Score Higher on Verified JN0-364 | Service Provider Routing and Switching - Specialist (JNCIS-SP) Exam Questions with Answers

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, 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 theBorder Gateway Protocol (BGP)finite state machine (FSM), theEstablishedstate is the final and functional stage of a BGP peering session. According to Juniper Networks technical documentation, once a session reaches this state, the two peers have successfully exchanged Open messages and agreed upon session parameters (such as AS numbers, hold timers, and BGP identifiers). Only after the session is "Established" can the routers begin the actual exchange of network layer reachability information (NLRI).

The most frequent message type exchanged in the Established state is theUPDATEmessage. These messages are the heart of BGP operations; they are used to advertise new feasible routes to a peer or to withdraw routes that are no longer reachable. An UPDATE message contains path attributes (like AS-Path, Next-Hop, and Local Preference) and the associated prefixes. In a stable network, UPDATE messages are only sent when there is a change in the topology, adhering to BGP’s incremental update philosophy.

The second message type that can be exchanged in this state is theNOTIFICATIONmessage. While ideally, a session stays established, any detected error—such as a hold timer expiration, a malformed update, or a manual "clear" command—will trigger the transmission of a NOTIFICATION message. This message informs the peer of the specific error code and immediately causes the BGP session to transition back to the Idle state, tearing down the TCP connection.

It is important to note thatOPENmessages (Option A) are only used during the session initialization phase to transition from the OpenConfirm state to Established.REQUEST(Option B) is not a valid BGP message type defined in the standard (RFC 4271); the closest equivalent in functionality would be a Route-Refresh message, which is a separate extension. Therefore, in the context of standard BGP operations within the Established state, Updates and Notifications are the correct answers.