JNCIS-ENT Protocol Independent Routing Practice Test 3 — 15 Questions

Question 1

What is the primary function of a Junos routing instance of type 'forwarding'?

Accepted Answer

To support filter-based forwarding without running routing protocols. A Junos routing instance of type 'forwarding' is used to support filter-based forwarding (FBF) without running routing protocols. It allows traffic to be directed to specific next-hops based on firewall filter match conditions, using a separate routing table (the forwarding instance's table) that contains only static routes or interface routes. This is ideal for policy-based routing scenarios where you don't need a full routing protocol.

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To terminate BGP sessions from multiple external peers

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To run multiple independent OSPF instances

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To provide transparent bridging across WAN circuits

Question 2

You need to configure a static floating route that will only become active if the primary dynamic route learned via OSPF fails. Which configuration parameter must be adjusted to achieve this behavior?

Accepted Answer

preference. In Junos, route preference (also called administrative distance) determines which route wins when multiple protocols offer a path to the same destination. OSPF's default preference is 10, while a static route's default preference is 5 — meaning the static route would normally win. To make the static route a floating (backup) route, you must raise its preference above OSPF's (e.g., 'set routing-options static route 10.0.0.0/24 next-hop x.x.x.x preference 200') so it is only installed when the OSPF route disappears.

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as-path prepend

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local-preference

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metric

Question 3

Which command allows you to inspect the active forwarding table entries and verify next-hops on a Junos router?

Accepted Answer

show route forwarding-table. 'show route forwarding-table' displays the active forwarding table entries that the Packet Forwarding Engine (PFE) actually uses to forward packets, including next-hop information. This is the command that verifies what the router is really doing at the forwarding plane, not just what the routing protocol advertised.

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show interfaces extensive

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show pfe traffic

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show routing-engine status

Question 4

You are configuring Filter-Based Forwarding (FBF) on an interface. You apply a firewall filter that matches specific source addresses and uses 'then routing-instance' as an action. What must be configured in that target routing instance to ensure return traffic is handled correctly?

Accepted Answer

A default route pointing back to the core gateway within that routing instance's routing table.. When using Filter-Based Forwarding (FBF) to direct traffic to a specific routing instance, the target routing instance must have a route back to the source to handle return traffic. Typically, a default route pointing back to the core gateway within that routing instance's routing table ensures that return traffic is sent back correctly. Without such a route, return traffic may be dropped or misrouted.

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A static ARP entry for every device on the network.

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An RSVP LSP bridging the source and destination instances.

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A BGP peering session with the loopback address.

Question 5

An administrator implements filter-based forwarding and applies a firewall filter on the ingress interface. However, traffic matching the filter is dropped rather than routed to the alternate routing instance. Upon investigation, what is the most likely cause of this issue?

Accepted Answer

The destination IP address does not have a matching route in the alternate routing instance's table.. Filter-based forwarding (FBF) works by matching traffic with a firewall filter and then using a 'routing-instance' action to steer it into an alternate routing table. If the alternate routing instance has no route for the destination IP, the packet cannot be forwarded and is dropped, which matches the symptom described.

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The ingress interface is configured as a trunk port.

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The firewall filter term lacks a 'count' action statement.

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The routing instance type is set to virtual-router instead of forwarding.

Question 6

Which command is used to display all configured static routes along with their active/inactive status in Junos?

Accepted Answer

show route protocol static. The command `show route protocol static` queries the routing table (by default inet.0) and filters for routes whose protocol is static. This output includes both active routes (marked with `*`) and inactive routes (marked with `-`), along with next-hop, preference, and metric information. It directly answers the question by showing all configured static routes and their active/inactive status.

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show static routes summary

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show configuration routing-options static

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show route table inet.0 static

Question 7

When configuring a routing instance of type 'virtual-router', what is required to allow communication between the virtual router instance and the default routing instance?

Accepted Answer

Configure leaking routes between instances using rib-groups or routing policies.. To allow communication between a virtual-router routing instance and the default routing instance, you must configure route leaking using rib-groups or routing policies. Virtual routers have separate routing tables, so routes are not shared by default. Rib-groups can copy routes from one table to another, or routing policies can export/import routes between instances.

Answer

Connect a physical cable between two ports on the router (cable-patch).

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Configure static routes with identical preferences in both instances.

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Disable firewall filters on all loopback interfaces.

Question 8

You want to ensure that a static route is only installed in the routing table when a specific tracked object or ping test succeeds. Which Junos feature should you pair with the static route?

Accepted Answer

RPM (Real-time Performance Monitoring) with static route tracking. RPM (Real-time Performance Monitoring) in Junos allows you to configure probes that periodically test reachability of a target (e.g., via ICMP ping) and track the results. When you associate a static route with an RPM probe using the `track` option, the route is only installed in the routing table if the probe succeeds. This provides conditional route installation based on real-time performance monitoring, ensuring the static route is active only when the tracked object is reachable.

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OSPF sham-links

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Keepalive timers on static routes

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BFD synchronized static routes

Question 9

An administrator configures filter-based forwarding. The firewall filter uses the action 'then routing-instance custom-vr'. Where must the custom-vr routing instance be defined for the filter to accept this configuration without committing errors?

Accepted Answer

Under the [edit routing-instances] hierarchy.. Filter-based forwarding (FBF) requires the target routing instance to exist under the [edit routing-instances] hierarchy before a firewall filter can reference it with the 'then routing-instance' action. Junos validates this reference at commit time, so if custom-vr is not defined as a routing instance, the commit fails with an error. The routing instance is typically of type 'forwarding' or 'virtual-router', and the filter action simply directs matching packets into that instance's forwarding table.

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Under the [edit system FBF] hierarchy.

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Under the [edit interfaces interface-name unit 0 family inet] hierarchy.

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Under the [edit firewall family inet filter] hierarchy.

Question 10

You need to ensure that traffic entering interface ge-0/0/1 is forced into a specific forwarding routing instance named 'corp-fwd', regardless of the destination IP address in the packet header. Which configuration steps must you perform?

Accepted Answer

Create a forwarding instance, write a firewall filter with 'then routing-instance corp-fwd', and apply the filter as input on ge-0/0/1.. To force traffic from an interface into a specific forwarding routing instance regardless of destination, you create a forwarding instance (type 'forwarding'), define a firewall filter with the action 'then routing-instance corp-fwd', and apply that filter as an input filter on ge-0/0/1. This is the standard Junos method for policy-based routing into a forwarding instance, often used for service chaining or VRF selection.

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Create a virtual-router instance and apply a static default route pointing to ge-0/0/1.

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Configure a routing policy with 'then export corp-fwd' and apply it under protocols.

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Enable proxy-ARP on ge-0/0/1 and assign the corp-fwd routing instance to the physical port.

Question 11

Which Junos routing instance type is best suited when you want to run MPLS L3VPNs on a Provider Edge (PE) router?

Accepted Answer

vrf. L3VPN routing instances are used on PE routers to maintain customer-specific VRF routing tables and exchange VPN-IPv4 routes via MP-BGP.

Answer

forwarding

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virtual-router

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no-structural-instance

Question 12

What is the default preference value for a direct route in Junos OS?

Accepted Answer

0. In Junos OS, the default preference (administrative distance) for a direct route is 0. This is the highest priority among all route types, ensuring that directly connected networks are always preferred over routes learned via dynamic protocols or static configuration. The value 0 reflects that a directly connected interface is the most trusted source of routing information.

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5

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20

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10

Question 13

An administrator configures a static route with a next-hop that points to an indirect (recursive) address. What condition must be met for this static route to become active in the inet.0 routing table?

Accepted Answer

A more direct route to the next-hop IP must already exist and be active in inet.0.. For a static route with an indirect (recursive) next-hop to become active in inet.0, the router must already have an active route to that next-hop IP in inet.0. This is the definition of recursive resolution: the next-hop must be reachable via another route in the same routing table. Without this, the static route remains hidden.

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The static route preference must be set lower than 1.

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The next-hop IP must be directly connected to a physical interface on the local router.

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BGP multipath must be enabled under routing-options.

Question 14

An engineer configures multiple static routes to the same destination with different preference values. When all paths are up, how does Junos select which route is active in the routing table?

Accepted Answer

The route with the lowest preference value is installed as active.. Junos uses route preference (also called administrative distance) to select the active route when multiple routes to the same destination exist. Lower preference values are preferred, so a static route with preference 5 beats one with preference 10. Only the route with the lowest preference is installed as active in the routing table and subsequently into the forwarding table (unless ECMP conditions apply).

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The route configured last in the candidate configuration file becomes active.

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The route with the highest metric value is preferred.

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All routes are installed into the forwarding table for equal-cost multi-path load balancing.

Question 15

You are troubleshooting an aggregate route that is not being advertised to BGP peers. You verify that contributing routes are active in the routing table. What command should you use to check if the aggregate route itself is successfully generated and active in the local routing table?

Accepted Answer

show route protocol aggregate. The command 'show route protocol aggregate' displays all routes in the routing table that were learned via the aggregate protocol. This allows you to verify if the aggregate route is active and installed. If the aggregate is not shown, it means it is not active, likely due to no contributing routes. The other commands are either invalid or do not show the aggregate route in the local routing table.

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show route summary aggregate

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show bgp aggregate-routes

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show routing-options aggregate detail