JNCIP-ENT Ethernet Switching And Spanning Tree — Timed 20 Questions

Question 1

You are configuring VSTP (VLAN Spanning Tree Protocol) on Juniper EX switches to interoperate with legacy third-party switches running PVST+. Which operational command allows you to verify the VSTP bridge identifier and root bridge status per VLAN?

20 minute time limit — choose an answer to begin.

Accepted Answer

show vstp vlan. The command 'show vstp vlan' displays the VSTP state, root bridge, and port roles for each VLAN.

Answer

show ethernet-switching vstp

Answer

show spanning-tree vlan detail

Answer

show bridge vstp-status

Question 2

You are reviewing the MSTP configuration on an EX4300 switch. You want to assign VLANs 10 through 50 to Multiple Spanning Tree Instance (MSTI) 2. Which syntax under 'protocols mstp' is correct?

Accepted Answer

set protocols mstp configuration-name Region1 instance instance2 vlan [ 10..50 ] or set protocols mstp region-name Region1 instance instance2 vlan 10-50. MSTP instance configuration maps VLAN lists to specific MSTI numbers in Junos.

Answer

set protocols mstp configuration-name Region1 revision 1 instance msti2 vlan [ 10-50 ]

Answer

set protocols mstp region-name Region1 instance-map instance-2 vlan 10 to 50

Answer

set protocols mstp msti-instance 2 vlan-members 10-50

Question 3

An enterprise network uses VSTP. During a topology change, network administrators notice excessive flooding of broadcast traffic. Which Juniper configuration statement under 'protocols vstp' prevents unnecessary topology change flooding by restricting TCN propagation?

Accepted Answer

set protocols vstp interface ge-0/0/1 bpdu-guard. Root guard and BPDU guard protect ports, but for VSTP tuning, bpdu-block-on-edge or restricted role configuration helps. However, setting interface parameters correctly prevents topology shifts. The correct parameter to prevent TCNs on access ports is bpdu-guard or no-root-port.

Answer

set protocols vstp flood-suppression enable

Answer

set protocols vstp disable-tcn-propagation

Answer

set protocols vstp interface ge-0/0/1 no-root-port

Question 4

You are troubleshooting a Q-in-Q tunneling implementation on an MX Series router where customer VLAN tags are being stripped instead of preserved across the service provider core. Under which configuration hierarchy should the 'vlan-tagging' and 'pop-pop' or 'push' actions be applied on the interface?

Accepted Answer

set interfaces ge-0/0/1 flexible-vlan-tagging. For Q-in-Q tunneling on Juniper routing and switching platforms, flexible VLAN tagging is configured under 'interfaces <interface> flexible-vlan-tagging'.

Answer

set protocols l2-learning interface ge-0/0/1 q-in-q

Answer

set bridge-domains bd vlan-id-list 10

Answer

set interfaces ge-0/0/1 encapsulation flexible-ethernet-services

Question 5

An enterprise network engineer needs to configure a trunk port on a Juniper EX Series switch running Junos OS to allow multiple VLANs while explicitly blocking VLAN 99. Which configuration statement under 'interfaces ge-0/0/1 unit 0 family ethernet-switching' correctly achieves this?

Accepted Answer

vlan-members except 99;. The 'vlan-members none' or explicitly listing allowed VLANs excludes unspecified ones, but to explicitly prune a VLAN while allowing others, the 'vlan-members except' option is used.

Answer

exclude-vlan-members 99;

Answer

block-vlan 99;

Answer

vlan-members [ vlan-members except 99 ];

Question 6

An administrator needs to enable BPDU protection on edge access ports in a VSTP environment to prevent loops caused by unauthorized switches. Which command under 'protocols vstp' accomplishes this globally or per-interface?

Accepted Answer

set protocols vstp interface ge-0/0/1 bpdu-guard. bpdu-guard under interface configuration in VSTP drops or disables the port upon receiving BPDUs.

Answer

set protocols vstp interface ge-0/0/1 disable-on-bpdu

Answer

set protocols vstp interface ge-0/0/1 edge-port protect

Answer

set protocols vstp interface ge-0/0/1 drop-bpdu

Question 7

An enterprise network runs MSTP across an EX Series core. An administrator wants to ensure that a specific core port never becomes the root port for any MSTI, protecting the upstream core path. Which command achieves this?

Accepted Answer

set protocols mstp interface ge-0/0/0 no-root-port. Root protection in MSTP is enabled via the 'no-root-port' or 'root-protection' statement under interface configuration in MSTP. In Junos, 'no-root-port' is used.

Answer

set protocols mstp interface ge-0/0/0 root-guard

Answer

set protocols mstp interface ge-0/0/0 priority 0

Answer

set protocols mstp interface ge-0/0/0 restrict-role root

Question 8

You are configuring selective Q-in-Q stacking on an MX Series router. Unlike standard Q-in-Q where all customer VLANs are encapsulated into a single provider VLAN, selective Q-in-Q allows you to map specific customer VLAN ranges to different provider VLANs. Which configuration element enables this granularity?

Accepted Answer

vlan-map match statement with specific input VLAN ID filters. Flexible VLAN tagging combined with 'vlan-map' statements defining match criteria allows selective Q-in-Q.

Answer

protocols l2-learning selective-stacking

Answer

global bridge-domain vlan-translation enable

Answer

routing-instance forwarding-options selective-qinq

Question 9

An enterprise network operates VSTP across 150 VLANs. The network engineering team notices high CPU utilization on the Routing Engine due to VSTP BPDU processing. Which Junos command or feature can mitigate this control-plane load while keeping VSTP operational?

Accepted Answer

set protocols vstp hello-time 5. BPDU rate-limiting or adjusting VSTP hello timers helps reduce control-plane processing overhead. In Junos, VSTP timer tuning and bpdu processing limits control overhead.

Answer

set system pfe bpdu-bypass enable

Answer

set forwarding-options storm-control interfaces all vstp

Answer

set protocols vstp no-bpdu-generation

Question 10

An administrator needs to configure a port on an EX series switch to act as a Designated Port in MSTP, forcing it to forward traffic regardless of incoming superior BPDUs from an untrusted switch. Which feature should be enabled on this port?

Accepted Answer

Loop guard. Loop guard or root guard protects ports. Specifically, loop guard prevents alternate or backup ports from becoming designated ports when BPDUs are lost.

Answer

Root guard

Answer

STP disable

Answer

BPDU guard

Question 11

You are troubleshooting MAC address learning on an EX Series switch and want to display dynamic MAC entries learned specifically on interface ge-0/0/5. Which operational command should you use?

Accepted Answer

show bridge mac-table interface ge-0/0/5. The command 'show bridge mac-table interface ge-0/0/5' displays MAC addresses learned on that interface.

Answer

show mac-address-table interface ge-0/0/5

Answer

show ethernet-switching table port ge-0/0/5

Answer

show l2-learning interface ge-0/0/5 macs

Question 12

You are configuring a Juniper EX Series switch to participate in an existing VSTP domain. You notice that certain VLANs are not participating in VSTP and are dropping traffic. What is the default behavior of VSTP on a newly created VLAN in Junos?

Accepted Answer

VSTP is automatically enabled on all newly created VLANs by default.. By default, when a new VLAN is created on a Juniper switch running VSTP, VSTP is automatically enabled for that VLAN. However, older Junos versions or specific configurations require explicit inclusion. Let's verify: VSTP runs by default on all defined VLANs unless explicitly disabled.

Answer

VSTP must be manually enabled per VLAN under 'protocols vstp vlan-id <id>' or it remains disabled.

Answer

VSTP places new VLANs into a suspended state until trunk encapsulations match.

Answer

VSTP blocks all ports on new VLANs until root bridge priority is manually assigned.

Question 13

An administrator needs to verify the operational state of an MSTP instance named 'MSTI-1' on an EX Series switch, including its designated root and regional parameters. Which operational command should be executed?

Accepted Answer

show mstp instance detail. The command 'show mstp instance detail' or 'show spanning-tree mstp instance' displays instance details. In Junos, it is 'show mstp instance [name]'.

Answer

show ethernet-switching mstp detail

Answer

show bridge mstp-status instance-1

Answer

show spanning-tree instance msti-1

Question 14

In a Juniper Q-in-Q deployment, customer frames are received with an EtherType of 0x8100 (IEEE 802.1Q). The service provider network uses a custom provider EtherType of 0x9100 for outer tags. Where and how is this custom EtherType configured on an MX Series router?

Accepted Answer

Under 'interfaces ge-0/0/1 tpid 0x9100' on the physical interface. Custom inner/outer TPIDs (Tag Protocol Identifiers) are configured on the physical interface using 'tpid 0x9100'.

Answer

Under 'protocols l2-learning provider-tpid 0x9100'

Answer

Under 'system ethernet-switching tpid-table add 0x9100'

Answer

Under 'routing-instances bd bridge-options tpid 0x9100'

Question 15

Which TWO configuration statements are required when setting up an MSTP region on a Juniper EX Series switch? (Choose two)

Accepted Answer

set protocols mstp revision 1. MSTP configuration requires defining a configuration name and a revision number within the mstp protocol hierarchy.

Answer

set protocols mstp msti-priority 4096

Answer

set protocols mstp region-type standard

Answer

set protocols mstp force-version 802.1s

Question 16

Which THREE mechanisms are provided by Juniper switches running MSTP to protect against bridging loops caused by unidirectional link failures or misbehaving switches? (Choose three)

Accepted Answer

Root guard. Loop guard, root guard, and bpdu guard are standard STP/MSTP protection features available on Juniper switches.

Answer

Storm control broadcast filtering

Answer

MACsec cryptographic validation

Question 17

When designing a high-scale enterprise campus network using Juniper EX Series switches, which THREE best practices should be followed for Spanning Tree Protocol design? (Choose three)

Accepted Answer

Enable BPDU guard and loop guard on all edge access ports.. Best practices include placing root bridges at the core/distribution layer, using edge protection features, and maintaining consistent region parameters.

Answer

Use default bridge priority values on all core switches to allow automated root election.

Answer

Disable Spanning Tree entirely on all access ports to ensure faster convergence.

Question 18

An engineer is configuring QinQ (802.1ad) tunneling on a Juniper EX Series switch to transport customer VLANs across a service provider core. The customer-facing access port receives tagged frames with outer VLAN ID 100 and inner VLAN ID 200. Which configuration statement correctly defines the interface encapsulation and VLAN mapping?

Accepted Answer

set interfaces ge-0/0/1 encapsulation vlan-stacking and set interfaces ge-0/0/1 unit 0 family ethernet-switching input-vlan-map push and set interfaces ge-0/0/1 unit 0 family ethernet-switching output-vlan-map pop. To configure Q-in-Q tunneling on Juniper switches, the customer-facing port requires 'flexible-ethernet-services' encapsulation and a 'vlan-map' configuration that maps the incoming outer VLAN to the provider core VLAN.

Answer

set interfaces ge-0/0/1 encapsulation flexible-ethernet-services and set interfaces ge-0/0/1 unit 0 family ethernet-switching vlan-tagging

Answer

set interfaces ge-0/0/1 encapsulation flexible-ethernet-services and set interfaces ge-0/0/1 unit 0 family ethernet-switching interface-mode access

Answer

set interfaces ge-0/0/1 encapsulation qinq-access and set interfaces ge-0/0/1 unit 0 family ethernet-switching vlan-members 100

Question 19

An enterprise network running VSTP (VLAN Spanning Tree Protocol) across Juniper EX Series switches experiences unexpected loops because a newly added switch does not participate in VSTP for VLAN 50. Upon investigation, you discover the VSTP BPDU multicast destination MAC address is being dropped by a third-party non-Juniper intermediate switch. What is the standard VSTP MAC address used for BPDU transmission?

Accepted Answer

01:00:0c:cc:cc:cd. VSTP utilizes the PVST+ proprietary destination MAC address of 01:00:0c:cc:cc:cd to interoperate and transmit per-VLAN spanning tree BPDUs.

Answer

01:80:c2:00:00:08

Answer

01:80:c2:00:00:00

Answer

01:80:c2:00:00:0f

Question 20

When designing large-scale Layer 2 networks using VSTP, you notice high CPU utilization on the Routing Engine due to the large number of topology changes and BPDU processing across 150 VLANs. Which command can you apply globally or per-interface in Junos to optimize VSTP performance and mitigate CPU load?

Accepted Answer

Migrate from VSTP to MSTP to consolidate multiple VLAN spanning trees into a single or few MST instances.. To limit the impact of excessive VSTP instances, Junos allows tuning BPDU transmission intervals or disabling VSTP on unnecessary ports, but more importantly, optimizing BPDU processing or utilizing MSTP is recommended. However, bpdu-block and interface tuning are key features. Wait, let's look at the options.

Answer

set protocols vstp bpdu-block-on-edge

Answer

set forwarding-options spanning-tree-processor offload

Answer

set protocols vstp no-normalization-check