JNCIS-ENT Spanning Tree — 40 Questions

Question 1

Which command allows an operator to view the designated root bridge identifier and port roles for all active VLANs under RSTP?

Accepted Answer

show spanning-tree bridge. The command 'show spanning-tree bridge' displays the designated root bridge identifier and port roles for all active VLANs under RSTP. It provides a summary of the spanning tree topology, including the root bridge ID, root path cost, and port roles for each VLAN. This is the correct command to view the root bridge and port roles.

Answer

show spanning-tree statistics

Answer

show spanning-treemstp region

Answer

show spanning-tree interface detail

Question 2

An EX Series switch running RSTP receives BPDUs with superior path cost information from an unauthorized access switch. Which feature should you enable on access-facing ports to prevent these unauthorized BPDUs from altering the spanning tree topology?

Accepted Answer

set protocols rstp interface ge-0/0/5.0 bpdu-guard. BPDU guard is the correct feature to enable on access-facing ports to prevent unauthorized BPDUs from altering the spanning tree topology. When BPDU guard is enabled on an interface, the port is shut down (err-disabled) if it receives a BPDU, which stops an unauthorized switch from participating in STP. This is the standard defense against rogue switches on access ports.

Answer

set protocols rstp interface ge-0/0/5.0 no-root-port

Answer

set protocols rstp interface ge-0/0/5.0 root-protection

Answer

set protocols rstp interface ge-0/0/5.0 loop-protection

Question 3

You are troubleshooting a suboptimal spanning tree topology where an EX4300 switch is chosen as the STP Root Bridge despite having a high bridge priority. What command helps verify the current bridge priority and local bridge identifier parameters?

Accepted Answer

show spanning-tree bridge. The 'show spanning-tree bridge' command on Junos EX switches displays the local bridge identifier, bridge priority, and related STP parameters for the switch. This directly answers the question of verifying the current bridge priority and local bridge ID when troubleshooting root bridge election.

Answer

show spanning-tree statistics

Answer

show spanning-tree interface

Answer

show spanning-treemstp configuration

Question 4

An administrator has deployed a new EX Series switch running RSTP and notices that access ports connected to end-user workstations take up to 30 seconds to transition to the forwarding state. Which configuration command should be applied to immediately resolve this delay for access ports?

Accepted Answer

set protocols rstp interface ge-0/0/1.0 edge. RSTP (802.1w) still uses the original 802.1D timers (Listening/Learning) on ports that are not explicitly identified as edge ports, which is why the access port waits ~30 seconds. Configuring 'set protocols rstp interface ge-0/0/1.0 edge' tells RSTP that no BPDUs will ever be received on that port, so it can transition directly to forwarding (like PortFast in Cisco). This is the correct, targeted fix for end-user access ports.

Answer

set protocols rstp force-version stp

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set protocols rstp interface ge-0/0/1.0 mode point-to-point

Answer

set protocols rstp bridge-priority 0

Question 5

You are configuring MSTP and want to assign VLANs 10 through 50 to MST instance 2. Which configuration hierarchy is correct in Junos OS?

Accepted Answer

set protocols mstp configuration-name RegionA revision 1 instance msti2 vlan 10-50. In Junos OS, MSTP configuration is done under the [edit protocols mstp] hierarchy. To assign VLANs to an MST instance, you use the 'instance' statement with a name (e.g., msti2) and then specify the VLANs using 'vlan 10-50'. The correct syntax is 'set protocols mstp configuration-name RegionA revision 1 instance msti2 vlan 10-50'. This creates an MST instance named msti2 and maps VLANs 10 through 50 to it.

Answer

set bridge-domains domain1 mstp-instance 2 vlan 10-50

Answer

set protocols rstp msti 2 vlan-list 10-50

Answer

set vlans vlan-range 10-50 mstp instance 2

Question 6

You suspect that a network loop has formed due to a misconfigured RSTP topology on your EX Series switches. Which command shows real-time topology change notifications and counts on the switch?

Accepted Answer

show spanning-tree statistics. The 'show spanning-tree statistics' command on Juniper EX Series switches displays real-time topology change notifications and counts, including TC (topology change) and TCN (topology change notification) counters. This directly addresses the requirement to see topology change notifications and counts. The other commands show different spanning-tree information.

Answer

show bridge mac-table

Answer

show spanning-tree interface detail

Answer

show rstp neighbor

Question 7

Which RSTP port role replaces the legacy STP blocking and listening port states to provide rapid convergence?

Accepted Answer

Alternate and Backup roles. RSTP (802.1w) replaced the legacy STP blocking and listening states with the discarding state, and introduced the alternate and backup port roles to provide rapid failover. The alternate role provides an alternate path to the root bridge, while the backup role provides a backup path to the same segment. These roles enable immediate transition to forwarding when the root or designated port fails.

Answer

Disabled and Discarding roles

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Master and Slave roles

Answer

Root and Designated roles

Question 8

Which command allows you to verify whether a specific interface on an EX Series switch is operating as an edge port?

Accepted Answer

show spanning-tree interface. The command 'show spanning-tree interface' displays detailed spanning-tree information for a specific interface, including whether it is configured as an edge port. On EX Series switches, this output includes a field like 'Edge: Yes' or 'Edge: No' to indicate the edge port status. This is the correct way to verify edge port operation for a given interface.

Answer

show protocols rstp edge

Answer

show spanning-tree edge-ports

Answer

show interfaces edge

Question 9

Which protocol standard defines Rapid Spanning Tree Protocol (RSTP)?

Accepted Answer

IEEE 802.1W. RSTP is standardized under IEEE 802.1W, which was later incorporated into IEEE 802.1Q.

Answer

IEEE 802.3ad

Answer

IEEE 802.1X

Question 10

Which command is used to display comprehensive spanning tree status, including port roles, states, and priorities across all interfaces on an EX Series switch?

Accepted Answer

show spanning-tree interface. On EX Series switches, the 'show spanning-tree interface' command displays comprehensive spanning-tree information for all interfaces, including port roles (Root, Designated, Alternate), states (Forwarding, Blocking, Learning), priorities, costs, and designated bridge information. This is the go-to command for detailed STP verification.

Answer

show protocols rstp status

Answer

show spanning-tree summary

Answer

show bridge spanning-tree

Question 11

You are troubleshooting an RSTP topology where a port remains in the discarding state. You suspect a proposal-agreement handshake is failing. Which mechanism is responsible for accelerating this handshake in RSTP?

Accepted Answer

The proposal-agreement handshake mechanism. RSTP uses the proposal-agreement handshake to rapidly transition ports to forwarding without relying on timers. When a port is designated and wants to move to forwarding, it sends a proposal BPDU; the downstream bridge responds with an agreement BPDU, allowing immediate transition. This mechanism is responsible for accelerating convergence and resolving discarding states quickly.

Answer

The dynamic MAC address flushing timer

Answer

The portfast edge negotiation protocol

Answer

The root bridge hello timer synchronization protocol

Question 12

You notice that an EX Series switch is experiencing frequent topology changes, causing intermittent packet drops across VLANs. Which command helps identify which specific interface is generating the topology changes?

Accepted Answer

show spanning-tree statistics. The command 'show spanning-tree statistics' on Juniper EX switches displays per-interface counts of topology change notifications (TCNs) and topology changes (TCs), allowing you to pinpoint which interface is generating the frequent topology changes. This is the correct tool because it provides interface-level granularity for troubleshooting intermittent packet drops caused by STP reconvergence.

Answer

show spanning-tree bridge detail

Answer

show interfaces diagnostics optics

Answer

show rstp topology-changes

Question 13

An administrator implements MSTP and wants to configure a maximum hop count of 20 for instance 1. Which configuration command is used in Junos OS?

Accepted Answer

set protocols mstp configuration-name RegionA max-hops 20. The correct command is 'set protocols mstp configuration-name RegionA max-hops 20'. In Junos OS, MSTP configuration requires a region name (configuration-name) and a revision level. The maximum hop count is configured under the MSTP protocol hierarchy using the 'max-hops' parameter. This command sets the maximum hops to 20 for the MSTP region, which applies to all instances within that region, including instance 1. The other options are incorrect because they either use the wrong parameter name, apply to a different protocol, or configure a different timer.

Answer

set protocols mstp instance 1 hop-limit 20

Answer

set protocols rstp max-hops 20

Answer

set protocols mstp max-age 20

Question 14

You need to verify the regional configuration name and revision number currently active on an EX Series switch running MSTP. Which operational command should you use?

Accepted Answer

show spanning-tree mstp configuration. The command 'show spanning-tree mstp configuration' on Juniper EX switches displays the MSTP region configuration, including the region name, revision number, and VLAN-to-instance mappings. This directly answers the requirement to verify the regional configuration name and revision number currently active. Thus, option D is correct.

Answer

show protocols mstp status

Answer

show mstp region detail

Answer

show spanning-tree bridge

Question 15

Which TWO actions occur when you configure an interface as an edge port in RSTP? (Choose two.)

Accepted Answer

The port does not generate topology change notifications when its link goes up or down.. Option A is correct because an RSTP edge port is expected to connect to an end station, so when its link toggles it does not trigger a topology change notification (TCN) that would cause other switches to flush MAC address entries. Option C is correct because edge ports are configured with PortFast-like behavior, allowing them to bypass the listening and learning states and transition directly to forwarding when the link comes up. Option B is wrong because a backup port is a discarding RSTP role elected on a shared segment when a switch has multiple ports to the same segment, not a result of edge configuration. Option D is wrong because bridge priority is a separate STP parameter set on the bridge, not automatically changed to 0 by making a port an edge port. Option E is wrong because edge ports still participate in RSTP and send BPDUs; they do not stop transmitting BPDUs entirely.

Answer

The port automatically becomes a backup port.

Answer

The port's bridge priority is automatically set to 0.

Answer

The port stops transmitting BPDUs entirely under all circumstances.

Question 16

Which THREE parameters can be configured manually to influence root bridge selection in an RSTP network? (Choose three.)

Accepted Answer

Bridge priority increments (multiples of 4096). In RSTP (and STP/MSTP), root bridge election is driven by the Bridge ID, which is the concatenation of the bridge priority (a 4-bit value in multiples of 4096) and the bridge MAC address, so configuring the bridge priority (option D) directly influences which switch becomes root. Option A is also correct because the bridge priority can only be set in increments of 4096 (e.g., 0, 4096, 8192, 12288, 32768), and this granularity is what allows an administrator to manually tune root selection. Option B is correct because interface port cost (path cost) is a manually configurable parameter that influences the root port and designated port selection process, effectively steering the topology toward or away from a given root bridge. Option C (configuration revision number) and option E (MST regional configuration name) are MSTP-specific parameters used to define region membership and version consistency, not to influence root bridge election, so they do not belong.

Answer

Configuration revision number

Answer

MST regional configuration name

Question 17

Which TWO statements are true regarding loop protection in RSTP? (Choose two.)

Accepted Answer

It protects against loops caused by unidirectional link failures.. Option C is correct because RSTP loop protection (configured with the 'spanning-tree loopguard' or 'loop-protect' feature depending on vendor) is specifically designed to prevent Layer 2 loops that arise from unidirectional link failures, where a port stops receiving BPDUs but can still forward traffic. Option D is correct because when loop protection detects that BPDUs have unexpectedly stopped arriving on a non-designated port, it moves that port into the discarding (blocking) state rather than letting it transition to forwarding, thereby breaking the potential loop. Option A is incorrect because increasing bridge priority to maintain root status describes root guard or priority tuning, not loop protection. Option B is incorrect because loop protection operates independently of MSTP instance mapping and does not require full instance mapping before activation. Option E is incorrect because shutting down access ports upon receiving any BPDU describes BPDU Guard, not loop protection, and BPDU Guard typically err-disables the port rather than merely shutting it down.

Answer

It increases the bridge priority of the local switch to maintain root status.

Answer

It requires MSTP instances to be fully mapped before activation.

Answer

It shuts down access ports immediately upon receiving any BPDU.

Question 18

Which TWO conditions will cause an MSTP boundary port to drop BPDUs or recognize a region separation? (Choose two.)

Accepted Answer

Receipt of a legacy 802.1D or RSTP BPDU from outside the region. Option D is correct because when an MSTP boundary port receives a legacy 802.1D or RSTP BPDU from a switch outside the MST region, the receiving switch treats that neighbor as being in a different spanning-tree domain; the port becomes a boundary port and the BPDU is not interpreted as an internal MSTP BPDU, effectively recognizing a region separation. Option E is correct because MSTP switches include an MST configuration digest (a hash of the VLAN-to-instance mapping, region name, and revision level) in their BPDUs; if a received BPDU carries a mismatched digest, the switch recognizes that the sender belongs to a different MST region, so the port is treated as a boundary port and the internal MSTI information is not merged. Option A is incorrect because the regional root's bridge priority is an internal MSTI attribute, not part of the MST region identity, so a mismatch does not by itself cause region separation or BPDU dropping. Option B is incorrect because the root bridge's local MAC address is not part of the MST configuration identifier and does not define region membership. Option C is incorrect because exceeding the maximum regional hop count affects the stability and size of the MST region's spanning tree, but it does not cause a boundary port to drop BPDUs or recognize a region separation.

Answer

A mismatch in the bridge priority of the regional root

Answer

A mismatch in the local bridge MAC address of the root bridge

Answer

Exceeding the maximum configured regional hop count

Question 19

Which TWO parameters are typically configured when tuning RSTP timers on Junos EX Series switches? (Choose two.)

Accepted Answer

max-age. On Junos EX Series switches, RSTP timers are tuned under the spanning-tree protocols rstp interface or global configuration using the parameters hello-time and max-age. Option E (hello-time) is correct because it sets the interval between BPDU transmissions, defaulting to 2 seconds, and is a standard RSTP timer that can be adjusted to control convergence behavior. Option C (max-age) is correct because it defines the maximum time a BPDU is retained before being considered expired, defaulting to 20 seconds, and is a core RSTP timer configurable on EX switches. Option A (forward-delay-timer-scale) is not a valid Junos RSTP timer parameter; forward delay is not scaled this way in RSTP configuration. Option B (edge-port-delay) is not a recognized Junos RSTP timer; edge ports are configured with the edge keyword rather than a delay timer. Option D (topology-change-interval) is not a configurable RSTP timer on Junos EX switches; topology change handling is internal and not tuned via this parameter.

Answer

forward-delay-timer-scale

Answer

edge-port-delay

Answer

topology-change-interval

Question 20

Which TWO operational commands are useful for troubleshooting MSTP configuration mismatches and instance status on Junos switches? (Choose two.)

Accepted Answer

show spanning-tree interface. Option A, 'show spanning-tree interface', is correct because it displays per-interface spanning-tree operational state, including the MSTP instance, role, state, and any configuration mismatch indicators for each interface, which is exactly what you need to troubleshoot MSTP mismatches and instance status on Junos switches. Option D, 'show spanning-tree mstp configuration', is correct because it shows the operational MSTP configuration, including region name, revision level, and the VLAN-to-instance mapping, allowing you to compare the local MSTP parameters against peers and detect configuration mismatches. Option B, 'show bridge-domain mstp-status', is not a valid Junos operational command for this purpose. Option C, 'show mstp topology-digest', is not a valid Junos command; the topology digest is viewed via spanning-tree MSTP configuration output, not this syntax. Option E, 'show protocols mstp neighbors detail', is not a valid Junos command, as MSTP neighbor information is not exposed through the protocols hierarchy in that form.

Answer

show bridge-domain mstp-status

Answer

show mstp topology-digest

Answer

show protocols mstp neighbors detail