JNCIS-ENT Protocol Independent Routing — 15 Questions

Question 1

You need to create a virtual router routing instance to isolate traffic for a specific department. Which routing-instance type should you configure in the Junos configuration?

Accepted Answer

virtual-router. A virtual-router routing instance in Junos creates a separate routing table and separate instances of routing protocols, effectively isolating traffic like a separate logical router. This is ideal for isolating a department's traffic. The virtual-router type provides independent routing and forwarding, making it suitable for multi-tenant or departmental isolation.

Answer

forwarding

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no-structural-instance

Answer

l2vpn

Question 2

An administrator configures an aggregate route for 172.16.0.0/16. However, no contributing routes are present in the routing table. What is the default behavior of this aggregate route in Junos OS?

Accepted Answer

It is suppressed and will not be installed in the routing table.. By default, an aggregate route in Junos OS is not installed in the routing table unless it has at least one contributing route. If no contributing routes are present, the aggregate route is suppressed and will not be installed. This prevents blackholing traffic when no more specific routes exist.

Answer

It is installed in the routing table with a reject discard next-hop.

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It installs into the routing table with a next-hop pointing to the local loopback.

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It floods a default route to all routing instances.

Question 3

You are configuring an aggregate route and want to ensure that a discard next-hop is automatically applied when there are no contributing routes, preventing routing loops. Which configuration statement should you use under routing-options?

Accepted Answer

set routing-options aggregate route 10.0.0.0/16 discard. The `discard` option under an aggregate route in Junos OS automatically installs a discard next-hop for the aggregate when no more-specific contributing routes are present. This prevents routing loops by silently dropping traffic destined to the aggregate prefix instead of forwarding it to a default route. The `discard` action is specifically designed for aggregate routes to provide a black hole without generating ICMP unreachable messages, which is exactly what the question requires.

Answer

set routing-options static route 10.0.0.0/16 drop

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set routing-options aggregate route 10.0.0.0/16 passive

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set routing-options aggregate route 10.0.0.0/16 reject

Question 4

An engineer has configured a static route to the 192.168.100.0/24 network via 10.0.1.1. However, the route is not installing into the forwarding table because the recursive next-hop cannot be resolved. Which Junos command should the engineer use to verify the resolution status of the recursive next hop?

Accepted Answer

show route protocol static detail. When a static route's next-hop is not directly connected, Junos performs recursive resolution — it looks up the next-hop IP in the routing table to find a usable path. If that resolution fails, the route is marked hidden and not installed in the forwarding table. The 'show route protocol static detail' command displays the route's status including the 'Next hop' resolution state and any 'Hidden' reason, making it the right tool to diagnose recursive next-hop failures.

Answer

show route resolution

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show route forwarding-table destination 10.0.1.1

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show system route summary

Question 5

When configuring load balancing on a Junos device, you want to include Layer 4 source and destination ports in the hash calculation to achieve finer-grained load distribution. Which configuration statement achieves this under the forwarding-options hierarchy?

Accepted Answer

set forwarding-options hash-key layer-3 layer-4. The 'set forwarding-options hash-key layer-3 layer-4' statement tells Junos to include both Layer 3 (source/destination IP) and Layer 4 (source/destination port) fields in the ECMP/LAG hash computation. This produces finer-grained distribution because flows differing only in TCP/UDP port numbers will hash to different next-hops. It is configured under the [edit forwarding-options] hierarchy, which is where Junos controls per-packet load-balancing hash inputs.

Answer

set system services load-balancing ports

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set routing-options forwarding-table export load-balance-policy

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set protocols ospf ecmp-hash-ports

Question 6

You are troubleshooting a load-balancing configuration across multiple equal-cost paths (ECMP) on a Juniper MX Series router. Traffic is hashing persistently onto only one link. Which action should you take to enable per-packet or per-flow load balancing across the ECMP paths?

Accepted Answer

Configure a routing policy with 'then load-balance' action and apply it to forwarding-options.. On Juniper MX routers, ECMP load balancing behavior is controlled via a routing policy with the 'then load-balance' action, applied under [edit forwarding-options] or per-policy. By default, Junos uses per-flow (hash-based) load balancing, but if traffic is hashing to a single link, the hash algorithm or policy needs adjustment. Configuring a policy with 'then load-balance per-packet' (or ensuring the correct hash fields) and applying it to forwarding-options enables the desired distribution across ECMP paths.

Answer

Increase the router's MTU across all core interfaces.

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Enable BGP multipath under protocols bgp.

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Configure a static discard route for the loopback interface.

Question 7

What is the key difference between a 'discard' static route and a 'reject' static route in Junos OS?

Accepted Answer

Discard drops packets silently, whereas reject returns an ICMP unreachable message.. In Junos OS, a discard static route silently drops matching packets without sending any ICMP error message, while a reject static route drops the packet and returns an ICMP unreachable message (typically ICMP type 3, code 3 for port unreachable or code 13 for administratively prohibited). This difference is crucial for security and troubleshooting: discard is used to blackhole traffic quietly, whereas reject provides feedback to the sender that the destination is unreachable.

Answer

Reject installs the route into the forwarding table, while discard does not.

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Discard is used for IPv4 and reject is exclusively used for IPv6.

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Discard requires an active next-hop interface, while reject does not.

Question 8

You need to configure a static route that acts as a discard route, ensuring that any traffic destined for a specific unassigned subnet is silently dropped. Which configuration syntax should you use?

Accepted Answer

set routing-options static route 192.168.50.0/24 discard. In Junos, a static route with the 'discard' next-hop silently drops matching traffic without generating ICMP unreachable messages. This is the correct syntax for a discard route that blackholes traffic destined to an unassigned subnet.

Answer

set routing-options static route 192.168.50.0/24 drop

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set routing-options static route 192.168.50.0/24 reject

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set routing-options static route 192.168.50.0/24 null0

Question 9

You are configuring a qualified next-hop for a static route in Junos to provide redundancy. Which parameter allows you to specify a secondary next-hop with a different preference value?

Accepted Answer

qualified-next-hop. In Junos, a qualified next-hop allows you to specify multiple next-hops for a static route, each with its own preference value. This provides redundancy: if the primary next-hop becomes unreachable, the secondary next-hop with a higher preference (lower numerical value) can be used. The 'qualified-next-hop' parameter is used within the static route configuration to define these next-hops and their preferences.

Answer

alternate-path

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secondary-route

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backup-next-hop

Question 10

An engineer configures a static route pointing to an interface rather than an IP address. What potential issue can occur with interface-based static routes on multi-access Ethernet networks in Junos OS?

Accepted Answer

The router will generate an ARP request for every destination IP, potentially exhausting ARP cache.. When a static route points to an interface (e.g., 'set routing-options static route 10.0.0.0/8 next-hop ge-0/0/0.0') on a multi-access Ethernet network, Junos treats it as a route to the connected subnet and will ARP for every destination IP in that range. This can cause the ARP cache to fill with entries for every host the router tries to reach, potentially exhausting the cache and degrading performance.

Answer

The packet forwarding engine will crash due to unresolvable MAC addresses.

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The static route will immediately enter a hidden state due to missing next-hop IP.

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BGP will automatically re-advertise the interface route into external ASNs.

Question 11

You configure an aggregate route and want to control which specific attributes or policies are applied when the aggregate is advertised by BGP. Which routing option is used to assign a specific contributing route tag or policy to an aggregate route?

Accepted Answer

set routing-options aggregate route 10.10.0.0/16 metric 100. Aggregate routes can have specific attributes like preference, metric, and tags assigned using the routing-options aggregate hierarchy.

Answer

set routing-instances aggregate-property

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set protocols bgp aggregate-attribute 10.10.0.0/16

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set routing-options aggregate route 10.10.0.0/16 policy

Question 12

You need to verify that load balancing is functioning properly across your core interfaces and wish to see the specific hash buckets assigned in the forwarding table. Which command should you execute?

Accepted Answer

show route forwarding-table destination 192.168.1.1. The show route forwarding-table destination command displays the next-hops and whether multiple equal-cost paths (load balancing) are active for a destination.

Answer

show system switchover status

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show forwarding-options hash-results

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show pfe statistics load-balance

Question 13

You have configured load balancing across two uplinks using a routing policy. After committing the configuration, you notice that traffic is still exiting only via the primary link. What missing command under forwarding-options is necessary to activate the policy for forwarding table generation?

Accepted Answer

set routing-options forwarding-table export . On Junos, a routing policy that contains the 'load-balance per-packet' or 'load-balance' action only takes effect for forwarding-table (FIB) next-hop selection when it is referenced as an export policy under [routing-options forwarding-table]. Without this export statement, the policy is defined but never applied to the forwarding table, so the router continues to install only the primary next hop and traffic egresses via a single uplink. The command 'set routing-options forwarding-table export ' is therefore the missing piece that activates the load-balancing behavior.

Answer

set protocols load-balancing enable

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set forwarding-options family inet load-balance

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set interfaces load-balance-ecmp

Question 14

You want to configure load balancing across four distinct next-hops. By default, how many paths will Junos install into the forwarding table for ECMP if you configure a load-balancing policy without specifying path limits?

Accepted Answer

Up to 16 paths. By default, Junos OS installs up to 16 ECMP paths into the forwarding table when a load-balancing policy is configured without specifying a path limit. This default applies to both IPv4 and IPv6 and is controlled by the `maximum-paths` statement under `forwarding-options` or `routing-options`. The 16-path limit is a software default, not a hardware limitation, and can be increased up to 64 on supported platforms.

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Exactly 4 paths

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Up to 64 paths

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Only 1 path (no load balancing)

Question 15

You need to isolate a set of interfaces into a separate routing table without running dynamic routing protocols inside that table. Which routing instance type should you deploy?

Accepted Answer

virtual-router. A virtual-router routing instance in Junos creates a completely separate routing table (RIB) and forwarding table (FIB) with its own interfaces and routing protocols, isolated from the master instance. It is the correct choice when you need a separate routing table without running dynamic protocols inside it — you can simply add static routes.

Answer

l2vpn

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no-structural-instance

Answer

nat