JNCIS-ENT Spanning Tree Practice Test 2 — 15 Questions

Question 1

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 2

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 3

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 4

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 5

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

Question 6

Which THREE actions are performed by an RSTP switch when a topology change is detected? (Choose three.)

Accepted Answer

Starting a TC while timer to flood notifications. Option A is correct because RSTP uses a TC While timer (typically twice the Hello time) on the port where the topology change was detected, during which TC notifications are flooded. Option B is correct because the switch sends topology change notifications (TCN BPDUs with the TC flag set) out of all its designated and root ports, except the port on which the change was received. Option E is correct because upon receiving a TC, RSTP switches flush dynamic MAC address entries learned on all non-edge ports to ensure rapid reconvergence. Option C is incorrect because root bridge election only occurs during initial convergence or when the current root fails, not as a direct response to a topology change. Option D is incorrect because edge ports remain in forwarding and are not reset to discarding; RSTP only flushes non-edge port MAC entries.

Answer

Electing a new root bridge automatically

Answer

Resetting all edge ports to the discarding state

Question 7

Which TWO conditions will cause an RSTP port configured with loop protection to transition into a discarding state? (Choose two.)

Accepted Answer

Absence of incoming BPDUs on a point-to-point root port. Loop protection triggers when BPDUs stop arriving on a point-to-point link or when an alternate/root port loses expected BPDU keepalives.

Answer

Receipt of any BPDU on an access edge port

Answer

Exhaustion of the regional hop count limit

Question 8

An administrator needs to configure a Juniper EX Series switch to participate in an existing Spanning Tree topology without risking the switch becoming the root bridge. Which command should be used to achieve this?

Accepted Answer

set protocols rstp bridge-priority 61440. Setting the RSTP bridge priority to 61440 ensures the switch has a very low priority (higher numerical value means lower priority), making it unlikely to become the root bridge. In RSTP, the bridge with the lowest priority becomes root; 61440 is the highest configurable value, so this switch will not be elected root if other switches have lower priorities.

Answer

set protocols rstp forward-delay 30

Answer

set protocols rstp root-protection

Answer

set protocols rstp disable

Question 9

While troubleshooting an RSTP topology on EX Series switches, you notice that a designated port has stopped receiving BPDUs from its upstream neighbor due to a unidirectional link failure. Which feature should you enable to prevent this port from transitioning to a Designated Forwarding state and causing a loop?

Accepted Answer

set protocols rstp interface ge-0/0/5.0 loop-protection. Loop protection (also called loop guard) on RSTP prevents a designated port from transitioning to forwarding when it stops receiving BPDUs due to a unidirectional link failure. Instead of moving to forwarding and potentially creating a loop, the port is placed in a loop-inconsistent state (blocking) until BPDUs resume.

Answer

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

Answer

set protocols rstp interface ge-0/0/5.0 no-negotiation

Answer

set protocols rstp interface ge-0/0/5.0 error-disable

Question 10

When verifying RSTP operation on a Junos switch, which operational command should you use to view the bridge identifier, root bridge identifier, and local port states?

Accepted Answer

show rstp bridge. The command 'show rstp bridge' on Junos switches displays the bridge identifier, root bridge identifier, and local port states for RSTP. This command is specifically designed to provide a comprehensive view of the RSTP topology and port roles. It is the correct operational command to verify RSTP operation as it includes all the required information.

Answer

show rstp statistics interface

Answer

show bridgestp detail

Answer

show spanning-tree summary

Question 11

An administrator is configuring MSTP on a Junos device and needs to map VLANs 10 through 20 to MST instance 2. Which configuration hierarchy is required to achieve this?

Accepted Answer

set protocols mstp configuration region-name Corp revision 1 instance 2 vlan 10-20. On Junos devices, MSTP instance-to-VLAN mapping is configured under the 'protocols mstp' hierarchy using the 'configuration' stanza, which includes the region name, revision level, and instance definitions with their associated VLAN lists. Option D correctly shows the full hierarchy: 'set protocols mstp configuration region-name Corp revision 1 instance 2 vlan 10-20'. The region name and revision must match on all switches in the same MST region for the instances to align.

Answer

set vlans vlan-range 10-20 mst-instance 2

Answer

set protocols msti instance-id 2 vlan-list 10-20

Answer

set protocols rstp instance 2 vlan 10-20

Question 12

In an RSTP-enabled network, which port role immediately transitions to the forwarding state upon failure of the designated port on the segment, bypassing the traditional listening and learning states?

Accepted Answer

Alternate port. In RSTP (IEEE 802.1w), an Alternate port is a discarding port that receives superior BPDUs from another bridge, meaning it provides an alternate path to the root. When the designated port on a segment fails, the Alternate port can immediately transition to forwarding because it has already pre-negotiated the topology change and can bypass the listening and learning states. This rapid failover is a key RSTP improvement over STP, where such a port would need to go through listening and learning (taking up to 30 seconds).

Answer

Root port

Answer

Disabled port

Answer

Designated port

Question 13

Which TWO actions occur when you configure an interface as an edge port using 'set protocols rstp interface ge-0/0/10.0 edge' on a Juniper switch? (Choose two.)

Accepted Answer

The port immediately transitions to the forwarding state, bypassing listening and learning phases.. Edge ports immediately transition to the forwarding state and do not generate topology change notifications (TCNs) when their link status toggles.

Answer

The port automatically assumes a Root Port role for the local switch.

Answer

The port will drop any incoming configuration BPDUs.

Question 14

When configuring and verifying MSTP on Junos devices, which THREE statements are correct regarding regional boundaries and communication? (Choose three.)

Accepted Answer

All switches within the same MST region share a common MST configuration identifier digest.. MSTP uses an Internal Spanning Tree (IST) inside a region and communicates with other regions and legacy STP via the Common Spanning Tree (CST) and region boundary ports.

Answer

MSTP regions must have identical bridge priorities for instance 0 to function.

Question 15

You administer an EX Series switch running RSTP. Interface ge-0/0/12 is connected to a downstream switch that is the designated bridge for that segment. However, you notice that ge-0/0/12 is in a discarding state and not forwarding traffic. You want to verify the port's RSTP role and state. Which command displays the RSTP role and state for ge-0/0/12?

Accepted Answer

show spanning-tree interface ge-0/0/12 detail. The correct command is 'show spanning-tree interface ge-0/0/12 detail'. This command provides detailed RSTP information for the specified interface, including its role and state. In RSTP, a port can be in discarding state even if it is a designated port, for example during synchronization or if it is an alternate port. The detail view helps diagnose such conditions by showing the port role, state, and other parameters.

Answer

show spanning-tree interface ge-0/0/12 brief

Answer

show spanning-tree bridge interface ge-0/0/12

Answer

show spanning-tree interface ge-0/0/12