JN0-364 Juniper Service Provider Routing and Switching - Specialist (JNCIS-SP) Free Practice Exam Questions (2026 Updated)

In the IS-IS protocol,TLVs (Type, Length, Value)are the fundamental building blocks used to carry information withinLink-State PDUs (LSPs). Unlike some other protocols that have a fixed, rigid packet format, IS-IS was designed from the ground up to be modular and extensible. This extensibility is achieved through the use of TLVs, which allow the protocol to carry different types of data without requiring changes to the core protocol state machine.

According to Juniper Networks technical documentation,TLVs allow flexible encoding of routing information (Option C). Each TLV specifies the "Type" of information it carries (such as neighbor information or IP reachability), the "Length" of that information, and the "Value" (the actual data). This architecture is what allowed IS-IS to easily support IPv6 by simply adding new TLVs (like TLV 236 for IPv6 reachability) without redesigning the protocol. It also supports Traffic Engineering (TE) extensions used in MPLS environments by adding TLVs that describe link bandwidth and administrative groups.

Furthermore, a singleLSP can contain multiple TLVs (Option D). When a Juniper router generates an LSP, it packs all the necessary information—such as the router's area addresses, its neighbors, and its local interface prefixes—into various TLVs and places them into a single PDU. If the amount of information exceeds the Maximum Transmission Unit (MTU) of the interface, the router will generate additional LSPs (fragmented LSPs) to carry the remaining TLVs.

Options A and B are incorrect because restricting an LSP to a single TLV would make the protocol incredibly inefficient, and the very nature of IS-IS is its ability to support multiple network layer protocols (not just IPv4) through its agnostic TLV-based transport.

In a Juniper Networks MPLS environment, the selection of a signaling protocol depends heavily on the requirement for traffic engineering and path control. To satisfy the requirement for anexplicit path—where the network architect defines specific hop-by-hop routers that the traffic must traverse—theResource Reservation Protocol (RSVP)is the necessary choice.

According to Juniper documentation, RSVP (specifically RSVP-TE) supports the use ofExplicit Route Objects (EROs). When you configure an LSP in Junos OS, you can define a path consisting of a series of IP addresses (strict or loose hops). RSVP then signals the LSP along that exact sequence of routers, reserving resources and establishing labels as it goes. This allows for precise control over the network's traffic patterns, enabling administrators to steer traffic away from congested links or toward specific high-bandwidth paths.

In contrast,LDP (Label Distribution Protocol)(Option D) is a "best-effort" signaling protocol. LDP strictly follows the Interior Gateway Protocol (IGP) shortest path. It does not support explicit paths or traffic engineering constraints; it simply builds a "mesh" of labels based on the existing routing table.IS-IS(Option C) is an IGP used to populate the routing table and TED but does not signal labels.BGP(Option A) is used for service delivery (like L3VPNs) but relies on an underlying transport LSP (built by RSVP or LDP) to reach its next hop. Therefore, only RSVP provides the mechanism for explicit path manipulation.

In the Juniper Networks Junos operating system, the management of routing information is partitioned into several distinct routing tables (RIBs), each serving a specific architectural purpose. When dealing withMultiprotocol Label Switching (MPLS), understanding the distinction between inet.0 and inet.3 is fundamental for troubleshooting and traffic engineering.

Theinet.3routing table is specifically designed to store the egress IPv4 addresses ofLabel-Switched Paths (LSPs). When an ingress router successfully establishes an LSP (via RSVP or LDP), it places the host address of the egress router (the tail-end) into the inet.3 table. This table is not used for general packet forwarding; instead, it is primarily used by theBorder Gateway Protocol (BGP)for next-hop resolution. When BGP receives a route, it checks both inet.0 and inet.3 to resolve the next hop. If a matching entry exists in inet.3, the router knows it can reach that destination via an MPLS tunnel, allowing for the encapsulation of BGP traffic within MPLS.

In contrast,inet.0is the default unicast routing table used for standard IPv4 forwarding and contains routes learned via IGPs (OSPF, IS-IS) or static routing.inet.1is utilized for multicast forwarding (MBGP), andinet.2is typically used for Multicast Source Discovery Protocol (MSDP) or RPF checks in multicast environments. By isolating LSP egress points in inet.3, Junos prevents MPLS-specific paths from interfering with standard IGP path selection unless the administrator explicitly chooses to merge them (e.g., using the traffic-engineering bgp-igp command). Therefore, by default, the ingress router maintains its list of reachable LSP endpoints in inet.3.

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 a Juniper Networks environment running Junos OS, understanding the interaction between different versions of OSPF is essential for multi-protocol environments.OSPFv2(defined in RFC 2328) is the standard protocol used for routing IPv4 unicast traffic.OSPFv3(defined in RFC 5340) was originally developed to support IPv6 routing. However, OSPFv3 was later extended via RFC 5838 to support multiple address families (AF), allowing it to carry IPv4 unicast, IPv4 multicast, and other address types within a single OSPF instance.

According to Juniper technical documentation, Junos OS implements this multi-AF support in OSPFv3 through the use ofrealms. When the realm ipv4-unicast statement is configured under the [edit protocols ospf3] hierarchy, the OSPFv3 process becomes capable of calculating and advertising IPv4 routes.

In the provided exhibit, routerR2has a dual-protocol configuration. First, it is running standard OSPFv2, with the ge-0/0/1.0 interface (which is directly connected to the 172.16.2.0/24 network) participating in Area 0. This ensures that the prefix is advertised as a standard IPv4 LSA to its neighbor,R1. Second, R2 is running OSPFv3 with the realm ipv4-unicast specifically enabled on that same ge-0/0/1.0 interface. Because of this realm, OSPFv3 also treats the 172.16.2.0/24 prefix as a reachable IPv4 destination and advertises it to R1 as an OSPFv3 IPv4-unicast LSA.

As a result, whenR1(which is also running both protocols) receives these routing updates, it will see the same destination prefix advertised by two different protocols. Its routing table (inet.0) will contain one entry learned from the OSPFv2 process and a second, separate entry learned from the OSPFv3 process. While the Junos Routing Engine will ultimately select one as the "active" route based on route preference (both protocols have a default preference of 10), both entries will technically exist within the Routing Information Base (RIB). This confirms that statementBis the correct description of the operational state of the network.

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In the context ofJuniper Networks Junos OSand theIS-IS (Intermediate System to Intermediate System)protocol, understanding the hierarchical relationship between router levels is critical for effective troubleshooting and design. IS-IS uses a two-level hierarchy to manage scalability:Level 1 (L1), which represents intra-area routing, andLevel 2 (L2), which represents inter-area backbone routing.

When a router is configured as aLevel 1/2 (L1/L2)device, it acts as a bridge between the two levels. According to Juniper technical documentation, an L1/L2 router maintains two completelyseparate Link-State Databases (LSDB)—one for Level 1 and one for Level 2. It does not merge these into a single topology. This separation ensures that local area topology changes (L1) do not necessarily flood into the backbone (L2) unless specific redistribution is configured, and vice versa. Therefore, statementCis correct because the L1/L2 router maintains distinct SPF (Shortest Path First) computations for each level.

Regarding the visibility of the Level 1 router, IS-IS is designed to keep L1 areas "stubby" by default. ALevel 1 routeronly possesses the topology information for its own area (the Level 1 LSDB). It does not receive specific L2 routes or the L2 topology. Instead, the L1/L2 router sets theAttached (ATT) bitin its L1 Link-State PDUs (LSPs) to signal to L1-only routers that it has a connection to the backbone. The L1 router then generates a default route pointing to the L1/L2 router to reach inter-area destinations. This confirms that statementDis correct: the L1 router's knowledge is limited to its local L1 topology.

Conversely, statements A and B are incorrect because merging topologies would violate the hierarchical scaling principles of IS-IS, and L1 routers never learn the full L2 topology without explicit, non-standard route leaking.

In theIS-IS (Intermediate System to Intermediate System)protocol, communication between routers is performed usingProtocol Data Units (PDUs). To discover neighbors and maintain adjacencies, IS-IS relies on theHello PDU (IIH - IS-IS Hello).

According to Juniper Networks technical documentation, when IS-IS is enabled on an interface, the router begins transmitting Hello PDUs to a multi-destination address (multicast). These PDUs contain essential information such as the router'sSystem ID, its configuredArea Addresses, and itsLevel capability(Level 1, Level 2, or both). For two routers to become neighbors, they must exchange these Hello PDUs and agree on specific parameters, such as the MTU of the link and the hello/hold timers.

Once an adjacency is established, the Hello PDU serves as a "keepalive" mechanism. If a router stops receiving Hello PDUs from a neighbor for a duration exceeding theHolding Time, it assumes the neighbor is down and flushes the associated Link-State PDUs (LSPs) from its database.

To clarify the other options:

Link-State PDU (Option A):These are used to distribute actual topology and reachability information, not to form adjacencies.

Partial Sequence Number PDU (Option C):PSNPs are used on point-to-point links to acknowledge the receipt of LSPs or to request missing LSPs.

Update PDU (Option D):This is not a standard IS-IS term; in IS-IS, updates are handled via the flooding of LSPs.

BGP attributes are categorized into four distinct types based on how they are handled by a BGP speaker:Well-known mandatory,Well-known discretionary,Optional transitive, andOptional non-transitive. Understanding these categories is essential for traffic engineering and ensuring consistent policy across an Autonomous System.

According to Juniper Networks technical documentation, theCommunityattribute is classified as anoptional transitiveattribute. The term "optional" implies that a BGP implementation is not required to support or recognize the attribute. However, because it is "transitive," if a Juniper router receives an update containing a community tag that it does not recognize or has no specific policy for, it must accept the attribute and pass it along to other BGP peers unchanged. This ensures that community-based policies can be signaled across intermediate ASes that may not be configured to act upon those specific tags.

In contrast:

Origin (Option A)andAS Path (Option B)arewell-known mandatoryattributes. Every BGP update must include these, and every BGP-compliant router must recognize them.

MED (Option D)(Multi-Exit Discriminator) is anoptional non-transitiveattribute. If a router receives a MED and advertises that route to an EBGP peer, the MED is typically stripped away (unless specific configurations like path-selection cisco-non-deterministic are used), as it is intended only to influence the immediate neighboring AS.

The Community attribute (defined in RFC 1997) is a powerful tool in Junos OS, often used for tagging routes to trigger specific routing policies, such as setting local preference or identifying the geographic origin of a prefix. By being transitive, it allows for sophisticated administrative control across complex multi-provider environments.

Multiple Spanning Tree Protocol (MSTP), as defined in IEEE 802.1s and implemented in Juniper Networks Junos OS, allows for the grouping of VLANs into specific spanning tree instances. This provides significant scalability and load-balancing advantages over traditional STP or RSTP. To achieve this, switches must be grouped into logical "Regions."

According to Juniper documentation, for two or more switches to be considered part of the sameMSTP Region, they must possess an identicalMSTP Configuration Identifier. This identifier consists of three specific attributes that must match exactly across all participating switches:

MSTI Name (Region Name):A descriptive string (up to 32 characters) that identifies the region.

MSTI Revision Level:A numerical value (0–65535) used to track configuration changes.

VLAN-to-Instance Mapping:The specific table that defines which VLAN IDs are associated with which Multiple Spanning Tree Instances (MSTIs).

If even one of these parameters—such as theRegion name(Option A)—differs, the switches will treat each other as being in separate regions. When switches are in different regions, they interact using theCommon Spanning Tree (CST), effectively seeing the other region as a single "virtual bridge," which limits the granularity of traffic engineering.

TheExtended system ID(Option B) is a component of the Bridge ID used to carry VLAN information in PVST+ but is not a region-matching requirement.Root bridge priority(Option C) andRoot bridge ID(Option D) are variables used during the STP election process to determine the topology's root, but they do not define the boundaries of an MSTP region itself.

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.

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 a Juniper Networks environment, deployingRSVP-signaled LSPsrequires a functionalTraffic Engineering Database (TED). This database is populated by the Interior Gateway Protocol (IGP) using specific extensions that carry link-state information beyond simple reachability, such as available bandwidth, administrative groups (link coloring), and Maximum Reservable Bandwidth.

The behavior of these extensions differs betweenOSPFandIS-ISin Junos OS:

OSPF (Option C):By default, OSPF is a "pure" routing protocol. To support RSVP-TE, it must carry Opaque LSAs (Type 10). According to Juniper documentation, you mustexplicitly configuretraffic engineering within the OSPF protocol hierarchy using the set protocols ospf traffic-engineering command. Without this command, OSPF will not flood the TE information required by the Constrained Shortest Path First (CSPF) algorithm, and LSPs will fail to establish.

IS-IS (Option D):IS-IS was designed to be extensible through the use of TLVs (Type, Length, Value). In Junos OS,IS-IS traffic engineering extensions are enabled by defaultonce the protocol is active. As soon as you enable IS-IS on an interface, it begins to advertise the wide metrics and TE TLVs (like TLV 22 and 135) necessary for building the TED.

This distinction is a common design consideration for network architects. While IS-IS simplifies the rollout of MPLS by having TE enabled "out of the box," OSPF requires that extra configuration step to transition from a standard IGP to a TE-aware protocol.

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.

On multi-access network segments, such asEthernet, OSPF could potentially face a scalability issue. If every router on a segment formed a full adjacency with every other router, the number of adjacencies would follow the formula $n(n-1)/2$. In a segment with 10 routers, this would result in 45 adjacencies, each generating redundant flooding of Link-State Advertisements (LSAs) and excessive Hello traffic.

To solve this, OSPF elects aDesignated Router (DR)and aBackup Designated Router (BDR). According to Juniper Networks documentation, the primary purpose of the DR is to act as a central point of contact for the segment, therebyreducing OSPF traffic (Option C).

Instead of every router syncing with every other router, they all form aFull adjacencyonly with the DR and BDR. When a router (a DR-Other) has an update, it sends it to the multicast address224.0.0.6(All DR Routers). The DR then acknowledges the update and floods it to all other routers on the segment using the multicast address224.0.0.5(All OSPF Routers). This "hub-and-spoke" signaling model within the local segment significantly minimizes the bandwidth consumed by protocol overhead and reduces the CPU load on the participating routers.

It is important to note that the DR's scope is limited to the local segment; it does not "assign IDs" (Option A) nor does it flood routes to the "entire domain" (Option D), as that is the responsibility of individual routers within their respective areas.

Graceful Restart (GR)is a high-availability mechanism designed to minimize the impact of a routing protocol process (rpd) restart or a Routing Engine (RE) switchover. It allows a router to continue forwarding traffic while the control plane is recovering, provided that the data plane (Packet Forwarding Engine) remains intact.

According to Juniper Networks documentation, Graceful Restart operates in two distinct roles:

Restarting Mode:This is the role of the router that is actually undergoing the restart. In Junos OS, this mode isnot enabled by default (Option A). An administrator must explicitly configure graceful-restart under the [edit routing-options] hierarchy to allow the router to signal its neighbors that it is attempting a graceful recovery.

Helper Mode:This is the role of the neighboring routers. When a neighbor sees a router restart, if it is in "helper mode," it will continue to forward traffic toward the restarting router and will not flush the associated routes from its forwarding table for a specified period. In Junos,helper mode is enabled by default (Option B)for most protocols (OSPF, BGP, IS-IS). This means that even if you haven't configured GR on your own router, it will automatically assist its neighbors if they perform a graceful restart.

Why other options are incorrect:

Option C:WhileGRES (Graceful Routing Engine Switchover)is often usedwithGraceful Restart to handle hardware-level RE failures, they are independent features. GR can function during a simple software process restart without dual REs or GRES.

Option D:Nonstop Bridging (NSB)is a separate high-availability feature for Layer 2 protocols (like STP). While it shares a similar goal, Graceful Restart is specifically a Layer 3 protocol mechanism (Layer 2 does not use "helper" routers in the same way).

In Junos OS, astatic routeis a manually configured entry in the routing table. Unlike dynamic routes, which have built-in timers and aging mechanisms, static routes are generally "permanent" as long as their conditions for validity are met.

1. Manual Removal (Option A):

Since static routes are explicitly defined by the administrator, the most direct way to remove one is through a configuration change. Using the delete routing-options static route command followed by a commit will immediately remove the route from the Routing Information Base (RIB).

2. Next-Hop Reachability (Option B):

For a static route to be "active" and installed in the forwarding table, itsnext-hop must be reachable. If a static route points to a specific physical interface or an IP address on a local segment, and thatoutbound interface becomes unavailable(e.g., the link goes "Down"), the Junos kernel detects that the next-hop is no longer viable. Consequently, the route is marked as "hidden" or "inactive" and is removed from the active forwarding table to prevent traffic from being black-holed.

Why other options are incorrect:

Aging (Option C):Static routes do not have an expiration timer based on traffic. Even if no packet is sent for years, the route remains as long as the interface is up.

Remote Reachability (Option D):Standard static routes only track the status of the local interface or the immediate next-hop. They do not possess "end-to-end" visibility. If a host two hops away fails, the local router has no way of knowing this via the static route itself. To achieve this level of tracking, features likeRPM (Real-time Performance Monitoring)orBFD (Bidirectional Forwarding Detection)must be linked to the static route.

When implementingMultiprotocol BGP (MP-BGP)for IPv6, several architectural constants remain consistent with the original BGP design, while others have evolved to accommodate larger network scales.

Router ID (Option C):

A critical point in Juniper's Service Provider documentation is that theBGP Router IDremains a32-bit value, even when the protocol is carrying 128-bit IPv6 prefixes. The Router ID is typically represented in dotted-quad notation (e.g., 192.168.1.1). In an IPv6-only environment, a Juniper router cannot automatically derive this ID from an interface address, so it must be manually defined under [edit routing-options]. This 32-bit ID is essential for BGP tie-breaking and loop prevention within the AS.

Autonomous System Number (Option D):

TheAutonomous System Number (ASN)was originally a 16-bit value (0 to 65535). However, to address the exhaustion of available ASNs, the standard was extended to32-bit ASNs(documented in RFC 6793). In Junos OS, you can configure BGP using either the older 16-bit format or the newer 32-bit format (often represented in "asplain" or "asdot" notation). While the question mentions a 64-bit value, there is currently no standard for a 64-bit ASN in BGP; the transition from 16-bit to 32-bit satisfies current global scalability needs. Therefore, Option D is the most accurate within the context of current networking standards, as it acknowledges the coexistence of different ASN lengths.

TheBorder Gateway Protocol (BGP)operates based on a strict set of advertisement rules designed to prevent routing loops while ensuring global reachability. These rules differ significantly depending on whether the relationship isExternal BGP (EBGP)orInternal BGP (IBGP).

1. EBGP Advertisement (Option A):In a standard EBGP scenario, a router acts as an exit/entry point for an Autonomous System. When an EBGP speaker receives a valid route from any peer (Internal or External), it will, by default, advertise that route to all of its other EBGP peers. This is the primary mechanism that allows prefixes to propagate across the global internet from one AS to another.

2. IBGP Split Horizon (Option D):

The most critical rule within an AS is theIBGP Split Horizonrule. To prevent loops within an AS, BGP dictates that a route learned from an IBGP peermust notbe advertised to any other IBGP peer. This is why BGP requires a "full mesh" of IBGP sessions or the use ofRoute Reflectorsto ensure all internal routers learn all routes. Without this rule, a route could circulate infinitely within the AS because IBGP does not update the AS_PATH attribute.

3. EBGP to IBGP Propagation (Option B):

When a router learns a route from an EBGP peer, it is permitted to advertise that route to all of its IBGP peers. This ensures that everyone inside the network knows how to reach external destinations. However, it is important to remember that in Junos OS, theBGP Next Hopis not modified by default when sending routes to IBGP peers, often requiring a "next-hop-self" policy to ensure internal reachability.

Options C and E are incorrect because they directly contradict these fundamental BGP loop-prevention and propagation mechanisms.