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

The transition to IPv6 requires routing protocols that are capable of carrying 128-bit address information. Juniper Networks Junos OS supports several "IPv6-ready" protocols for dynamic routing.

1. IS-IS (Option B):

As discussed in previous questions,IS-ISis inherently extensible due to its use ofTLVs (Type, Length, Value). To support IPv6, the protocol did not need a major rewrite; instead, new TLVs (such as TLV 236 for IPv6 reachability and TLV 232 for IPv6 interface addresses) were added. A single IS-IS process in Junos can simultaneously carry both IPv4 and IPv6 routing information, making it a highly efficient choice for "dual-stack" service provider backbones.

2. BGP (Option D):

BGP was updated to support multiple protocols throughMultiprotocol Extensions (MP-BGP), defined in RFC 4760. By usingAddress Family Identifiers (AFI)andSubsequent Address Family Identifiers (SAFI), a single BGP session can exchange NLRI (Network Layer Reachability Information) for IPv4 unicast, IPv6 unicast, and even VPNv4/VPNv6 routes. In Junos, this is configured under the family inet6 unicast hierarchy within the BGP protocols configuration.

Why other options are incorrect:

IGMP (Option A):This is a management protocol for IPv4 multicast (Internet Group Management Protocol). Its IPv6 equivalent isMLD (Multicast Listener Discovery).

OSPFv2 (Option C):OSPF version 2 is strictly for IPv4. To run OSPF in an IPv6 environment,OSPFv3must be used, as it was specifically redesigned to handle the IPv6 address space and link-local communication.

In OSPF networks, when multiple routers are connected to a shared multi-access broadcast segment (like an Ethernet switch), they undergo an election process to select aDesignated Router (DR)and aBackup Designated Router (BDR). This mechanism is essential for reducing the number of adjacencies and limiting the volume of Link State Advertisement (LSA) flooding on the segment.

The OSPF election process follows a strict hierarchy based on the following criteria:

Interface Priority:The router with the highest OSPF interface priority is elected as the DR. The router with the second-highest priority becomes the BDR. In Junos, the default priority is 128, but it can be manually configured between 0 and 255.

Router ID:If there is a tie in priority, the router with the numerically highest Router ID (RID) wins the election.

Analyzing the configuration provided in the exhibit:

R1:Priority 200, Router-ID 192.168.1.1

R2:Priority 100, Router-ID 192.168.1.2

R3:Priority 50, Router-ID 192.168.1.3

R4:Priority 90, Router-ID 192.168.1.4

Comparing the priority values,R1 has the highest priority (200)and therefore becomes theDR. The next highest priority value among the remaining routers is100, which belongs to R2, making it theBDR. Although R4 has a higher Router ID than R2, the priority value is evaluated first and takes precedence.

Since all routers came online within a short window (one minute), they participate in the same election cycle, ensuring the configured priorities dictate the outcome rather than "first-come, first-served" preemption behavior common in OSPF once a DR is already established.

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 standard routing, astatic routeis typically considered valid only if the specified next-hop IP address is directly reachable on a local subnet. However, in complex service provider designs, the next-hop might be a "distant" IP address that is reachable through another route (such as a BGP route or another static route). This process of looking up a next-hop within another routing entry is calledrecursive lookup.

In Junos OS, theresolve (Option A)parameter is explicitly used to enable this behavior for static routes. According to Juniper technical documentation, when you append the resolve keyword to a static route configuration, you are instructing the Routing Engine to search the routing table to find a path to that distant next-hop.

For example:

set routing-options static route 10.1.1.0/24 next-hop 192.168.100.1 resolve

If 192.168.100.1 is not on a local interface but is reachable via an OSPF route, the router will "resolve" the path and install the 10.1.1.0/24 route into the forwarding table using the OSPF path's exit interface.

Why other options are incorrect:

Discard (Option B)andReject (Option C)are "next-hop types" used to drop traffic, either silently (discard) or by sending an ICMP unreachable message (reject).

Next-table (Option D)is used forInter-VRF routing, where the router is told to look up the destination in a completely different routing instance (like a VRF table), which is a different architectural function than a recursive next-hop lookup within the same table.

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 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 Junos OS, the flow of routing information is managed by policies that sit between the protocol's database (the RIB-In/LSDB) and the main routing table (inet.0). Understanding the direction of these policies is critical for correct configuration.

Animport policy (Option B)is used to control the movement of routes from a routing protocol into the routing table. According to Juniper Service Provider documentation, even though IS-IS is a link-state protocol that requires all routers in an area to have an identical Link-State Database (LSDB), animport policycan be used to filter which of those validated routes are actually placed into inet.0 for forwarding. For external routes (routes leaked into IS-IS from other areas or protocols), an import policy allows an administrator to selectively accept or reject prefixes based on specific criteria like prefix-lists or community tags.

It is important to distinguish this from anexport policy (Option A). In Junos, an export policy is used to take routesalready inthe routing table and push themoutto a protocol to be advertised to neighbors. For example, you would use an export policy to redistribute static routes into IS-IS.Route preference (Option C)is a global value used to select between different protocols for the same prefix, and theinterface metric (Option D)is used by the SPF algorithm to calculate the shortest path within the IS-IS database itself. Therefore, to specifically control which learned external routes are "installed" into the forwarding table, theimport policyis the correct tool.

In the BGP finite state machine, theIdlestate is the "stop" or "start" point of the protocol. When a session is stuck in Idle, it means the BGP process is either administratively disabled or, more commonly, is unable to initiate the underlying TCP connection required for BGP.

According to Juniper Networks Service Provider documentation, the most common reason for a BGP session to remain in Idle is a lack ofrouting reachability. For BGP to move to theConnectstate, the Junos kernel must have a route to the IP address specified in the neighbor statement. If thepeer IP address is unreachable (Option A)—meaning there is no route in inet.0 (via OSPF, IS-IS, or static)—the router cannot initiate the TCP three-way handshake on port 179. Consequently, the state machine will never progress.

Analysis of incorrect options:

Option B:BGP does not care if the local AS is higher or lower than the peer's; it only cares if they match the configuration. AS numbers are identifiers, not priorities.

Option C:A mismatchedRouter IDdoes not prevent a session from leaving the Idle state. It would typically cause the session to reach theOpenConfirmstate, and then fail with a "Notification" message due to a collision or identification error.

Option D:While a mismatchedgroup type(internal vs. external) will cause the session to fail, it usually fails during theOpenmessage exchange (OpenSent state) because the AS numbers provided will not match the expected peer type (IBGP vs. EBGP).

Only the lack of a path to the neighbor (reachability) keeps the session at the very beginning of the process: theIdlestate.

In Junos OS, the Routing Engine maintains several different tables to manage various types of reachability and forwarding information. When a router is running MPLS, it must track both IP routes and label-to-label mappings.

Thempls.0table is the primary repository for theLabel Information Base (LIB)and theLabel Forwarding Information Base (LFIB). According to Juniper Networks documentation, mpls.0 is used by transit and egress routers to perform label lookups. When a labeled packet arrives at an interface, the router looks at the top label and references the mpls.0 table to determine the next action. This table stores the mapping of incoming labels to their corresponding operations:Pop(remove the label),Swap(replace the label), orPush(add an additional label).

It is crucial to understand the roles of the other tables to avoid confusion:

inet.0 (Option D):This is the default unicast routing table for IPv4, used for standard IP-to-IP forwarding.

inet.3 (Option C):This is theMPLS Path Table. It stores the egress loopback addresses of LSPs and is used by BGP for next-hop resolution to determine if a destination can be reached via an MPLS tunnel. While inet.3 knowsaboutLSPs, the actual label-switching instructions reside in mpls.0.

inet6.0 (Option A):This is the default unicast routing table for IPv6.

Therefore, for the specific purpose of storing the label base used for transit switching operations,mpls.0is the correct and only table used in the Junos architecture.

In Juniper Networks Junos OS,Generic Routing Encapsulation (GRE)andIP-in-IP (IP-IP)are common tunneling mechanisms used to transport packets across a network by encapsulating them within another protocol. Understanding the header structure and the limitations of these protocols is essential for proper MTU (Maximum Transmission Unit) management and security design.

Overhead (Option A):

Both GRE and IP-IP tunnels operate by adding an additional IP header to the original packet. An IP-IP tunnel (Protocol 4) adds a20-byteIPv4 header. A GRE tunnel (Protocol 47) adds the same20-bytedelivery IP header plus a minimum4-byteGRE header (totaling 24 bytes, which can increase if keys or sequencing are used). Because these headers are added to the payload, the total size of the packet increases. This "overhead" means that if the original packet was already at the MTU limit (e.g., 1500 bytes), the encapsulated packet will exceed it, potentially leading to fragmentation or the need to adjust theTCP MSS (Maximum Segment Size).

Encryption (Option D):

Crucially, according to Juniper Service Provider documentation, neither GRE nor IP-IP provides nativeencryptionor data confidentiality. They are encapsulation protocols, not security protocols. The payload remains in cleartext and is visible to any device along the path. If security and encryption are required for data traversing these tunnels, they must be combined withIPsec (IP Security). While GRE is often used as the "carrier" for IPsec (to allow multicast or dynamic routing protocols which IPsec alone does not support), the GRE protocol itself remains an unencrypted delivery mechanism. Therefore, statements A and D accurately describe the architectural behavior of these tunnel types.

IS-IS is a link-state protocol that relies on the rapid and consistent flooding ofLink-State PDUs (LSPs)to ensure that every router in an area has an identical view of the topology. To manage the "freshness" of information, IS-IS uses aSequence Number—a 32-bit unsigned integer that increments every time the originating router makes a change to its LSP.

According to Juniper Networks technical documentation, when a router receives an LSP, it performs a comparison between the received LSP and the version it currently holds in itsLink-State Database (LSDB). If the received LSP has ahigher sequence number, the router concludes that this is "newer" and more accurate information. The router will then perform two immediate actions:

Update:It replaces the older LSP in its LSDB with the newly received version.

Flood:It propagates the new LSP to all other neighbors (except the one that sent it) to ensure the entire area converges on the new data.

If the sequence numbers were equal, the router would ignore the incoming PDU as it already has the information. If the received sequence number werelower, the router would conclude its own database is more recent and would actually send its own "newer" version back to the neighbor to bring them up to date (a process called "poisoning" or refreshing the neighbor).Complete Sequence Number PDUs (CSNPs)(Option C) are used during initial database synchronization or periodic checks on broadcast links, but the primary response to a "newer" LSP is immediate database update and flooding.

In the context of theLabel Distribution Protocol (LDP), the method by which a router advertises labels to its neighbors is defined by itsLabel Advertisement Mode. According to Juniper Networks documentation and industry standards (RFC 5036), there are two primary modes:Downstream Unsolicited (DU)andDownstream on Demand (DoD).

InDownstream Unsolicited (DU)mode, which is the default behavior for Junos OS and most service provider implementations, an LSR (Label Switching Router) does not wait for a specific request from its neighbors. Instead, as soon as the LSR learns a prefix through its Interior Gateway Protocol (IGP) and establishes an LDP session, it automatically generates a label for that prefix and advertises it to all of its LDP peers. This explains the scenario where labels appear for every loopback address in the IGP as soon as LDP is enabled. DU mode is highly efficient for fast convergence because the labels are already present in the neighbors' databases before they are even needed for traffic forwarding.

By contrast,Downstream on Demand (DoD)requires a router to explicitly request a label for a specific prefix from its next-hop neighbor.Ordered Control (Option B)andIndependent Controlrefer to thetimingof label creation (whether a router waits for the next-hop to provide a label before creating its own), whileConservative Retention (Option A)refers to how a router stores labels it receives but doesn't currently use for forwarding. In the Junos default environment, LDP utilizesDownstream Unsolicitedadvertisement combined withOrdered ControlandLiberal Retentionto ensure a robust and rapidly converging MPLS control plane.

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.

When transitioning to an IPv6 environment usingOSPFv3(the version of OSPF designed for IPv6), there are significant architectural differences compared to OSPFv2 (IPv4). According to Juniper Networks technical documentation, OSPFv3 was redesigned to be more protocol-agnostic.

Router ID (Option C):

Despite OSPFv3 routing IPv6 (which uses 128-bit addresses), the OSPFRouter IDremains a32-bit valueformatted like an IPv4 address (e.g., 1.1.1.1). This is a common point of confusion. In a pure IPv6 environment where no IPv4 addresses are configured on any interfaces, a Juniper router cannot automatically derive a Router ID. Therefore, the administrator must manually configure a 32-bit Router ID under [edit routing-options] for the OSPFv3 process to initialize.

Interface Configuration (Option D):

OSPFv3 runs directly over the IPv6 link-local scope. Unlike OSPFv2, it does not require an IPv4 address to function. Therefore, interfaces areonly required to be configured with family inet6(Option D). You do not need "dual-stack" (both IPv4 and IPv6) functionality just to run OSPFv3. The protocol uses the link-local address (fe80::/10) of the interface for neighbor adjacencies and as the next hop for routing updates. This separation allows OSPFv3 to carry multiple "address families" (both IPv4 and IPv6 unicast) if needed, but the base requirement for an IPv6-only network is simply the family inet6 configuration.

In Juniper Networks Junos OS, theLabel Distribution Protocol (LDP)is specifically designed to automate the creation of Label Switched Paths (LSPs) based on the information provided by the underlying Interior Gateway Protocol (IGP), such as OSPF or IS-IS. When LDP is enabled on a set of interfaces within an OSPF area (as shown in the exhibit with Area 0.0.0.0), it automatically discovers neighbors and exchanges label mappings for all known unicast routes in the routing table.

The defining characteristic of LDP in this context is its "topology-driven" nature. Unlike RSVP (Resource Reservation Protocol), which typically requires the manual configuration of each LSP ingress point and destination, LDP follows the IGP's shortest path tree to automatically build afull meshof LSPs between all participating routers. This means that every Provider Edge (PE) and Provider (P) router in the exhibit—PE1, PE2, PE3, P1, P2, and P3—will establish label-switched connectivity to every other router without the administrator having to define individual tunnels.

LDP accomplishes this through a downstream-unsolicited label distribution mode by default in Junos. Each router assigns a local label for its loopback address and other prefixes and advertises these to its neighbors. Because every router is performing this action for every reachable prefix in the OSPF domain, a complete fabric of label-switched paths is formed. While RSVP is more robust for traffic engineering and bandwidth reservation, LDP is the preferred protocol for creating a simple, scalable full mesh of LSPs for applications like Layer 3 VPNs or internal BGP tunneling where complex path manipulation is not required. BFD is a failure detection protocol, and BGP is used for service signaling, making LDP the only correct choice for automatic mesh creation.

To supportdual VLAN tagging(often referred to as Q-in-Q or 802.1ad), a Juniper interface must be configured to process more than one 802.1Q header. In Junos OS, this is handled at the physical interface level ([edit interfaces ]).

According to Juniper Service Provider documents, two primary configuration statements enable this capability:

stacked-vlan-tagging (Option D):This is the traditional command used to enable an interface to accept frames with two VLAN tags. When this is enabled, the router expects an outer "service" tag and an inner "customer" tag. This is specifically used in provider edge scenarios where a service provider is tunneling multiple customer VLANs.

flexible-vlan-tagging (Option A):This is a more modern and versatile command. It allows the interface to support a mix of different encapsulation types across different logical units. For example, with flexible-vlan-tagging, you can have one logical unit (unit 10) doing standard single-tagging and another logical unit (unit 20) doing dual-tagging (vlan-tags outer X inner Y). This is the preferred method on newer hardware (like the MX Series) because it provides the highest level of configuration flexibility.

Vlan-tagging (Option C)only enables the interface to support a single 802.1Q tag, andgigether-options (Option B)contains physical-layer settings like auto-negotiation or flow control, which do not influence VLAN encapsulation. Therefore, A and D are the correct mechanisms for enabling dual-tag support.