions) to identify gaps, then work through explanations. Repeat with a longer session once the weak areas feel solid.
- Can I practise only Spanning Tree questions?
- Yes — the session launcher on this page filters questions to this domain only. Choose any session length for inline explanations and scoring.