Free JN0-364 Practice Test Questions | Know You're Ready for Service Provider Routing and Switching - Specialist (JNCIS-SP)

Explanation:

The exhibit shows a discrepancy between the Routing Table (RIB) and the Forwarding Table (FIB), which is the key to understanding how Junos handles multiple equal-cost paths by default.

Routing Table Behavior (C):
In the show route output, two next hops are listed for the 172.24.0.0/24 route. However, notice the > symbol next to the first next hop (to 172.20.0.2 via ge-0/0/2.0). In Junos, this symbol identifies the selected path that is actually installed in the forwarding engine. Even though OSPF has provided two equal-cost paths, the routing engine selects only one to be active by default.

Default Load-Balancing (A):
By default, Junos performs "per-prefix" load balancing (which essentially means no load balancing for a single destination). Even if multiple equal-cost paths exist in the RIB, only one next hop is pushed to the Forwarding Table (FIB). The show route forwarding-table output confirms this: although it lists both potential next hops, it lacks the "ulst" (unicast list) or "indr" (indirect) structures typically seen when active load balancing is configured.

Why the Other Options are Incorrect

B. The default route load-balancing behavior of this router has been modified:
If the behavior had been modified (typically via a policy-statement with then load-balance per-packet applied to the forwarding table), the show route forwarding-table output would show a multipath or list entry, and the show route output would likely show both routes as active (both having the * and > symbols).

D. This router will choose both next hops in the routing table:
As shown by the single > in the routing table and the lack of a balanced structure in the forwarding table, the router is only using one physical interface to reach that destination.

References

Juniper Networks Technical Publication: Junos OS Routing Protocols Configuration Guide, section on "Configuring Load Balancing."

JNCIS-SP Study Guide: Domain: Routing Policy and Load Balancing.

Explanation:

In Junos OS, the distinction between "import" and "export" is defined by the direction of data flow relative to the Routing Table (RIB).

Import Policy (B):
When a routing protocol (like IS-IS) receives information from the network, it must decide whether to "import" that information into the routing table. To control which external routes (or any routes) are accepted and installed into the RIB from the IS-IS process, you apply an import policy. This is used to filter specific prefixes or modify attributes as they enter the routing table.

Why the Other Options are Incorrect

A. export policy:
In Junos, an export policy is used to take routes from the routing table and "export" (advertise) them into a protocol. For example, if you wanted to advertise your static routes into IS-IS, you would use an export policy. It does not control what gets installed into the local routing table.

C. route preference:
Route preference (administrative distance) is used to choose between the same prefix learned from two different routing protocols (e.g., choosing OSPF over RIP). It is a tie-breaker for the best path selection, not a mechanism to filter which routes from a specific protocol are installed.

D. interface metric:
This is used to influence the SPF (Shortest Path First) calculation by making a specific path look more or less expensive. While it influences which path is chosen, it is not a tool for controlling the installation of specific external routes based on their prefix or properties.

References

Juniper Networks Technical Publication: Junos OS Policy Framework User Guide, section on "Import and Export Policies."

JNCIS-SP Study Materials: Domain: Routing Policy and IS-IS Operations.

Explanation:

To support IPv6 routing, dynamic routing protocols must be capable of carrying 128-bit addresses and handling IPv6-specific next-hop attributes.

IS-IS (B):
IS-IS is a link-state protocol that uses Type-Length-Values (TLVs) to carry routing information. Because it runs directly on Layer 2 (using its own CLNS encapsulation) rather than on top of IP, it was easily extended to support IPv6. By simply adding new TLVs (specifically TLV 236 for IPv6 Reachability and TLV 232 for IPv6 Interface Address), IS-IS can carry both IPv4 and IPv6 information simultaneously within the same adjacencies (Multi-Topology IS-IS).

BGP (D):
Multiprotocol BGP (MP-BGP), defined in RFC 4760, allows BGP to carry routing information for multiple "Address Families." By enabling the family inet6 unicast NLRI (Network Layer Reachability Information), BGP can exchange IPv6 prefixes. It remains the standard protocol for routing IPv6 across Autonomous Systems.

Why the Other Options are Incorrect

A. IGMP:
The Internet Group Management Protocol is used for IPv4 multicast group management (hosts signaling to routers that they want to receive a multicast stream). In IPv6, this function is replaced by MLD (Multicast Listener Discovery). Neither is a dynamic routing protocol.

C. OSPFv2:
OSPF version 2 was designed strictly for IPv4. While it is a dynamic routing protocol, it cannot carry IPv6 prefixes. To route IPv6 using OSPF, you must use OSPFv3, which was rewritten to be address-family independent.

References

RFC 4760: Multiprotocol Extensions for BGP-4.
RFC 5308: Routing IPv6 with IS-IS.
Juniper Networks Technical Publication: Junos OS IPv6 Configuration Guide.

Explanation:

RSVP-TE (Resource Reservation Protocol for Traffic Engineering) relies on CSPF (Constrained Shortest Path First) to calculate the optimal path for an LSP based on specific requirements or constraints.

Why the Other Options are Incorrect

A. CSPF is not required for LSPs using admin-groups:
This is incorrect. Admin-groups (link coloring) are a specific constraint that CSPF uses to filter the topology. To use them effectively to influence a path, CSPF must be enabled.

C. The paths used by LSPs are always calculated using the SRGB:
This is incorrect. The SRGB (Segment Routing Global Block) is a concept used in Segment Routing (SR-MPLS), not RSVP-signaled LSPs. RSVP uses a label distribution mechanism based on protocol signaling (PATH and RESV messages).

D. The paths used by LSPs are always calculated using the TED:
While this sounds plausible, it is technically incorrect to say they are always calculated using the TED. The TED (Traffic Engineering Database) is the source of data (containing link states and available bandwidth), but the calculation itself is performed by the CSPF algorithm. Furthermore, an RSVP LSP can be configured with a strict hop explicit path, in which case a TED-based CSPF calculation is bypassed entirely for those hops.

References:

RFC 3209: RSVP-TE: Extensions to RSVP for LSP Tunnels.

Juniper Networks Technical Publication: Junos OS MPLS Configuration Guide, section on "How CSPF Selects a Path."

Explanation:

The Sequence Number in an IS-IS Link State PDU (LSP) is the primary mechanism used to ensure that all routers in an area maintain the most current version of the Link State Database (LSDB).

Why the Other Options are Incorrect

A. It ignores the link-state PDU:
This only happens if the sequence numbers are identical. Ignoring a higher sequence number would prevent the network from converging on new topology changes.

C. It sends a CSNP to request confirmation:
Complete Sequence Number PDUs (CSNPs) are used to describe the entire database (usually on broadcast links or during initial synchronization). They are not used as an immediate "confirmation" for a single received LSP; the standard flooding process handles the update.

D. It resets the adjacency:
Receiving a new routing update is a normal operational event. Resetting an adjacency is a drastic measure typically reserved for authentication failures, hold-timer expirations, or physical link drops.

References

ISO/IEC 10589: The IS-IS standard, which defines the "Update Process" and how sequence numbers are used to manage LSP aging and synchronization.

Juniper Networks Technical Publication: Junos OS Routing Protocols Configuration Guide, Chapter: "IS-IS Link-State PDU Transmission."

Explanation:

In Junos OS, the output of show route identifies the egress interface for a specific destination. To determine which traffic is sent through a tunnel, you must look for an interface prefix associated with tunneling protocols.

Why the Other Options are Incorrect

B. 203.0.113.65:
This is the gateway address (next hop) for the route to 198.51.100.1/32. It is an IP address, not a destination network, and the traffic to reach this gateway goes over a physical interface (ge-0/0/3.0).

C. 0.0.0.0/0:
While a default route could be pointed at a tunnel, there is no evidence of a default route configuration in the provided exhibit.

D. 198.51.100.1/32:
As shown in the first show route output, this traffic exits via a standard Gigabit Ethernet interface (ge-0/0/3.0).

References

Juniper Networks Technical Publication: Junos OS Interface Classification, detailing interface naming conventions (e.g., ge for Gigabit, gr for GRE, ip for IP-IP).

JNCIS-SP Study Materials: Domain: IP Tunneling and GRE Configuration.

Explanation:

Label Distribution Protocol (LDP) can operate in different modes to determine how and when labels are shared between Label Switching Routers (LSRs).

Downstream Unsolicited (DU) Mode:
In this mode, an LSR does not wait for a specific request from its neighbors. Instead, it automatically advertises labels for all routes it knows about (typically all IGP prefixes) to all of its LDP peers. This is the default behavior for Junos OS and most major networking vendors.

Behavioral Match:
The scenario describes labels being advertised for "every loopback IPv4 address" as soon as LDP is enabled. This proactive advertisement is the hallmark of Downstream Unsolicited mode.

Why the Other Options are Incorrect

A. conservative retention:
This refers to Label Retention Mode, not distribution mode. Conservative retention means a router only keeps labels for the next hop chosen by the routing table. Junos actually uses liberal retention by default, keeping all labels from all neighbors to allow for faster convergence if the primary path fails.

B. ordered control:
This refers to Label Control Mode. It dictates that an LSR only advertises a label for a prefix once it has received a label from the next hop for that prefix (or if it is the egress). While Junos uses ordered control, the question specifically asks about the distribution mechanism (how the advertisement is triggered), which is DU.

D. downstream on demand (DoD):
In this mode, an LSR only advertises a label for a prefix if a neighbor specifically sends a "Label Request" message for it. This is much more restrictive and results in fewer labels in the network, but it is not the default behavior described in the scenario.

References

RFC 5036: LDP Specification, detailing the mechanics of DU and DoD modes.

Juniper Networks Technical Publication: Junos OS MPLS Configuration Guide, Section: "LDP Label Distribution, Retention, and Control."

Explanation:

OSPF (Open Shortest Path First) is a link-state routing protocol. Within a single area, the primary mechanism for preventing loops is the fundamental way link-state protocols process map data.

Complete Topology Map: Every router in a single OSPF area maintains an identical Link-State Database (LSDB). This database is essentially a map of every router and every link in the area.

The Dijkstra Algorithm (Shortest Path First): Each router independently runs the Dijkstra algorithm using itself as the root (the starting point). Because every router is calculating the shortest path based on a complete, identical map of the network, the resulting paths are mathematically guaranteed to be loop-free.

Loop Prevention vs. Distance Vector:
Unlike Distance Vector protocols, which only know the "distance" and "direction" to a destination (and thus can fall victim to "rumors" or count-to-infinity loops), OSPF routers know exactly how every piece of the puzzle fits together.

Why the Other Options are Incorrect

B. Routing policies:
Routing policies in Junos are used to manipulate which routes are accepted or advertised, but they are not the underlying mechanism that prevents loops in a link-state protocol.

C. The Bellman-Ford algorithm:
This algorithm is used by Distance Vector protocols (like RIP). Bellman-Ford is susceptible to routing loops, which is why protocols using it require additional mechanisms like Split Horizon, Poison Reverse, and Hold-down timers.

D. Forwarding policies:
Forwarding policies (such as Filter-Based Forwarding) dictate how packets are handled at the hardware level but do not participate in the calculation of a loop-free control plane topology.

References:

RFC 2328:OSPF Version 2, which specifies the use of the Dijkstra algorithm for path calculation.

Juniper Networks Technical Publication: Junos OS Routing Protocols Configuration Guide, Chapter: "OSPF Overview."