JN0-364 Service Provider Routing and Switching - Specialist (JNCIS-SP) Questions and Answers

In the architecture of aLabel-Switched Path (LSP), routers are categorized based on their role in the handling of a specific packet's lifecycle through the MPLS network. Juniper Networks documentation defines these roles clearly:

TheIngress Router (Option D), also known as theIngress Label Edge Router (LER), is the entry point of the LSP. Its primary responsibility is to take an incoming "unlabeled" packet (usually a standard IPv4 or IPv6 packet), perform a route lookup, and determine which LSP the packet should follow. Once determined, the Ingress router performs aPushoperation, where it encapsulates the packet with an MPLS label header and forwards it toward the next hop. This is where the transition from IP-based forwarding to Label-based switching occurs.

To contrast this with the other options:

Transit Router (Option B):These are routers located between the ingress and egress. They performSwapoperations, replacing an incoming label with an outgoing label based on the Label Forwarding Information Base (LFIB).

Egress Router (Option A):This is the "tail-end" of the LSP where the packet exits the MPLS domain and the final label is removed (if it hasn't been removed already by the penultimate hop).

Penultimate Router (Option C):This is the second-to-last router in the path. As discussed in previous questions, it often performs thePopoperation (Penultimate Hop Popping) to remove the transport label before sending the packet to the Egress LER.

Therefore, the router where traffic first "enters" the LSP and receives its initial label is strictly defined as the Ingress router.

In theIS-IS (Intermediate System to Intermediate System)protocol as implemented in Junos OS, the output of operational commands uses specific numerical representations to denote the hierarchy levels of a neighbor adjacency. Understanding these values is crucial for troubleshooting peering relationships in a multi-level IS-IS network.

According to Juniper Networks technical documentation, the show isis adjacency command displays the status of the neighbors. The "L" column indicates the level of the adjacency:

Level 1:Indicates the adjacency is strictly for intra-area routing.

Level 2:Indicates the adjacency is strictly for backbone/inter-area routing.

Level 3:This is ashorthand representationused by Junos to indicate that a single adjacency has been established forboth Level 1 and Level 2 simultaneously.

The critical distinction in this question lies in the interface type. On abroadcast interface(such as standard Ethernet), IS-IS typically establishes and maintains separate adjacencies for Level 1 and Level 2. In the CLI output for a broadcast link, you would generally see two separate lines for the same neighbor—one for Level 1 and one for Level 2.

However, on apoint-to-point (P2P)interface, IS-IS can negotiate both levels within a single adjacency. When this occurs, Junos consolidates the output into a single entry and usesLevel 3to signify that the adjacency is functional for both levels. Since the exhibit shows ge-0/0/0.0 as Level 3, it confirms that the link is configured with a point-to-point encapsulation (either natively or via the interface-type p2p command) and is acting as a Level 1/2 adjacency.

Option B is incorrect as the number "3" refers to protocol levels, not the count of neighbors. Option C is a reference to data center architectures that does not influence IS-IS level nomenclature. Option D is incorrect because, as noted, broadcast interfaces display these levels separately rather than using the Level 3 shorthand.

One of the most significant architectural improvements in IPv6 is the move from a complex, variable-length header (as seen in IPv4) to a streamlined, fixed-length base header of 40 bytes. Additional functionality that was previously handled by "Options" in IPv4 is now moved toExtension Headers, which are inserted between the IPv6 base header and the upper-layer protocol (TCP/UDP).

According to Juniper Networks technical documentation and RFC 8200, the following are valid IPv6 Extension Headers:

Hop-by-Hop Options (Option B):This header carries optional information that must be examined by every node along the delivery path. It is used for features like the Router Alert and Jumbo Payload options.

Fragment (Option E):Unlike IPv4, where any router can fragment a packet, in IPv6, fragmentation is performed only by the source node. The Fragment header contains the information necessary for the destination to reassemble the packet (Offset, Identification, and More Fragments flag).

Destination Options (Option A):This header carries information intended only for the destination node. It can appear twice: once before a routing header and once after.

Why other options are incorrect:

Protocol (Option C):In IPv4, this was a field in the header. In IPv6, this is replaced by theNext Headerfield, which identifies the type of the following header (whether it's an extension header or the upper-layer protocol).

Header Checksum (Option D):This field was entirely removed in IPv6. IPv6 relies on the data link layer (Ethernet) and the transport layer (TCP/UDP) to perform error detection, significantly reducing the processing overhead for routers in the core of a service provider network.

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.

In theBorder Gateway Protocol (BGP), theSplit Horizonrule is a fundamental loop-prevention mechanism for internal sessions. This rule dictates that a BGP speaker must not advertise a route learned from anInternal BGP (IBGP)peer to any other IBGP peer within the same Autonomous System (AS). This ensures that routes do not circulate infinitely inside a network, as IBGP does not modify the AS_PATH attribute. Consequently, to maintain full reachability, a network normally requires a "full mesh" of IBGP sessions, where every BGP-speaking router is directly peered with every other router.

In the provided exhibit,ROUTER_1is part of AS 64523. The requirement is for ROUTER_1 to take prefixes learned from its internal peers and re-advertise them to other internal peers in the same AS. This behavior is a direct violation of the standard Split Horizon rule. According to Juniper Networks technical documentation, the standard solution to scale IBGP without a full mesh is to configureRoute Reflection.

When a router is configured as aRoute Reflector (RR), it is permitted to "reflect" (re-advertise) routes learned from one IBGP peer to another. In Junos OS, the mechanism to enable Route Reflection is to configure acluster IDwithin the BGP group. By adding the cluster keyword followed by a unique 32-bit identifier (usually the router's loopback address) to the internal BGP group configuration, the router assumes the role of an RR. It then follows specific reflection rules:

Routes learned from anEBGP peerare reflected to all IBGP peers.

Routes learned from aRoute Reflector Clientare reflected to all other clients and non-clients.

Routes learned from anon-clientare reflected to all clients.

Option A is incorrect because BGP advertisement rules are hard-coded; a standard export policy cannot override the Split Horizon rule. Option C handles traffic engineering tags but does not enable route reflection. Option D would change the session to EBGP, which does not address the internal reachability requirement within AS 64523. Therefore, configuring the cluster ID is the only valid way to achieve the desired re-advertisement behavior.

In theIS-IS (Intermediate System to Intermediate System)protocol, the process of forming an adjacency between two neighbors follows a specific sequence of states. While OSPF uses states like "Init," "Two-Way," and "Full," IS-IS uses a slightly different nomenclature within its state machine.

According to Juniper Networks technical documentation, when a router first sends anIS-IS Hello (IIH) PDUand receives one back from a neighbor, but has not yet confirmed that the neighbor "sees" it back, the adjacency enters theInitializingstate. Specifically, on a point-to-point link, the state transitions fromDowntoInitializingas soon as the first PDU is received. On a broadcast network (like Ethernet), the Initializing state indicates that the local router has received a Hello PDU from the neighbor, but the local router's own System ID is not yet listed in the neighbor's list of "seen" neighbors (the neighbor's Hello PDU does not yet contain the local router's MAC address).

The adjacency only moves to theUpstate (Option C) once bi-directional communication is confirmed—meaning both routers have seen each other's System IDs in the incoming Hello PDUs.

Why other options are incorrect:

Loading (Option A):This is an OSPF state, not an IS-IS state. In IS-IS, database synchronization happens after the adjacency is Up.

Two-Way (Option D):While functionally similar to the state IS-IS is achieving, "Two-Way" is the specific terminology for OSPF. In IS-IS, the intermediate step between knowing a neighbor exists and having a fully functional adjacency is strictly calledInitializing.

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.

In a Multiprotocol Label Switching (MPLS) environment, label operations are categorized into three primary actions:Push(adding a label),Swap(replacing a label), andPop(removing a label). The specific behavior described in the question refers to a mechanism calledPenultimate Hop Popping (PHP).

According to Juniper Networks technical documentation, the goal of PHP is to improve forwarding efficiency at the egress point of a Label-Switched Path (LSP). TheEgress Label Edge Router (LER), which is the final destination for the LSP, would normally have to perform two lookups if it received a labeled packet: first, it would look up the label in its MPLS table to see it is the destination, and second, it would look up the underlying IP payload in its IP routing table (inet.0) to forward the packet.

To alleviate this burden, the Egress LER signals a special label value calledImplicit Null (Label 3)to its upstream neighbor (the penultimate router) during the signaling process (RSVP or LDP). When thepenultimate routerreceives a packet destined for that egress LER, it sees the instruction to pop the transport label. Consequently, the penultimate router performs aPopoperation, stripping away the outer MPLS label and sending the raw IP packet (or the remaining inner service label) to the Egress LER.

This allows the Egress LER to perform only a single lookup. If the transport label was the only label, the Egress LER simply performs a standard IP lookup. If there is a VPN label remaining, it performs a single MPLS lookup for the VRF. This "default" behavior in Junos OS optimizes the performance of the egress router by offloading the final label removal to the penultimate hop. Note that ifUltimate Hop Popping (UHP)were configured (via the explicit-null command), the penultimate router would perform aSwapto Label 0 instead of a Pop.

In aSegment Routing (SR-MPLS)architecture using IS-IS as the control plane, routers exchange labels (segments) to build Label-Switched Paths (LSPs) without the need for traditional signaling protocols like LDP or RSVP. According to Juniper Networks technical documentation, for a router to be reachable via a shortest-path LSP from other nodes in the network, it must advertise aPrefix Segment Identifier (Prefix SID).

A specific type of Prefix SID is theNode SID, which is assigned to a loopback address (typically lo0.0) to uniquely identify the router within the SR domain. In the provided exhibit, routerR10has been configured with aSegment Routing Global Block (SRGB)starting at label 300000. This configuration tells the router which label range to use for global segments, but it does not automatically assign a label to its own loopback interface.

Without aNode SIDconfiguration, R10 is not telling its neighbors which specific index or label within that SRGB corresponds to its own address. Consequently, the other nine routers in the IS-IS area can calculate the shortest path to R10 using standard SPF, but they cannot perform the "label-binding" necessary to push an SR-MPLS label onto the packets.

To solve this, a Node SID must be explicitly configured under the loopback interface within the IS-IS protocol hierarchy, such as:

set protocols isis interface lo0.0 level 2 ipv4-node-sid index

Analysis of incorrect options:

Binding SID (Option A):This is used to encapsulate or steer traffic into a specific policy or tunnel (like a TE-LSP) and is not required for basic shortest-path reachability.

Adjacency SID (Option B):These are generated automatically by Junos for each link and represent a specific local hop; they are not used for global "shortest-path" forwarding to a distant node.

BGP Prefix SID (Option C):This is used for BGP Egress Peer Engineering (EPE) or prefix advertisement via BGP, which is not relevant for building the underlying IS-IS SR-MPLS transport.

Therefore, configuring anexplicit IPv4 node SIDis the mandatory step to enable the rest of the network to build a shortest-path SR-LSP toward R10.

In the Junos OS, routing policy behavior is governed bydefault import and export rulesthat vary significantly between different protocols. ForIS-IS (Intermediate System to Intermediate System), the default export policy is "reject all." This means that, by default, an IS-IS process willexport nothingfrom the routing table into the IS-IS database.

According to Juniper Networks technical documentation, IS-IS automatically advertises its own direct interfaces that are configured under the [edit protocols isis] hierarchy. However, it does not automatically redistribute routes learned from other sources—such asStatic routes,OSPF, orBGP—into the IS-IS domain. This is a safety mechanism designed to prevent accidental routing loops or the flooding of unnecessary prefixes into the link-state database (LSDB), which could impact the stability of the SPF (Shortest Path First) algorithm.

To move routes from the routing table (inet.0) into IS-IS, an administrator must explicitly create arouting policyand apply it as anexport policywithin the IS-IS configuration. For example:

Code snippet

set policy-options policy-statement REDIST-STATIC term 1 from protocol static

set policy-options policy-statement REDIST-STATIC term 1 then accept

set protocols isis export REDIST-STATIC

Without such a policy, the IS-IS LSPs (Link-State PDUs) will only contain information about the IS-IS enabled interfaces and the reachability of other IS-IS neighbors. This behavior contrasts with protocols like BGP, which has different default rules for exporting active BGP routes to EBGP peers. In the context of IS-IS in a Juniper environment, "export nothing" is the standard operational baseline.

In a Juniper Networks environment, when a router receives multiple BGP paths for the same destination prefix, it utilizes theBGP Path Selection Algorithmto determine the single "best" path to install in the routing table and advertise to other peers. This selection process follows a strict hierarchy of attributes.

According to Juniper Networks technical documentation, the very first attribute evaluated by the BGP process (after ensuring the next hop is reachable) is theLocal Preference. Local preference is a well-known discretionary attribute used to communicate a preference for a specific exit point from the local Autonomous System (AS). A higher local preference value is always preferred over a lower one.

Analyzing the exhibit:

R3receives the prefix from R1 and applies an export policy to its IBGP session that sets thelocal preference to 150.

R4receives the same prefix from R2 and applies an export policy to its IBGP session that sets thelocal preference to 200.

R5receives both of these IBGP updates from R3 and R4.

When R5 runs the best-path algorithm for the 203.0.113.0/24 prefix, it compares the local preference values. Since the path from R4 has a local preference of 200 and the path from R3 has a local preference of 150, R5 immediately selects the path fromR4as the best route. Because BGP is designed to prevent loops and maintain a consistent view, only this single best path is installed as the active route in R5's routing table (inet.0). Options B and D are incorrect because they imply multiple paths are installed for forwarding, which only occurs if specific multipath load-balancing is configured, which is not indicated here.

In an MPLS network, routers are categorized by their role along a Label Switched Path (LSP). In this scenario, the LSP originates onR1 (Ingress LER)and terminates onR5 (Egress LER). Between these two endpoints are the Provider (P) routers, also known as Transit Label Switching Routers (LSRs), which include R2, R3, and R4.

To identify which router performsonlylabel swap operations, we must look at the standard Junos data plane behavior:

R1 (Ingress LER):Performs aPushoperation. It receives native IP traffic from Networks 1 or 2, looks up the destination, and imposes (pushes) an MPLS label onto the packet before sending it to R2.

R2 and R3 (Transit LSRs):These routers perform aSwapoperation. They receive a labeled packet, look up the incoming label in their Label Forwarding Information Base (LFIB), replace it with an outgoing label provided by the downstream neighbor, and forward it.

R4 (Penultimate Hop):By default, Junos usesPenultimate Hop Popping (PHP). Because R4 is the second-to-last router before the egress (R5), the egress router R5 advertises an "implicit-null" label (Label 3) to R4. This instructs R4 to perform aPopoperation—removing the MPLS label entirely—and sending the native IP packet to R5.

R5 (Egress LER):Receives the packet (which is already unlabeled due to PHP) and performs a standard IP route lookup to reach the final destination in Network 3 or 4.

Among the options provided,R3is the only router that is a transit LSR butnotthe penultimate hop. While R2 also performs a swap, it is not an option. R4 performs a Pop (due to PHP), R1 performs a Push, and R5 performs an IP lookup. Therefore, R3 is the correct answer as it solely performs the label swap operation.

In OSPF, loop prevention within a single area is achieved through the fundamental nature of its link-state architecture. Unlike distance-vector protocols that rely on "routing by rumor," OSPF ensures that every router within an area maintains an identicalLink-State Database (LSDB). This database acts as a complete map of the network topology.

Once the LSDB is synchronized, each router independently executes theShortest Path First (SPF) algorithm, which is formally known as theDijkstra algorithm. This mathematical process treats the local router as the "root" of a tree and calculates the shortest path to every other node (router) and prefix in the area based on the cumulative interface costs. Because every router uses the same synchronized map (the LSDB) and the same deterministic algorithm, they all arrive at a consistent, loop-free view of the best paths.

According to Juniper Networks technical documentation, the Dijkstra algorithm is superior to theBellman-Ford algorithm(used by distance-vector protocols like RIP) in this regard. Bellman-Ford is susceptible to "count-to-infinity" problems and loops because routers only know the distance and direction to a destination provided by their neighbors, rather than the full topology. In OSPF, even if a link fails, the updated Link-State Advertisement (LSA) is flooded rapidly, and the Dijkstra algorithm is re-run to find a new loop-free path.Routing policies(Option B) are used to manipulate path selection or filter routes but are not the primary mechanism for fundamental loop prevention in OSPF. Similarly,forwarding policies(Option D) govern how traffic is handled at the data plane level rather than determining the control plane's loop-free topology.

In the IPv6 architecture, the base header is kept at a fixed size of 40 bytes to streamline processing. Any additional features or options are handled byExtension Headers, which are inserted between the IPv6 header and the upper-layer protocol. According to Juniper Networks technical documentation and RFC 8200, when a source node needs to list one or more intermediate nodes to be "visited" on the way to the final destination, it utilizes theRouting extension header (Option B).

The Routing header is functionally similar to the "Source Route" option in IPv4. When a packet contains a Routing header, it is addressed to the first intermediate node listed in the header. That node examines the header, swaps its own address with the next address in the list, and forwards the packet. This process continues until the packet reaches the final destination. This is a foundational component for technologies likeSegment Routing over IPv6 (SRv6), where the Routing header (specifically the Segment Routing Header or SRH) is used to steer traffic through a specific set of service instructions or nodes.

To distinguish this from the other options:

Hop-by-hop options (Option A):These carry information that must be examined byeverynode along the path (such as Router Alert), not just specific intermediate nodes.

Fragment (Option C):This is used only when the source node needs to fragment a packet that exceeds the path MTU.

Destination options (Option D):These carry optional information intended specifically for the destination node (or nodes listed in a Routing header), but they do not dictate the path themselves.

For Juniper platforms equipped with dualRouting Engines (REs), the fundamental technology required to provide high availability during a hardware or software failure of the primary RE isGraceful Routing Engine Switchover (GRES).

According to Juniper Networks technical documentation, GRES allows the backup RE to stay in a "hot" standby state. When GRES is enabled, the primary RE synchronizes critical state information with the backup RE, specifically thechassis stateand theinterface state. This synchronization includes the Packet Forwarding Engine (PFE) configuration.

When the primary RE fails, the backup RE takes over immediately. Because the PFE (which resides on the line cards) was already synchronized and is not restarted during the switchover, the routercontinues to forward packetsthat are already in flight or part of established flows. This prevents a complete network outage during an RE failover.

Comparison with other options:

NSB (Non-Stop Bridging - Option A):Focuses specifically on maintaining Layer 2 protocol states (like STP) during a switchover.

LAG (Link Aggregation - Option B):Provides redundancy for physical links, not the control plane or the RE.

BFD (Bidirectional Forwarding Detection - Option C):Is a protocol used for rapid detection of link or neighbor failures; it does not protect the RE or maintain forwarding during an internal switchover.

It is important to note that while GRES maintains theforwardingstate, it does not by itself maintain therouting protocolstate (adjacencies). To keep OSPF or BGP sessions from dropping during the switchover, GRES must be paired withNon-Stop Active Routing (NSR). However, as the question focuses on the core requirement of continuing to forward packets,GRESis the foundational technology.

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.