JNCIS-ENT Spanning Tree Practice Test 4 — 15 Questions

Question 1

You 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?

Accepted Answer

set protocols rstp bridge-priority 4096. On Juniper EX Series switches, RSTP is configured under the [edit protocols rstp] hierarchy. The bridge priority is set using the 'bridge-priority' command, so the correct full command is 'set protocols rstp bridge-priority 4096'. This value is used in the bridge ID and must be a multiple of 4096 (ranging from 0 to 61440). Setting it to 4096 makes the switch highly likely to become the root bridge, assuming no other switch has a lower priority.

Answer

set spanning-tree priority 4096

Answer

set protocols rstp priority 4096

Answer

set bridge-priority 4096

Question 2

When a topology change occurs in an RSTP network, how are MAC address forwarding table entries affected across the switches?

Accepted Answer

MAC address table entries associated with non-edge ports are flushed immediately.. When a topology change occurs in RSTP, MAC address table entries associated with non-edge ports are flushed immediately. This allows the switches to relearn MAC addresses on the new topology, preventing forwarding to stale locations. Edge ports are not flushed because they connect to end devices and are not affected by topology changes.

Answer

MAC addresses are relearned only after the root bridge sends a direct clearing command.

Answer

MAC address entries are retained until the standard 300-second aging timer expires.

Answer

MAC address tables are completely cleared for all ports, including edge ports.

Question 3

An administrator configures link aggregation (ae0) using LACP between two EX Series switches running RSTP. How does RSTP treat the aggregated interface ae0?

Accepted Answer

RSTP treats the aggregate interface as a single logical port.. RSTP operates on the aggregate interface (ae0) as a single logical port, not on the individual physical member links. This means the entire bundle is either in a forwarding or discarding state based on the spanning tree topology, and LACP handles the distribution of traffic across the active member links. The aggregated interface presents a single MAC address and a single port cost to RSTP, so the spanning tree calculation sees it as one link.

Answer

RSTP places all member links except the primary active link into a permanent discarding state.

Answer

RSTP disables itself on aggregated links to prevent conflicts with LACP.

Answer

RSTP runs an independent spanning tree instance on each physical member link of the aggregate.

Question 4

An 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?

Accepted Answer

It is automatically mapped to Instance 0 (the CIST).. In MSTP (Multiple Spanning Tree Protocol, IEEE 802.1s), VLANs that are not explicitly mapped to a specific MST instance are automatically assigned to the Common and Internal Spanning Tree (CIST), which is MST instance 0. The CIST provides connectivity for all VLANs that are not mapped to other instances and also interconnects MST regions. Therefore, the packet is processed by Instance 0, not dropped or flooded indiscriminately.

Answer

It is processed by the default RSTP instance running outside MSTP.

Answer

It is dropped by the switch because unmapped VLANs are considered invalid in MSTP.

Answer

It floods to all ports regardless of spanning tree port states.

Question 5

What is the primary purpose of the Hello timer in RSTP?

Accepted Answer

It dictates how often configuration BPDUs are generated and sent by the root bridge.. In RSTP (IEEE 802.1w), the Hello timer defines the interval at which the root bridge generates and transmits configuration BPDUs. Non-root bridges do not originate BPDUs; they relay the root's BPDUs downstream after updating them. The default Hello time is 2 seconds, and this timer is configured only on the root bridge (or the intended root) and propagated throughout the spanning tree via the BPDU fields. Thus, the Hello timer directly controls the frequency of configuration BPDU generation and transmission by the root bridge.

Answer

It sets the interval between topology change notifications.

Answer

It determines the maximum time a switch waits before declaring a neighbor dead.

Answer

It defines the duration of the listening and learning states during convergence.

Question 6

An 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?

Accepted Answer

By comparing the received MST configuration digest and region parameters with its own.. MSTP (IEEE 802.1s) identifies region boundaries by comparing the MST Configuration Identifier carried in BPDUs, which consists of the configuration name, revision level, and a 16-byte configuration digest computed via HMAC-MD5 over the VLAN-to-instance mapping. If any of these parameters differ from the local switch's own MST configuration, the receiving switch treats the port as a boundary port and interoperates with the neighbor using CIST (MST instance 0) rather than a shared MSTI. This is why option C is correct — the digest and region parameters are the definitive boundary markers.

Answer

When the incoming BPDU contains an instance count greater than the local switch capacity.

Answer

When the CIST external path cost exceeds the maximum hop count.

Answer

By detecting a mismatch in the system MAC address of the root bridge.

Question 7

You 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?

Accepted Answer

set protocols rstp interface ge-0/0/10.0 disable. Junos supports per-interface RSTP disable via 'set protocols rstp interface <name> disable', which stops BPDU processing and STP participation on that specific interface while leaving RSTP running globally on the rest of the switch. This is the correct way to exempt an access port from spanning tree without affecting other ports.

Answer

delete protocols rstp

Answer

set interfaces ge-0/0/10.0 spanning-tree disable

Answer

set protocols rstp interface ge-0/0/10.0 mode off

Question 8

An enterprise network uses MSTP. What is the function of the Internal Root in an MST region?

Accepted Answer

It acts as the root switch for a specific MSTI within the local MST region.. In MSTP (IEEE 802.1Q), each MST instance (MSTI) within a region elects its own root bridge, called the Internal Root (or regional root for that MSTI). The Internal Root is the bridge with the lowest bridge ID for that specific MSTI inside the local region, and it is distinct from the CIST Regional Root, which connects the region to the outside. The Internal Root governs topology within the MSTI's scope inside the region.

Answer

It translates MST BPDUs into legacy STP BPDUs at the regional boundary.

Answer

It manages the CIST instance across all connected foreign MST regions.

Answer

It serves as the sole bridge responsible for generating the MST configuration digest.

Question 9

What action does the 'set protocols rstp max-age <seconds>' command perform on a Juniper switch?

Accepted Answer

It defines how long BPDU information is stored before being considered expired.. The 'max-age' timer in RSTP/MSTP (IEEE 802.1D-2004 / 802.1Q) defines how long a bridge retains received BPDU information before it declares that information stale and ages it out. When the max-age timer expires without receiving a fresh BPDU from the designated bridge, the port transitions and topology information is recalculated. On Juniper switches, 'set protocols rstp max-age <seconds>' tunes this BPDU-aging timer, with a default of 20 seconds and a valid range of 6–40 seconds.

Answer

It sets the maximum time a port can remain in the discarding state during bootup.

Answer

It limits the maximum age of MAC table entries learned via spanning tree.

Answer

It restricts the maximum number of hops an MSTP BPDU can traverse.

Question 10

When configuring MSTP, how is the regional maximum hop count enforced across switches within the same MST region?

Accepted Answer

BPDUs carry a hop count that is decremented by each bridge; when it reaches zero, the BPDU is dropped.. In MSTP, the regional maximum hop count is enforced by the hop count field in BPDUs. The regional root sends BPDUs with a hop count equal to the maximum hop count (default 20). Each bridge that receives the BPDU decrements the hop count by one before forwarding it. When the hop count reaches zero, the BPDU is dropped, preventing loops and limiting the network diameter within the MST region.

Answer

The regional root periodically floods drop-commands if the network diameter exceeds the limit.

Answer

Switches drop any incoming BPDUs that exceed the statically configured Max Age timer.

Answer

Ports exceeding the hop count are automatically placed into the error-disabled state.

Question 11

You are configuring an EX Series switch and want to set the forward delay timer to 15 seconds. Which command is used?

Accepted Answer

set protocols rstp forward-delay 15. On Junos EX Series switches, RSTP is configured under the [edit protocols rstp] hierarchy, and the forward-delay timer is set with 'set protocols rstp forward-delay 15'. This command sets the RSTP forward delay to 15 seconds, controlling how long a port stays in the Listening and Learning states before transitioning to Forwarding.

Answer

set bridge forward-delay 15

Answer

set spanning-tree forward-delay 15

Answer

set protocols rstp delay 15

Question 12

An 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?

Accepted Answer

Root-inconsistent (discarding) state. When root protection is configured on a designated port and that port receives a superior BPDU, the port is placed into the root-inconsistent (discarding) state rather than being error-disabled. This prevents the port from becoming a root port and preserves the intended root bridge placement.

Answer

Alternate port state

Answer

Error-disabled state

Answer

Listening state

Question 13

Which TWO port roles are defined in Rapid Spanning Tree Protocol (RSTP) but did not exist in legacy 802.1D STP? (Choose two.)

Accepted Answer

Backup port. RSTP (IEEE 802.1w) introduced two new port roles that did not exist in legacy 802.1D STP: the Alternate port (option E) and the Backup port (option C). An Alternate port is a discarding port that receives superior BPDUs from another bridge and serves as an immediate replacement for the Root port if it fails, while a Backup port is a discarding port that receives superior BPDUs from its own bridge (a redundant connection to the same shared segment) and can immediately replace a Designated port. Both roles enable RSTP's rapid failover by allowing pre-computed, ready-to-use backup paths instead of waiting for STP timers. The other options are not new to RSTP: Root port (B) and Designated port (D) are the two forwarding roles already defined in 802.1D STP, and Master port (A) is a role specific to Multiple Spanning Tree Protocol (MSTP, 802.1s) used on boundary ports toward the Common Spanning Tree region, not a role introduced by RSTP.

Answer

Master port

Answer

Root port

Answer

Designated port

Question 14

Which TWO commands can be used to view spanning tree information on a Juniper EX Series switch? (Choose two.)

Accepted Answer

show spanning-tree interface. Option B, 'show spanning-tree interface', is correct because on Juniper EX Series switches running Junos, this operational command displays per-interface spanning-tree information such as role, state, port cost, and edge/point-to-point settings for the specified interface. Option C, 'show spanning-tree bridge', is correct because it displays bridge-level spanning-tree parameters, including the bridge ID, root bridge information, timers, and topology change counters. These two commands are the standard Junos CLI forms for inspecting STP/RSTP/MSTP on EX switches. Option A is not a valid Junos command; the 'show bridge' hierarchy is not used for spanning tree on EX. Option D is invalid because 'show rstp summary' is not a Junos operational command. Option E is invalid because 'show protocols spanning-tree summary' is not a Junos command; 'show protocols' is used for routing protocol status, not spanning tree.

Answer

show bridge spanning-tree status

Answer

show rstp summary

Answer

show protocols spanning-tree summary

Question 15

What is the primary benefit of enabling edge port configuration on switch access ports connected to end devices?

Accepted Answer

It allows the port to transition immediately to the forwarding state, avoiding startup delays.. Enabling edge port configuration on switch access ports connected to end devices allows the port to transition immediately to the forwarding state, bypassing the listening and learning states of STP/RSTP. This avoids startup delays (up to 30 seconds with classic STP) and improves convergence for end-user devices. Edge ports are also known as PortFast in Cisco terminology.

Answer

It forces the port to operate exclusively in full-duplex mode.

Answer

It increases the interface bandwidth by disabling spanning tree calculations entirely.

Answer

It enables automatic detection of unauthorized rogue bridges on access links.