Free Juniper JN0-364 Practice Exam with Questions & Answers | Set: 2

To determine which traffic is being sent through a tunnel in a Junos OS environment, an administrator must analyze the routing table output for the exit interface associated with each destination prefix. The provided exhibit shows the results of the show route command on routerR2for two specific destination networks.

In the first output, the destination198.51.100.1/32is an active static route. The next-hop information specifies that traffic for this address is sent to the gateway 203.0.113.65 via the interfacege-0/0/3.0. According to Juniper Networks interface naming conventions, the prefixge-denotes aGigabit Ethernetinterface, which represents a standard physical connection. Therefore, this traffic does not traverse a tunnel.

In the second output, the destination172.20.110.0/24is also an active static route. However, the next-hop for this network is listed asvia gr-0/0/0.0. In the Junos operating system, thegr-prefix explicitly identifies aGeneric Routing Encapsulation (GRE) tunnel interface. GRE is a widely used protocol in service provider networks to encapsulate various network layer protocols over an IP backbone, effectively creating a virtual point-to-point link. Because the routing table has installed the route for 172.20.110.0/24 specifically via the gr- interface, all traffic destined for this network will be encapsulated and sent through the tunnel.

The other choices are incorrect for the following reasons:

203.0.113.65 (Option B):This is the next-hop IP address for the physical Gigabit Ethernet path; it is not a destination network directed to a tunnel.

0.0.0.0/0 (Option C):There is no information in the exhibit regarding a default route.

198.51.100.1/32 (Option D):As identified by thege-interface prefix in the exhibit, traffic for this destination is sent via a physical Ethernet link.

In a Juniper Networks environment, establishing a functionalMultiprotocol Label Switching (MPLS)Label-Switched Path (LSP) requires synchronized control plane operations. According to Juniper technical documentation, the most common reason for an LSP to remain in the "Down" state at the ingress router is a failure of theConstrained Shortest Path First (CSPF)algorithm during the path computation phase.

The provided exhibit for routerR1reveals a critical error in the show mpls lsp detail output: "CSPF: could not determine self". This specific error indicates that the CSPF process is unable to find its own local router ID within theTraffic Engineering Database (TED). For CSPF to build a valid TED, the underlying Interior Gateway Protocol (IGP), such as OSPF, must be configured to flood opaque link-state advertisements (Type 10 LSAs) that carry traffic engineering attributes. As seen in the OSPF configuration, traffic engineering is not enabled. Therefore, issuing theset protocols ospf traffic-engineeringcommand (Option D) will allow R1 to populate the TED with its own local information and that of its neighbors, enabling CSPF to calculate a valid path.

Alternatively, an administrator can choose to bypass the requirement for a TED entirely by disabling CSPF on the specific LSP. By issuing theset protocols mpls label-switched-path to-r3 no-cspfcommand (Option B), the router will stop attempting to perform a constrained path calculation. Instead, the signaling protocol (RSVP) will rely on the standardinet.0routing table to determine the hop-by-hop path to the egress destination (192.168.100.3), allowing the LSP to establish without traffic engineering constraints.

Regarding the other options, whilefamily mplsis required on all transit interfaces, the ingress loopback interface (lo0) generally does not require it for standard LSP signaling unless it's used as a transit hop. Furthermore, adding a static route toinet.3(Option A) is used for next-hop resolution of BGP routes over LSPs but does not assist in the signaling or establishment of the LSP itself.

In a Juniper Networks environment,Resource Reservation Protocol (RSVP)is a signaling protocol used to establish Label-Switched Paths (LSPs). While RSVP handles the actual signaling (requesting labels and reserving bandwidth along a path), it does not inherently know which path to take. This is whereConstrained Shortest Path First (CSPF)comes into play.

CSPFis an advanced version of the Dijkstra algorithm used specifically for traffic engineering. Unlike the standard SPF used by IGPs, which only considers the shortest metric, CSPF takes into account multiple constraints such as available bandwidth, link coloring (administrative groups), and explicit hop requirements. According to Juniper technical documentation, when an LSP is configured, the Ingress router uses CSPF to calculate a loop-free path that satisfies all these constraints before RSVP begins signaling. This is why statementBis the correct description of the operational flow.

StatementDis a common distractor. While CSPF uses theTraffic Engineering Database (TED)to perform its calculations, the path is not "calculated by the TED" itself; the TED is merely the repository of link-state information (provided by OSPF or IS-IS extensions). StatementCrefers to Segment Routing Global Block (SRGB), which is relevant to Segment Routing (SR-TE), not standard RSVP-signaled LSPs. Finally, statementAis incorrect because admin-groups (link coloring) are actually one of the primary constraints thatrequireCSPF to determine a valid path.

When troubleshooting OSPF in a service provider environment, distinguishing between "stuck" adjacencies and "flapping" adjacencies is the first step. A session that transitions frequently betweenFullandDownindicates that the relationship can be established successfully (meaning parameters match), but it cannot be maintained.

According to Juniper Networks documentation, the most common cause for a session to drop from Full to Down is the expiration of theDead Interval. If a router does not receive a Hello packet within the Dead Interval (usually 40 seconds), it tears down the adjacency. Aphysical interface flapping (Option A)is the primary trigger for this. If the physical link or the underlying transport (like a leased line or a microwave link) goes down even momentarily, the OSPF process immediately detects the interface failure, flushes the neighbors, and moves the state to Down. As soon as the interface comes back up, the routers perform the Hello exchange and reach the Full state again, creating the flapping cycle.

Analysis of other options:

MTU Mismatch (Option D):This typically causes the adjacency to get "stuck" in theExchangeorExStartstate. The routers can exchange small Hello packets, but when they try to send larger Database Description (DBD) packets that exceed the MTU, the packets are dropped, preventing the session from ever reaching "Full."

Area ID Mismatch (Option C):This prevents the adjacency from even reaching theInitstate; the routers will never form a neighbor relationship.

Route Preference (Option B):This affects which route is chosen for the forwarding table but has no impact on the OSPF neighbor state machine itself.

When designing High Availability (HA) for Juniper Service Provider routers, understanding the interaction between the control plane and data plane is vital. The user has already enabledGraceful Routing Engine Switchover (GRES), which synchronizes the interface and kernel state between the primary and backup Routing Engines (REs). However, GRES by itself does not preserve the routing protocol state (like OSPF adjacencies or BGP sessions).

To achieve the preservation of the routing protocol state without relying on external "helper" routers, you must implementNon-Stop Active Routing (NSR). According to Juniper Networks documentation, NSR uses the infrastructure provided by GRES to also synchronize the routing protocol process (rpd) information. Under NSR, the backup RE maintains a "hot" standby state of all routing protocols. If the primary RE fails, the backup RE takes over immediately. Because it already possesses the full routing table and peer session states, the peering neighbors are unaware that a switchover occurred. No protocol adjacency resets occur, and traffic continues to flow uninterrupted.

It is crucial to differentiate NSR fromGraceful Restart (Option C). While Graceful Restart also aims to maintain traffic flow during a switchover, itdoesrequire help from neighboring routers (known as "helper mode"). If the neighbors do not support or are not configured for Graceful Restart, the sessions will drop. Since the user explicitly stated they want to "avoid relying on helper routers," Graceful Restart is not the correct solution.

Non-stop Active Bridging (Option A)provides a similar "hitless" failover but specifically for Layer 2 environments (STP/VLANs) rather than Layer 3 routing protocols.BFD (Option B)is a failure detection protocol used to speed up convergence but does not preserve state during an RE failover; in fact, without NSR, BFD would likely trigger a faster teardown of the session during a switchover. Therefore,NSRis the only feature that meets the requirement for independent control-plane preservation.

The prevention of routing loops within an Autonomous System (AS) is handled differently than loop prevention between ASes. While External BGP (EBGP) uses the AS_PATH attribute to detect loops,Internal BGP (IBGP)does not modify the AS_PATH. Therefore, a different mechanism is required to ensure that a route does not circulate infinitely inside the network.

This mechanism is known as theIBGP Split Horizon rule. According to Juniper Networks documentation and the BGP standard (RFC 4271), a BGP speakermust not advertise a route learned via an IBGP peer to any other IBGP peer. In simpler terms, "what is learned internally, stays local." This rule ensures that a route only travels one "hop" inside the AS—from the router that learned it from an external source to all other internal routers.

Because of this rule, IBGP routers do not naturally propagate routes through each other. This creates a requirement for afull meshof IBGP sessions, where every BGP-speaking router in the AS must have a direct peering session with every other BGP-speaking router. To mitigate the scaling issues of a full mesh in large service provider networks, architects useRoute ReflectorsorConfederations, which are authorized exceptions to the Split Horizon rule.

Option B is incorrect because EBGP peersdoadvertise EBGP routes to other EBGP peers (this is how the internet works). Option C is incorrect because EBGP-learned routesmustbe sent to IBGP peers so the internal network knows how to reach the outside world. Option D is incorrect because internal routesmustbe sent to external peers to advertise your network to the internet.

In the Juniper Networks Junos operating system,aggregate routesare used to represent a group of more specific routes with a single, shorter prefix. This technique is essential for reducing the size of routing tables and minimizing the volume of routing updates sent to neighbors. According to Juniper technical documentation, for a destination IP address to "match" a specific route, it must fall within the range defined by the network address and its associated CIDR mask.

The provided exhibit shows a detailed lookup for the aggregate route$10.16.2.0/23$. To determine the range of IP addresses covered by a $/23$ mask, we examine the binary representation of the third octet. A $/23$ mask means the first 23 bits are fixed. For the address $10.16.2.0$:

The first two octets ($10.16$) are fixed.

The third octet ($2$) is $00000010$ in binary.

The 23rd bit is the second-to-last bit of this octet.

The $/23$ range allows the 24th bit (the last bit of the third octet) and all 8 bits of the fourth octet to vary.

This results in a range where the third octet can be either $2$ ($00000010$) or $3$ ($00000011$). Therefore, the aggregate route $10.16.2.0/23$ covers all IP addresses from$10.16.2.0$ to $10.16.3.255$. The exhibit further confirms this by listing the "Contributing Routes": $10.16.2.0/24$ and $10.16.3.0/24$.

Analyzing the provided options against this range:

10.16.3.79 (Option A):This address falls squarely within the $10.16.2.0$ to $10.16.3.255$ range.

10.16.0.4 (Option B):This address falls in the $10.16.0.0/23$ range ($0.0$ to $1.255$).

10.16.4.183 (Option C):This address falls in the $10.16.4.0/23$ range ($4.0$ to $5.255$).

10.16.1.214 (Option D):This address also falls in the $10.16.0.0/23$ range.

Consequently,10.16.3.79is the only destination listed that matches the aggregate route shown. It is also important to note theNext hop type: Rejectin the exhibit; this means that if a packet matches the aggregate but does not match any of the more specific contributing routes, the router will drop the packet and send an ICMP unreachable message to the source.

In theOSPF (Open Shortest Path First)protocol, ensuring that all routers within an area have a synchronizedLink-State Database (LSDB)is fundamental to building a consistent loop-free topology. During the adjacency formation process—specifically when transitioning from theExStartstate to theExchangestate—routers must determine what information they are missing from their neighbors without sending the entire database at once, which would be highly inefficient.

TheDatabase Description (DBD)packet, also known as a DDP, is the mechanism used for this summary exchange. According to Juniper Networks technical documentation, the DBD packet does not contain full Link-State Advertisements (LSAs). Instead, it contains only theLSA headers, which include the LSA type, the ID of the advertising router, and the sequence number.

By exchanging these headers, a Juniper router can compare the neighbor's database summary against its own local LSDB. If the router identifies a header in the DBD packet that represents a newer or missing entry, it records that LSA in its "Link-State Request List." This collaborative "handshake" ensures that only the necessary, updated information is requested in the subsequentLink-State Request (LSR)phase. It is important to distinguish this from theLink-State PDU (LSP)mentioned in Option D, which is actually the term used in the IS-IS protocol, not OSPF. In OSPF, the functional unit is the LSA, and the transport vehicle for the initial summary is the DBD packet. This methodical synchronization is what allows OSPF to scale effectively in large service provider environments.

In a Juniper Networks MPLS environment, the process by which a router determines how to forward traffic involves both the control plane and the data plane. When R1 (acting as an Ingress Label Edge Router, or LER) receives an IP packet from Site A destined for Site B, it must perform a lookup to decide whether to forward the packet via standard IP routing or via an MPLS Label Switched Path (LSP).

The first criterion R1 uses is thedestination address of the traffic(Option C). Upon receiving the native IP packet, R1 looks up the destination IP in its routing table (typically inet.0). If the destination matches a prefix that is associated with an LSP—such as the loopback address of R5 or a prefix reachable via R5—the router identifies the appropriate Forwarding Equivalence Class (FEC). The FEC essentially groups packets that should be forwarded in the same manner over the same path. Without identifying the destination, the router cannot map the traffic to the correct MPLS tunnel.

The second criterion is thelabel number advertisement received from R2(Option D). MPLS relies on downstream label allocation. In this topology, R2 is the immediate downstream "next hop" for R1 on the path to Site B. For the LSP to be established, R2 must signal a label to R1 using a protocol like LDP (Label Distribution Protocol) or RSVP (Resource Reservation Protocol). This label (in this case, 7166) tells R1: "If you want to send traffic to the destination associated with this LSP, wrap it in this specific label so I know how to process it."

R1 does not use the source address (Option A) for standard label mapping, nor does it receive the label directly from R5 (Option B) in a hop-by-hop signaling model; it must use the label provided by its direct neighbor, R2. Therefore, by combining the destination IP (to find the path) and the label provided by the next hop (to encapsulate the packet), R1 successfully directs the traffic through the MPLS core.