Juniper Service Provider Routing and Switching Professional (JNCIP-SP) (JN0-664)

✅ Option A (Correct)

Reasoning: On IS-IS broadcast networks, such as Ethernet, the Designated Intermediate System (DIS) is responsible for ensuring link-state database (LSDB) synchronization. To achieve this, the DIS periodically multicasts Complete Sequence Number PDUs (CSNPs), typically every 10 seconds, to all other routers on the segment.

✅ Option D (Correct)

Reasoning: A CSNP serves as a summary or table of contents for the LSDB. It contains a list of all LSPs held by the sending router, but only includes the LSP headers (LSP ID, sequence number, checksum, lifetime), not the full topological details. This allows for efficient comparison and synchronization of databases between neighbors.

❌ Why the other choices are incorrect:

• Option B is incorrect: Partial Sequence Number PDUs (PSNPs) are event-driven, not sent periodically. A router sends a PSNP to either request a specific LSP it is missing or to acknowledge the receipt of an LSP on a point-to-point link.
• Option C is incorrect: While a PSNP contains LSP descriptions (headers), it only includes a partial list related to the specific LSPs being requested or acknowledged. The CSNP is the PDU that contains a complete list of LSP descriptions.

✅ Option A (Correct) Reasoning: Junos schedulers use a credit-based system, often a variant of Deficit Weighted Round-Robin (DWRR). When a queue transmits more data than its configured transmit-rate allows during its turn, its bandwidth credit becomes negative. This "debt" is tracked to ensure fairness over time, especially during periods of congestion when it must be "repaid" before the queue can transmit again.

✅ Option D (Correct) Reasoning: By default, Junos schedulers are work-conserving. If one or more queues are not using their full allocated bandwidth, that unused (spare) bandwidth is not wasted. It is made available to other active queues that have traffic to send, allowing them to temporarily exceed their guaranteed rate and maximizing overall link utilization.

❌ Why the other choices are incorrect:

• Option B is incorrect: Traffic is not discarded simply for exceeding the guaranteed transmit-rate. Discards occur when buffers are full (governed by drop profiles) or when a hard limit, such as a shaping-rate, is exceeded. Using spare bandwidth is the primary action, not discarding.
• Option C is incorrect: A queue accumulates positive bandwidth credit when it uses less than its allocated bandwidth, not when it exceeds it. This saved credit can be used for bursting later.

✅ Option B (Correct) Reasoning: BGP Flow Specification (FlowSpec), defined in RFC 8955, is Juniper's recommended method for mitigating amplification and other DDoS attacks. It allows a network operator to distribute traffic filtering rules as BGP Network Layer Reachability Information (NLRI). These rules can match on various packet header fields (e.g., source/destination IP, protocol, ports). When an amplification attack is detected, a FlowSpec rule can be injected to precisely match and drop or rate-limit the malicious traffic at the network edge. This is highly scalable and surgical, protecting the target customer and network infrastructure without causing a complete outage for the victim, which is a major advantage over coarser methods.

❌ Why the other choices are incorrect:

• Option A is incorrect: ASN prepending is a BGP traffic engineering technique used to influence inbound path selection by making an AS-PATH appear longer. It is not a security mechanism for mitigating DDoS attacks.
• Option C is incorrect: Destination-based Remote Triggered Black Hole (RTBH) is a DDoS mitigation technique, but it is a blunt instrument. It drops all traffic destined for the victim's IP address, including legitimate traffic, effectively taking the target offline. FlowSpec is preferred for its granularity.
• Option D is incorrect: Unicast Reverse Path Forwarding (uRPF) is a feature used to mitigate source IP address spoofing. It is most effective at the network edge for traffic originating from directly connected customers, but it is not the primary tool for stopping a large-scale amplification attack coming from multiple sources across the Internet.

✅ Option C (Correct) Reasoning: The exhibit shows the IS-IS database on R1. For router R3, R1 sees both IP prefix (TLV 128 for narrow metrics) and IP extended prefix (TLV 135 for wide metrics). For router R2, R1 only sees IP extended prefix. The presence of the narrow metric TLV from R3 indicates it does not have wide-metrics-only enabled. This metric style mismatch between R3 and other routers (like R2) causes inconsistent path calculations in the SPF algorithm, preventing Equal-Cost Multi-Path (ECMP) load balancing.

❌ Why the other choices are incorrect:

• Option A is incorrect: The exhibit shows R1's database, not R2's. We cannot determine what routes R2 is missing from this output.
• Option B is incorrect: The command output on R1 clearly shows that R1 has learned IS-IS routes from R2 (show isis database R2 ...).
• Option D is incorrect: R1 is correctly interpreting the IP extended prefix information from R2, including metrics greater than 63 (e.g., 1000). This confirms that R1 has wide-metrics enabled.

The analysis of the EVPN MAC Mobility feature confirms that the ground truth answers are correct.

✅ Option C (Correct) Reasoning: In EVPN, individual MAC addresses and their associated IP addresses are advertised using EVPN Type 2 (MAC/IP Advertisement) routes. The MAC Mobility extended community, which contains the sequence number as shown in the exhibit, is an attribute carried within these Type 2 routes.

✅ Option D (Correct) Reasoning: The primary function of the sequence number is to track the freshness of a MAC address advertisement. When a MAC address moves between PEs, the sequence number is incremented. A receiving PE will always prefer the route with the highest sequence number, as this indicates it is the most recent location update for that MAC address.

❌ Why the other choices are incorrect:

• Option A is incorrect: The sequence number does not identify MAC addresses to be discarded. A MAC address is removed from the network via a BGP UPDATE message containing a withdrawn route, not by a specific sequence number.
• Option B is incorrect: While the sequence number is part of the process of resolving MAC location conflicts, its direct mechanism is to determine which advertisement is the newest. Therefore, identifying the latest advertisement (Option D) is a more precise description of its function.

✅ Option A (Correct)

Reasoning: The Bootstrap Router (BSR) protocol is a dynamic mechanism for distributing Rendezvous Point (RP) information in a PIM sparse-mode domain. Candidate RPs (C-RPs) advertise their availability to the BSR. The elected BSR then compiles this information into an RP-Set and includes it in bootstrap messages, effectively distributing the RP-to-group mappings dynamically throughout the domain.

✅ Option D (Correct)

Reasoning: Bootstrap messages are destined for the ALL-PIM-ROUTERS multicast address (224.0.0.13). They are forwarded on a hop-by-hop basis from the BSR to all PIM neighbors, ensuring that every PIM-enabled router within the domain receives the same RP-Set information. This flooding mechanism is crucial for consistent RP knowledge across the network.

❌ Why the other choices are incorrect:

• Option B is incorrect: While bootstrap messages contain RP information, their purpose is to distribute the entire RP-Set, which can contain multiple RPs mapped to different multicast group ranges. Stating it notifies of 'the' PIM RP is an oversimplification and less accurate than option A, which describes the dynamic distribution process.
• Option C is incorrect: Bootstrap messages are proactively flooded throughout the PIM domain. Routers do not need to explicitly request them. The forwarding is based on the well-known 224.0.0.13 multicast address, not on-demand requests.

BGP Path Selection Process Audit

The Junos OS BGP path selection algorithm follows a specific, ordered set of steps to determine the best path to a destination. The question states that the paths are tied on Local Preference, AS-path length, and Origin code.

Let's review the subsequent steps in the algorithm:

1. Lowest MED: By default, Junos OS only compares MED values for paths originating from the same neighboring AS. Since the paths are from two different ASNs (64500 and 64501), this step is skipped.
2. EBGP over IBGP: Both paths are from EBGP peers, resulting in a tie.
3. Lowest IGP metric: The router compares the IGP cost to reach the BGP next-hop for each path. This is a critical step for hot-potato routing.
4. Oldest Path: For EBGP paths, the default tie-breaker is to prefer the path that was received first (the oldest path).
5. Lowest Router ID: If the paths are still tied (which can happen if the oldest-path behavior is modified or if the paths were learned simultaneously), the router prefers the path from the peer with the lowest router ID.

Given the provided options, and assuming a common scenario where symmetric peering links result in an identical IGP metric to the next-hops, the selection process would proceed past the IGP metric step. The next major attribute in the algorithm used as a tie-breaker is the peer's router ID.

✅ Option A (router ID) (Correct)
Reasoning: After the initial attributes (Local Pref, AS Path, Origin) are found to be identical, and assuming the IGP metric to both next-hops is also the same, the path selection process falls back to the final tie-breakers. The BGP peer with the lowest router ID will be preferred.

❌ Why the other choices are incorrect:

• Option B (MED value) is incorrect: The MED attribute is not compared by default when the paths originate from different autonomous systems, as is the case here (AS 64500 vs. AS 64501).
• Option C (peer IP address) is incorrect: The peer IP address is used as one of the final tie-breakers in the path selection process, but only after the peer router ID has been evaluated and also resulted in a tie.
• Option D (IGP metric) is incorrect: While the IGP metric is evaluated before the router ID, the question implies a continuing tie. If the IGP metrics were different, that would be the deciding factor. However, in a scenario where they are identical, the process continues to the next tie-breakers, which include the router ID.

The configuration applies a policy named block-igmp to IGMP joins on interface ge-0/0/0.0. This policy contains a term that matches on two conditions: a multicast group address of 224.7.7.7 and a source address of 192.168.100.10. If an IGMPv3 join report meets both of these criteria, the then reject action is taken, and the join request is dropped.

✅ Option A (Correct)
Reasoning: This statement accurately describes the explicit logic of the block-igmp policy. The from clause requires both the route-filter (group) and source-address-filter (source) to match before the reject action is triggered.

❌ Why the other choices are incorrect:

• Option B is incorrect: The rejection is not for all joins to group 224.7.7.7. It is conditional upon the join also coming from the source 192.168.100.10. Joins for this group from other sources would be accepted (assuming they don't exceed the group-limit).
• Option C is incorrect: The group-policy is evaluated before the group-limit. If the policy rejects the join, it is dropped immediately and the group count is not a factor for that specific join.
• Option D is incorrect: This statement is false because the policy explicitly rejects a specific source-group pair. Additionally, the group-limit 25 will cause joins for new groups to be rejected once 25 groups are already active on the interface.

The provided configuration analysis is correct based on the exhibit.

✅ Option A (Correct) Reasoning: The as-override statement is configured for the BGP neighbor. This feature is specifically used in Layer 3 VPNs when a customer uses the same AS number at multiple sites. It allows the provider edge (PE) router to replace the customer's AS number with its own, preventing the remote customer edge (CE) router from rejecting the routes due to BGP loop prevention.

✅ Option C (Correct) Reasoning: The configuration explicitly uses instance-type vrf. A VRF (Virtual Routing and Forwarding) instance creates a separate and isolated IP routing table for a specific customer or VPN. This is the defining characteristic of a Layer 3 VPN in Junos OS.

❌ Why the other choices are incorrect:

• Option B is incorrect: The presence of as-override directly contradicts this statement. This feature is unnecessary if the customer sites use different AS numbers.
• Option D is incorrect: The instance-type is vrf, which specifies a Layer 3 VPN. A Layer 2 VPN would be configured using a different instance type, such as l2vpn or vpls.

✅ Option A (Correct) Reasoning: The output of show ospf neighbor detail explicitly states Adjacency check: Hello timer mismatch. For an OSPF adjacency to form, the Hello interval must be identical between neighbors on the same network segment. This mismatch is a direct cause of the adjacency being stuck in the Init state.

✅ Option C (Correct) Reasoning: The exhibit also clearly shows Dead timer mismatch as an adjacency check failure. It further details the mismatched values: Neighbor Dead time: 10 and Local Dead time: 40. The OSPF Dead interval, which is typically four times the Hello interval, must also match between neighbors.

❌ Why the other choices are incorrect:

• Option B is incorrect: OSPF uses a Dead Interval, not a hold timer, to determine if a neighbor is unreachable. The term hold timer is primarily associated with protocols like BGP.
• Option D is incorrect: The poll interval is specific to OSPF operation on Non-Broadcast Multi-Access (NBMA) networks to poll manually configured, inactive neighbors. It is not a standard parameter that must match on broadcast or point-to-point links and is not identified as an issue in the exhibit.