JNCIS-ENT High Availability — 20 Questions

Question 1

You are troubleshooting a Virtual Chassis formed by four EX4300 switches. The primary Routing Engine (RE) unexpectedly rebooted. Upon recovery, you notice that a different member switch has taken over the role of primary. How can you prevent this specific member from becoming the primary RE in the future?

Choose an answer to begin — your selection is scored in the full session.

Accepted Answer

Modify the routing-engine mastership-priority value on the preferred member switch to be higher than the others.. In a Juniper Virtual Chassis, the primary Routing Engine (RE) is elected based on the mastership priority value configured on each member. The switch with the highest priority becomes primary; if there is a tie, the one with the lowest MAC address wins. To prevent a specific member from becoming primary, you must ensure that the preferred member has a higher mastership priority than all others. Option B directly addresses this by modifying the routing-engine mastership-priority value on the preferred member to be higher, thus ensuring it becomes primary upon election. This is the standard method to control primary RE selection in a Virtual Chassis.

Answer

Disable Graceful Routing Engine Switchover (GRES) on the backup routing engine.

Answer

Enable preemption globally on all members in the virtual-chassis configuration.

Answer

Configure the non-preferred switch with a routing-engine user account restriction.

Question 2

An administrator wants to verify that GRES is operating correctly on an MX series router with dual Routing Engines. Which operational command should be used?

Accepted Answer

show chassis routing-engine. The correct command is `show chassis routing-engine`, which displays the status of both Routing Engines, including mastership, uptime, and GRES (Graceful Routing Engine Switchover) state. On MX series routers with dual REs, this command shows whether GRES is enabled and operational, along with other HA parameters. It is the standard operational command to verify GRES functionality.

Answer

show system backup-router

Answer

show virtual-chassis gres

Answer

show gres statistics

Question 3

You notice frequent mastership transitions in a VRRP group configured between two EX4300 switches. The logs indicate that VRRP advertisements are intermittently missing. Which troubleshooting step addresses the underlying cause without changing the master priority?

Accepted Answer

Check physical interface error counters and increase the VRRP advertisement interval if packet drops occur.. Intermittent missing VRRP advertisements usually stem from physical-layer packet loss (CRC errors, drops, congestion) rather than VRRP configuration itself. Checking interface error counters identifies the underlying link problem, and increasing the VRRP advertisement interval makes the protocol more tolerant of transient drops without altering mastership priority. This addresses the root cause while preserving the existing priority design.

Answer

Disable Virtual Chassis on both switches to allow VRRP to function properly.

Answer

Remove LACP from all aggregated interfaces connected to the VRRP routers.

Answer

Configure Nonstop Active Routing (NSR) under the VRRP hierarchy.

Question 4

During an audit of a Juniper EX4300 Virtual Chassis, you notice that member 1 and member 2 are connected via mixed 1G and 10G uplink ports. How does Junos handle mixed-speed Virtual Chassis port configurations?

Accepted Answer

Virtual Chassis ports (VCPs) must be explicitly configured to match supported transceiver speeds, and mixing unsupported interface types will result in port initialization failure.. Junos EX series virtual chassis ports can be configured for specific speeds, but mismatched or unsupported port combinations will cause initialization failures or link errors depending on the platform.

Answer

Mixed speed links automatically drop to 1G to maintain backward compatibility without administrative intervention.

Answer

Junos automatically load-balances stacking traffic across 1G and 10G links using weighted round-robin.

Answer

Only the 10G ports will carry control plane traffic while 1G ports are restricted to data transit.

Question 5

Which command displays the status of Virtual Chassis member switches, their roles, and their serial numbers on an EX series switch?

Accepted Answer

show virtual-chassis. The 'show virtual-chassis' command is the primary operational command to view member status, roles (Master/Backup/Linecard), and IDs.

Answer

show lacp statistics

Answer

show vrrp summary

Question 6

Which CLI command displays detailed information about VRRP groups, including master router status, virtual IP addresses, and advertisement intervals on an EX series switch?

Accepted Answer

show vrrp detail. The 'show vrrp interface' or 'show vrrp detail' commands provide granular details regarding VRRP instance status.

Answer

show lacp vrrp-status

Answer

show spanning-tree vrrp

Answer

show chassis vrrp

Question 7

You are troubleshooting a BGP session failure immediately following a manual Routing Engine switchover on an MX series router configured with GRES and NSR. BGP drops its peering session. What is the most likely reason for the failure?

Accepted Answer

NSR was not enabled under the BGP routing-options protocol hierarchy, causing the protocol daemon state to not synchronize to the backup Routing Engine.. NSR (Nonstop Active Routing) must be explicitly enabled under the [edit routing-options nonstop-routing] hierarchy for BGP to synchronize its state (peering sessions, routing tables, etc.) to the backup Routing Engine. Without NSR, a Routing Engine switchover causes the BGP process to restart on the new master, tearing down and re-establishing all BGP sessions. GRES alone only preserves the forwarding plane and kernel state, not the protocol daemon state. Therefore, the most likely reason for the BGP session drop is that NSR was not enabled.

Answer

The Virtual Chassis stacking cable was disconnected during the switchover.

Answer

VRRP preemption caused the default gateway IP to shift to the backup engine.

Answer

The LACP timeout on the aggregated uplink was set to slow mode.

Question 8

Which CLI command verifies that OSPF Graceful Restart helper mode is enabled by default on a Juniper routing platform?

Accepted Answer

show ospf overview. Running 'show ospf neighbor detail' or 'show ospf overview' displays protocol-level graceful restart helper status.

Answer

show chassis ospf-status

Answer

show lacp ospf-helper

Answer

show vrrp ospf-restart

Question 9

An engineer configures VRRP between two routers. Client traffic destined for the outside network is being dropped intermittently. Investigation reveals that the VRRP master is responding to ARP requests with its physical MAC address instead of the virtual router MAC address. What is the cause of this behavior?

Accepted Answer

The virtual-mac-address option was explicitly disabled, or an incorrect configuration caused the master to use its physical interface MAC address for virtual IP ARP responses.. VRRP masters normally reply to ARP for the virtual IP using the virtual MAC (00:00:5e:00:01:xx). If the master answers with its physical interface MAC instead, the virtual MAC behavior has been disabled or misconfigured — for example, the 'virtual-mac-address' option was turned off, or the VRRP group is not properly bound to the interface. This causes clients to cache the physical MAC, so when mastership changes, traffic is black-holed until ARP entries expire.

Answer

LACP fast mode was enabled on the VLAN interface.

Answer

GRES was disabled on the backup routing engine.

Answer

NSR was enabled without configuring BGP Graceful Restart.

Question 10

An engineer wants to configure a Link Aggregation Group (LAG) to load-balance traffic based on Layer 3 and Layer 4 header fields (source/destination IP and source/destination port) rather than just Layer 2 MAC addresses. How is this achieved in Junos?

Accepted Answer

Configure load-balance profiles under [edit forwarding-options hash-key] specifying inet protocols.. Junos allows configuring load-balancing profiles under the forwarding-options hierarchical level to include IP and port hash fields.

Answer

Enable LACP multi-chassis hashing under the chassis hierarchy.

Answer

Configure 'set interfaces ae0 load-balance layer-3-4' directly under the interface stanza.

Answer

Set the LACP periodic transmission interval to fast.

Question 11

You are troubleshooting a split-brain condition in an EX4300 Virtual Chassis where both partitions have elected a primary Routing Engine. Which feature can be configured to detect this condition and shut down downstream ports on the non-designated partition to prevent duplicate packet forwarding?

Accepted Answer

Virtual Chassis split-detection or auxiliary management connection mechanisms that evaluate reachability between partitions to disable the non-primary partition.. Dual-Routing Engine Advanced Support (DREAS) or specifically configured mechanisms like MAC-sec / split-detection scripts help, but let's look at Juniper's specific Virtual Chassis split-brain mitigation: Junos supports mechanisms such as MAC pinning or specifically configured split detection algorithms, or let's examine standard Juniper terminology. Wait, Junos Virtual Chassis uses Virtual Chassis Automatic Partition Detection or dedicated split-brain detection features. Let's see which option accurately names a valid approach.

Answer

Setting LACP mode to active on the stacking ports.

Answer

Enabling GRES on both partitions simultaneously.

Answer

Configuring VRRP with a priority of 255 on all member interfaces.

Question 12

An engineer configures Nonstop Active Routing (NSR) on an MX series router. What is required for NSR to function properly on BGP peering sessions?

Accepted Answer

GRES must be enabled at the chassis level, and NSR must be explicitly enabled under the BGP protocol hierarchy.. NSR requires GRES enabled at the chassis level and NSR configured specifically under the routing-options protocols bfgp hierarchy.

Answer

Graceful Restart must be disabled globally on all routers.

Answer

VRRP must be configured on all BGP peering interfaces.

Answer

LACP fast mode must be enabled on all physical uplinks.

Question 13

Which TWO operational commands are used to check the health and status of Routing Engines and GRES on an MX series router? (Choose two)

Accepted Answer

show chassis routing-engine. Option A, 'show chassis routing-engine', is correct because on Junos MX series routers this command displays the operational status, role (Master/Backup), temperature, memory, CPU utilization, uptime, and failover state of each Routing Engine, which is essential for verifying RE health and GRES (Graceful Routing Engine Switchover) readiness. Option D, 'show chassis alarms', is correct because it reports any chassis alarms, including RE-related alarms such as RE failure, GRES/NSR synchronization issues, or fabric/hardware faults, giving an immediate health indicator for the Routing Engines and redundancy state. Option B, 'show vrrp summary', is incorrect because VRRP is a first-hop redundancy protocol for interface/IP redundancy, not a tool for checking Routing Engine or GRES status. Option C, 'show lacp statistics', is incorrect because LACP statistics relate to link aggregation (LAG) bundle health, not Routing Engine or GRES operational state. Option E, 'show virtual-chassis', is incorrect because virtual chassis is a technology used on EX/QFX switches for interconnecting multiple devices, not an MX Routing Engine/GRES health command.

Answer

show vrrp summary

Answer

show lacp statistics

Answer

show virtual-chassis

Question 14

Which THREE factors must be considered when configuring a Link Aggregation Group (LAG) with LACP on Junos switches? (Choose three)

Accepted Answer

A minimum number of active links can be configured to ensure traffic drops if bandwidth falls below a threshold.. Option A is correct because Junos LACP LAGs support configuring a minimum number of active links (minimum-links) so that if the number of active member links falls below the threshold, the LAG is marked down and traffic is dropped rather than being forwarded over insufficient bandwidth. Option C is correct because LACP member links must be consistent in their physical characteristics and configuration — identical speed and duplex, and matching LACP mode (active/passive) — for the bundle to come up and pass traffic reliably. Option D is correct because on Junos switches you must define the maximum number of aggregated Ethernet interfaces (device count) under the [edit chassis aggregated-devices ethernet] hierarchy before configuring any LAG, otherwise the ae interfaces cannot be created. Option B is not required: VRRP is a first-hop redundancy protocol unrelated to LACP negotiation, and LACP operates on member links without VRRP. Option E is not required: GRES (Graceful Routing Engine Switchover) is a high-availability feature for dual Routing Engines and has no bearing on forming or operating a LAG.

Answer

VRRP must be enabled on every member port for LACP to negotiate successfully.

Answer

GRES must be enabled on all switches participating in a LAG.

Question 15

Which THREE statements regarding Juniper Virtual Chassis architecture and master election are correct? (Choose three)

Accepted Answer

Pre-provisioning member serial numbers ensures consistent member ID assignments regardless of boot order.. Option A is correct because pre-provisioning member serial numbers in the Virtual Chassis configuration binds each serial number to a fixed member ID, so member IDs remain consistent no matter which switch boots first. Option C is correct because with Graceful Routing Engine Switchover (GRES) enabled, the backup Routing Engine takes over the primary role when the primary RE fails, preserving routing state and forwarding for a seamless transition. Option E is correct because mastership priority is configurable from 0 to 255, and the member with the highest priority value is preferred to become the primary (master) in the election. Option B is not correct because split-brain detection does not automatically shut down all core routing protocols; it detects the split and can disable VCPs on the losing fragment, and recovery typically requires administrator action. Option D is not correct because Virtual Chassis ports are dedicated stacking links and do not require LACP active mode configuration on every stacking cable.

Answer

Split-brain detection automatically shuts down all core routing protocols without administrator intervention.

Answer

Virtual Chassis ports (VCPs) require LACP active mode configuration on every stacking cable.

Question 16

Which THREE statements are true regarding OSPF Graceful Restart helper mode on Juniper routers? (Choose three)

Accepted Answer

It helps prevent unnecessary SPF recalculations across the network during transient control plane restarts.. Option C is correct because OSPF Graceful Restart helper mode keeps the adjacency and LSDB state stable while a neighbor restarts its control plane, so other routers do not trigger unnecessary SPF recalculations and network-wide churn during the transient outage. Option D is correct because the helper router continues to advertise and retain the restarting neighbor's routing entries (treating the neighbor as still reachable) for the duration of the restart window, preserving forwarding while the control plane recovers. Option E is correct because in Junos OS, OSPF graceful restart helper mode is enabled by default, so a Juniper router will assist a restarting OSPF neighbor without requiring explicit helper configuration. Option A is incorrect because graceful restart is specifically designed to preserve data forwarding during the control plane restart, not to drop forwarding traffic immediately. Option B is incorrect because VRRP tracking on loopback interfaces is unrelated to OSPF graceful restart helper operation and is not a requirement for it.

Answer

It forces the restarting router to immediately drop all data forwarding traffic.

Answer

It requires manual configuration of VRRP tracking on all loopback interfaces.

Question 17

Which THREE potential issues can arise if LACP is misconfigured between a Juniper switch and a connected peer device? (Choose three)

Accepted Answer

Traffic loops may occur if static LAG mode is mismatched with LACP active mode on opposite ends.. Option A is correct because if one side is configured as a static LAG (no LACP) and the other side runs LACP active, the peer will not form a proper bundle and the resulting forwarding inconsistency can create traffic loops, especially with multiple links. Option B is correct because a LAG configured with a minimum-links threshold will bring the aggregate interface operationally down when the number of active member links falls below that threshold, which is a direct consequence of misconfigured or failed LACP negotiation. Option D is correct because when LACP protocol data units (PDUs) are not exchanged successfully, member links can enter 'Suspended' or 'Expired' states in the LACP state machine, preventing them from being used in the bundle. Option C is not correct because GRES (Graceful Routing Engine Switchover) is a high-availability feature independent of LACP and is not automatically disabled by an LACP mismatch. Option E is not correct because ISSU is a manual, administrator-initiated upgrade procedure and is never triggered automatically by an LACP configuration mismatch.

Answer

GRES will be disabled on the routing engine automatically.

Answer

The switch will automatically trigger an In-Service Software Upgrade (ISSU) to fix the mismatch.

Question 18

You administer a pair of Juniper EX4600 switches configured as a Virtual Chassis with two routing engines in a mixed mode (one master, one backup). You want to ensure that the backup routing engine is ready to take over immediately if the master fails, without requiring a full reboot. Which statement correctly describes the default behavior of the Virtual Chassis when the master Routing Engine fails?

Accepted Answer

The backup Routing Engine becomes master and maintains the forwarding state of the line cards, provided that graceful switchover is enabled.. Graceful switchover allows the backup Routing Engine to assume the master role without resetting the line cards, preserving forwarding state and minimizing traffic disruption. This is critical for high availability in a Virtual Chassis. Without graceful switchover, a failover might cause line card resets and longer outages.

Answer

The master Routing Engine must be manually failed over using the request chassis routing-engine master switch command before the backup can take over.

Answer

The backup Routing Engine automatically becomes master and the line cards are reset, causing a brief traffic disruption.

Answer

The backup Routing Engine reboots and then assumes the master role, causing a longer outage.

Question 19

You are configuring a Juniper EX4300 switch in a Virtual Chassis with two members. You want to enable graceful Routing Engine switchover (GRES) and ensure that the backup Routing Engine is synchronized with the master for kernel and forwarding state. Which additional configuration is required to achieve this synchronization?

Accepted Answer

Enable graceful-switchover under the [edit chassis redundancy] hierarchy.. Graceful Routing Engine switchover (GRES) is enabled by configuring the graceful-switchover statement under the [edit chassis redundancy] hierarchy. This allows the backup Routing Engine to synchronize kernel and forwarding state with the master, enabling a seamless takeover. Without this configuration, the backup cannot maintain synchronization.

Answer

Configure Nonstop Active Routing (NSR) under the [edit routing-options] hierarchy.

Answer

Enable VRRP on all interfaces to synchronize state between Routing Engines.

Answer

Configure the backup Routing Engine to be the master using the request chassis routing-engine master switch command.

Question 20

You are deploying a new EX4300 Virtual Chassis with four members. You want to ensure that the control plane remains available if the master Routing Engine fails. Which TWO actions should you perform? (Choose two.)

Accepted Answer

Configure a backup member by setting the mastership priority appropriately on at least one other member.. Control-plane redundancy in a Virtual Chassis requires a backup member that is ready to assume the master role. This is achieved by configuring mastership priorities so that at least one other member becomes the backup, and by ensuring that member is properly connected through Virtual Chassis ports. Together, these actions ensure the backup can take over if the master fails.

Answer

Set the master member's mastership priority to the highest possible value.

Answer

Configure Nonstop Active Routing (NSR) on all members.

Answer

Enable graceful Routing Engine switchover (GRES) on the Virtual Chassis.