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 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.

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.

In theIS-IS (Intermediate System to Intermediate System)protocol as implemented in Junos OS, routers can operate at two hierarchical levels:Level 1 (L1)for intra-area routing andLevel 2 (L2)for inter-area backbone routing. By default, a Juniper router and its interfaces are configured to act asLevel 1/2, meaning they will attempt to form adjacencies at both levels simultaneously.

According to Juniper Networks technical documentation, the show isis interface command provides a granular view of how the protocol is interacting with specific local links. In the provided exhibit, we must examine theL (Level)column and theDR (Designated Router)status columns to understand R1's operational state.

Level Configuration:Under the L column for both the physical interface ge-0/0/0.0 and the loopback lo0.0, the value is strictly2. This indicates that these interfaces have been explicitly configured to operate only at Level 2.

Adjacency Capabilities:For the interface ge-0/0/0.0, the Level 1 DR field is marked asDisabled. This confirms that R1 is not participating in Level 1 operations on this link; it will not transmit Level 1 Hello PDUs, nor will it listen for them. Consequently, R1 is incapable of forming a Level 1 adjacency with R2 on this segment.

Metric Implications:The exhibit shows an L1/L2 Metric of100/100. In Junos, "narrow" metrics (the default) are limited to a maximum value of 63 per interface. A metric of 100 indicates thatwide metrics(wide-metrics-only) have been enabled. Therefore, option A is incorrect because the routerisusing wide metrics.

Since the prompt states the adjacency is "up," and the interface is restricted to Level 2, we can conclude thatR1 only forms a Level 2 adjacency with R2 (Option B). Even though an L1 metric of 100 is displayed in the table as a configured value, it is not actually "advertised" in a Link-State PDU because the Level 1 protocol is disabled on that interface.

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.

TheBGP path selection algorithmis a deterministic process used by Juniper routers to select the single "best" path from the BGP table to be placed into the routing table (inet.0). This algorithm follows a specific, hierarchical set of rules. According to Juniper Networks technical documentation, the router evaluates attributes in a fixed order, and once a tie is broken at a specific step, the remaining steps are ignored.

The order of the primary BGP attributes in Junos OS is as follows:

Highest Local Preference:This is the first attribute evaluated after the basic check for a reachable next hop. Local preference is used within an Autonomous System (AS) to prioritize one exit point over another.

Shortest AS_PATH:If the local preference is equal, the router then evaluates the length of the AS_PATH attribute.

Lowest Origin Code:(IGP < EGP < Incomplete).

Lowest Multi-Exit Discriminator (MED).

In this specific scenario, the router compares a path with ahigher local preferenceagainst a path with ashorter AS path. Because theLocal Preferencecheck occurs at Step 1 and theAS_PATHcheck occurs later at Step 2, the router will select the path with the higher local preference immediately. The length of the AS path becomes irrelevant in this comparison because the tie was already broken by the local preference value. This allows network administrators to override the default "shortest path" logic of BGP to prefer specific providers or links based on business requirements.

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.