JN0-106 Routing Fundamentals — 15 Questions

Question 1

Which TWO statements about active routes in the Junos routing table are correct?

Accepted Answer

An active route is installed in the forwarding table.. An active route in Junos is one that is selected as the best route for a given prefix and is installed in the forwarding table (the FIB). The forwarding table is used by the Packet Forwarding Engine (PFE) to make actual forwarding decisions, so only active routes are placed there.

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All routes in the routing table are active.

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Multiple active routes can exist for the same prefix.

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An active route is always preferred over a passive route.

Question 2

Which THREE statements about static routes in Junos are correct?

Accepted Answer

Static routes have a default preference of 5.. Static routes in Junos have a default preference of 5, which is lower (more preferred) than routes learned from most dynamic routing protocols like OSPF (preference 10) or IS-IS (preference 18). This default value ensures that static routes are preferred over dynamically learned routes unless explicitly overridden.

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The preference of a static route cannot be changed.

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Static routes automatically update if the next hop becomes unreachable.

Question 3

Refer to the exhibit. How many static default routes are configured?

Accepted Answer

2. The exhibit shows two static routes with a next-hop of 0.0.0.0, which are static default routes. In JUNOS, a static default route is defined by a destination prefix of 0.0.0.0/0 (or ::/0 for IPv6). The configuration lists two such routes, each with a different next-hop address, confirming two static default routes.

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1

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0

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3

Question 4

You are the network engineer for a mid-sized enterprise with a Juniper MX router running Junos. The router has two uplinks to the internet: one to ISP-A via ge-0/0/0 (10.0.0.1/30) and one to ISP-B via ge-0/0/1 (10.0.0.5/30). You have configured static default routes to both ISPs: one to 10.0.0.2 and one to 10.0.0.6. Both routes have the same preference (default 5) and metric. You enabled ECMP to load-balance outbound traffic. After testing, you notice that all traffic is being sent only to ISP-A, and none to ISP-B. You verify that both interfaces are up and that the next-hop addresses are reachable. You check the routing table and see both routes are active but with different next-hop counts. What is the most likely cause of the traffic not being load-balanced?

Accepted Answer

One of the static routes resolves to multiple next hops, causing imbalance.. When both static default routes have the same preference and metric, Junos installs both in the routing table. However, ECMP load-balancing requires that each route resolve to a single next hop. If one static route resolves to multiple next hops (e.g., through a recursive lookup to a prefix with multiple paths), the route with a single next hop (ISP-A) becomes the sole active path for forwarding, while the multi-next-hop route (ISP-B) is not used for load balancing. This explains why all traffic goes to ISP-A despite both routes being present.

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ECMP is not enabled in the forwarding-options.

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The route to ISP-B is not active.

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Hash-based load balancing requires per-packet configuration.

Question 5

Which TWO attributes are used by Junos to select the active route among multiple routes to the same destination?

Accepted Answer

Metric. Junos uses the route preference (also known as administrative distance) as the primary attribute to select the active route among multiple routes to the same destination. If multiple routes have the same preference, Junos then compares the metric (also called cost or protocol-specific metric) to break the tie. Preference is a local, configurable value that ranks the trustworthiness of the routing protocol source, while metric is a protocol-specific value (e.g., OSPF cost, RIP hop count) used within the same protocol.

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Protocol type

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Next hop address

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AS path length

Question 6

What is the default preference of a direct route in Junos?

Accepted Answer

0. Direct routes in Junos have a default preference of 0, which is the highest possible preference (lowest numerical value). This ensures that directly connected routes are always preferred over any other route type, including static routes (preference 5) and OSPF internal routes (preference 10). The preference value is used by Junos to select the best route when multiple routes exist to the same destination.

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10

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5

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15

Question 7

A network engineer is troubleshooting a reachability issue between two directly connected routers. Both routers have IP addresses configured on their interfaces and the interfaces are up. Which command would the engineer use to verify the routing table entry for the remote network?

Accepted Answer

show route. The 'show route' command displays the routing table, which contains all active routes learned via static configuration, direct connections, or dynamic routing protocols. To verify the routing table entry for a specific remote network, this command shows the next-hop IP address, interface, and route preference/metric, directly confirming whether the remote network is reachable. Command 'show arp' (option D) only shows IP-to-MAC address mappings and does not display routing table information.

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show interfaces terse

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show configuration interfaces

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show arp

Question 8

Which THREE statements are true regarding static routes in Junos?

Accepted Answer

A static route can have a next-hop of a directly connected interface.. A static route can have a next-hop of a directly connected interface, meaning the router will consider the destination as directly reachable out that interface without needing an explicit IP next-hop. This is valid in Junos and is often used for point-to-point links or when the next-hop address is not required.

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Static routes require OSPF to be enabled on the router.

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Static routes automatically generate ICMP redirect messages.

Question 9

Which TWO statements are true about static routes in Junos?

Accepted Answer

Static routes remain in the routing table even if the next hop is unreachable.. Static routes in Junos have a default preference of 5, not 10 (which is the default for OSPF internal routes). They remain in the routing table even if the next hop becomes unreachable because Junos uses a 'passive' route model; the route is only removed if explicitly deleted or if a better route is learned via a routing protocol. This behavior is controlled by the 'no-readvertise' and 'passive' attributes of static routes.

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Static routes have a default preference value of 10.

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Static routes require a routing protocol to be activated.

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Static routes are automatically updated if the network topology changes.

Question 10

Your company operates a dual-homed network with two Juniper MX routers (R1 and R2) each connected to a different ISP. R1 uses BGP to receive a default route from ISP-A (preference 170), and R2 uses BGP to receive a default route from ISP-B (preference 170). Additionally, both routers have a static default route pointing to a local next-hop (192.0.2.1) with preference 5 for backup. R1 and R2 are connected via an internal link (10.0.0.0/30) and run OSPF to exchange internal routes. You notice that traffic from internal hosts is always exiting via R1's ISP-A link, even when R1's BGP session to ISP-A goes down. The OSPF routes are preferred. You want traffic to fail over to R2's ISP-B link when R1 loses its BGP default. Which configuration change should you make?

Accepted Answer

Configure the static default route with a preference of 180 so that the BGP default routes (pref 170) are preferred when available.. The static default route currently has a preference of 5, making it preferred over the BGP default (preference 170) even when the BGP route is available. By raising the static default's preference to 180 (higher than BGP's 170), the BGP default will be chosen when present, and the static default will only be used as a backup when BGP is unavailable. This ensures traffic fails over to R2's ISP-B link when R1 loses its BGP default, as OSPF will propagate the default from R2.

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Increase the preference of the BGP default routes to 180 so that the static default is always preferred.

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Configure a routing policy on R2 to reject the default route from ISP-B, forcing all traffic through R1.

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Remove the static default route and rely on OSPF to propagate a default route from the router with the active BGP session.

Question 11

Match each Junos routing protocol to its primary characteristic.

Accepted Answer

OSPF: Uses link-state advertisements to build a topology database. OSPF and IS-IS are link-state protocols. BGP is path-vector. RIP is distance-vector. Static routes are manually configured. Common confusions include mixing link-state with distance-vector or mislabeling BGP.

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IS-IS: Uses a distance-vector algorithm with split horizon

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Static route: Learned dynamically through routing protocols

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BGP: Uses distance-vector algorithm with link-state updates

Question 12

Match each Junos configuration hierarchy to its function.

Accepted Answer

system: Configures system-wide parameters such as hostname and time.. In Junos, 'system' manages hostname and time, 'interfaces' configures ports, 'protocols' handles routing protocols, and 'security' manages firewall policies. Common confusions include swapping these definitions.

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system: Configures routing protocols like OSPF and BGP.

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interfaces: Configures security policies and zones.

Question 13

A static route with a next-hop of 10.0.0.1 is configured, but the route is not appearing in the active routing table. The output of `show route 10.0.0.1` shows that 10.0.0.1 is reachable via an OSPF route with preference 10. What is the most likely reason the static route is not active?

Accepted Answer

The static route has a higher preference than OSPF.. In Junos, OSPF internal routes have a default preference of 10, while static routes have a default preference of 5 (lower is more preferred). If a static route is not active despite its next-hop being reachable, it is likely because the static route has been configured with a higher preference value (e.g., >10), making it less preferred than the OSPF route. The `show route` output confirms the OSPF route is active with preference 10, so the static route must have a preference higher than 10 to be inactive.

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The OSPF route has a lower metric than the static route.

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The static route's next-hop is not directly connected.

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The static route is configured with a qualified-next-hop that fails.

Question 14

A Juniper MX router is configured with multiple routing instances. In which scenario would an interface be automatically placed in a routing instance?

Accepted Answer

When the interface is associated with the default instance.. In Junos, all interfaces automatically belong to the default routing instance (inet.0) unless explicitly assigned to a different routing instance via the `interface` statement. Therefore, the scenario where an interface is automatically placed in a routing instance is when it is associated with the default instance, making option B correct.

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When the interface is configured with an IP address from the instance's address family.

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When the interface is configured under the instance's `interface` statement.

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When the routing instance type is virtual-router and the interface is a physical interface.

Question 15

A network engineer is troubleshooting a BGP routing issue. The router receives a route to 172.16.0.0/16 from two BGP peers with different local preferences. The route from peer A has local preference 200, and from peer B has local preference 100. The router selects the route from peer A. What is the next step in BGP path selection if the local preferences were equal?

Accepted Answer

Compare the AS path length.. B is correct because in BGP path selection, after comparing local preference (highest wins), the next tiebreaker is the shortest AS path length. If local preferences are equal, the router compares the AS path length, selecting the route with fewer AS numbers. This is defined in RFC 4271 and is the second step in the BGP decision process.

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Compare the MED.

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Compare the IGP metric to the next-hop.

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Compare the origin code.