JN0-664 Practice Exam Questions with Answers Service Provider Professional (JNCIP-SP) Certification

In a PIM Sparse Mode (PIM-SM) domain, Anycast RP is used for load balancing and redundancy by configuring multiple RPs with the same IP address. However, for active multicast sources to be shared between RPs, an additional mechanism is needed since PIM-SM does not automatically synchronize sources between RPs.

Evaluating the Answer Choices

Option A: "Configure MSDP on each RP router." ?

Multicast Source Discovery Protocol (MSDP) is required in an Anycast RP setup to share active source information between RPs.

MSDP allows RPs to exchange source-active (SA) messages, ensuring that multicast receivers in different regions can still receive traffic from sources registered with different RPs.

Juniper Documentation confirms that MSDP is used to synchronize active sources across multiple RPs in an Anycast RP deployment.

? This is a correct answer.

Option B: "Configure anycast PIM with the rp-set statement on each RP router." ?

Anycast PIM allows multiple RPs to share the same IP address, and the rp-set statement is used to define the set of Anycast RPs.

This enables receivers and sources to register with the closest RP.

However, Anycast PIM alone does not share active source information between RPs; MSDP is still needed for that.

The combination of Anycast PIM (rp-set) and MSDP is the correct approach.

? This is a correct answer.

In an EVPN (Ethernet Virtual Private Network) environment, MAC address mobility is a critical feature that allows devices to move across different locations while ensuring the network consistently tracks their MAC addresses. Let’s break down the components in the exhibit and analyze the correct statements.

Understanding MAC Mobility and Sequence Numbers in EVPN

In EVPN, MAC mobility is managed through sequence numbers that are included in Type 2 MAC/IP advertisements.

The sequence number tracks MAC movement events and is used to determine the most recent update when a MAC address appears on different PEs (Provider Edge devices).

When a MAC address moves between locations, the EVPN PEs increment the sequence number and advertise it to resolve conflicts and determine which PE has the most up-to-date information.

Now, Let’s Review the Options:

? C. It helps the local PE to identify the latest advertisement.

Correct:The sequence number plays a key role in resolving MAC address conflicts. If multiple PEs advertise the same MAC address, the PE compares the sequence numbers to determine which update is the latest.

A higher sequence number indicates a more recent MAC update.

? D. It is advertised using a Type 2 message.

Correct:EVPN MAC/IP advertisements use BGP EVPN Type 2 messages to carry MAC addresses, IP addresses (optional), and their associated sequence numbers.

Type 2 advertisements are used to track MAC mobility and IP reachability information in the EVPN.

Why the Other Options Are Incorrect:

? A. It identifies MAC addresses that should be discarded.

Incorrect:The sequence number doesn’t identify MAC addresses that need to be discarded.Instead, it resolves conflicts by determining the most recent MAC address advertisement based on the highest sequence number.

? B. It resolves conflicting MAC address ownership claims.

Partially true, but misleading:While it’s true that sequence numbers are used in conflict resolution, the sequence number itself doesn’t directly resolve ownership claims. It only helps determine which advertisement is more recent. The actual conflict resolution happens through the comparison of the advertisements and sequence numbers.

Final Answer:

? C. It helps the local PE to identify the latest advertisement.

? D. It is advertised using a Type 2 message.

Reference from Juniper Documentation:

Juniper EVPN Configuration Guide:

"In EVPN MAC/IP advertisements, sequence numbers track the mobility of MAC addresses and are used to resolve conflicts when the same MAC address is advertised by multiple PEs. The PE with the higher sequence number has the most recent information."

???? Juniper BGP EVPN Mobility Documentation

TLVs are tuples of (Type, Length, Value) that can be advertised in IS-IS packets. TLVs can carry different kinds of information in the Link State Packets (LSPs). IS-IS supports both narrow and wide metrics for link costs. Narrow metrics use a single octet to encode the link cost, while wide metrics use three octets. Narrow metrics have a maximum value of 63, while wide metrics have a maximum value of 16777215. If there are both narrow and wide metric devices in the topology, IS-IS will advertise two TLVs for each link: one with the narrow metric and one with the wide metric. This allows backward compatibility with older devices that only support narrow metrics12.

Understanding the Exhibit and the Problem Statement

The diagram shows a Layer 3 VPN (L3VPN) setup where multiple customers (Customer A, B, and C) are connected across a service provider network using PE (Provider Edge) routers.

PE1 and PE2 exchange VPN routes for all customers using BGP/MPLS Layer 3 VPN (L3VPN) routing.

The question asks how to verify if PE2 is receiving VPN routes for Customer C.

Evaluating the Answer Choices

? B. show route table bgp.l3vpn.0 (Correct Answer)

Why?

This command shows all VPN routes stored in the BGP Layer 3 VPN table (bgp.l3vpn.0).

Since PE routers exchange VPN routes using MP-BGP, this table contains the VPN-IPv4 or VPN-IPv6 routes for all customers.

If PE2 is receiving routes from PE1 for Customer C, they will appear in bgp.l3vpn.0.

? A. show route table inet.0 (Incorrect)

Why?

The inet.0 table contains global unicast routes for the service provider's network.

VPN routes do not appear here because they are stored in VRF-specific tables.

This command won’t help verify VPN route exchange between PE1 and PE2.

? C. show route table customer-c.inet.0 (Incorrect)

Why?

The table customer-c.inet.0 represents the VRF routing table for Customer C on the local PE.

It only shows locally installed routes for Customer C but does not confirm if PE2 is receiving routes from PE1.

This command is useful to check local VPN routing but not BGP route propagation.

? D. show route summary (Incorrect)

Why?

This command only provides a summary of route counts per protocol (BGP, OSPF, etc.).

It does not display specific VPN routes.

It is useful for general troubleshooting but doesn’t confirm VPN route receipt.

Final Answer: ? show route table bgp.l3vpn.0 (Option B)

Official Juniper Documentation Reference

Junos MPLS VPNs Configuration Guide???? Juniper Documentation

"The show route table bgp.l3vpn.0 command displays all VPN-IPv4 routes learned via MP-BGP for Layer 3 VPNs."

For a Layer 3 VPN to function correctly, an MPLS Label Switched Path (LSP) must be established between the Provider Edge (PE) routers. The MPLS LSP is necessary for the transport of VPN traffic across the service provider's backbone network. If the MPLS LSP is not established, the PE routers cannot forward the VPN traffic properly, causing the routes to be hidden in the BGP routing table.

Here’s a breakdown of why the other options are less likely:

A. The routes use overlapping IP addresses.

Overlapping IP addresses might cause issues with route advertisement and selection, but they would not typically cause routes to be hidden in the bgp.l3vpn.0 table.

C. There is a routing loop in the service provider backbone.

While routing loops are problematic, they would not specifically cause the routes to be hidden in the bgp.l3vpn.0 table. Routing loops would more likely result in dropped packets or increased latency.

D. Route targets are not configured.

Incorrect or missing route target configuration would prevent routes from being imported into the correct VRF, but it would not usually result in the routes being hidden. Instead, they would simply not appear in the relevant VRF.

Thus, the absence of an established MPLS LSP is the most plausible cause for the routes being hidden.

A sham link is a logical link between two PE routers that belong to the same OSPF area but are connected through an L3VPN. A sham link makes the PE routers appear as if they are directly connected, and prevents OSPF from preferring an intra-area back door link over the VPN backbone. A sham link creates an OSPF multihop neighborship between the PE routers using TCP port 646. The PEs exchange Type 1 OSPF LSAs instead of Type 3 OSPF LSAs for the L3VPN routes, which allows OSPF to use the correct metric for route selection1.

Let’s dive into this IS-IS routing problem with a Juniper Networks (JNCIP-SP) perspective, analyze the exhibit, and determine why load balancing isn’t working as expected after adding the new router R3. I’ll provide a verified answer and a detailed explanation step by step.

Answer: C. R3 does not have wide-metrics enabled.

Detailed Explanation

1. Understanding the Exhibit

The exhibit shows the output of IS-IS database commands on two routers, R1 and R2, at Level 2 (L2). IS-IS (Intermediate System to Intermediate System) is a link-state routing protocol commonly used in service provider networks. The output provides details about the IS-IS database, including IP prefixes, metrics, and whether the router supports "wide metrics."

R1’s IS-IS Database (Level 2):

IP prefix: 10.100.34.0/24

Internal, Metric: default 63, Up

IP extended prefix: 10.100.34.0/24, metric 63, Up

IP extended prefix: 10.100.13.0/24, metric 63, Up

R2’s IS-IS Database (Level 2):

IP extended prefix: 10.100.12.0/24, metric 1000, Up

IP extended prefix: 10.100.23.0/24, metric 1000, Up

Additionally, the exhibit mentions that R1 and R2 can "find TLVs" and "match prefix," indicating they are processing IS-IS Type-Length-Value (TLV) data correctly. However, R3 has been newly added, and load balancing across this router isn’t working as expected.

2. Key Concepts in IS-IS

To understand the issue, let’s break down some critical IS-IS concepts relevant to this scenario:

IS-IS Levels: IS-IS operates at two levels: Level 1 (L1) for intra-area routing and Level 2 (L2) for inter-area routing. The exhibit shows Level 2, so we’re dealing with backbone routing between areas.

Metrics in IS-IS:

Default Metrics (Narrow Metrics): By default, IS-IS uses narrow metrics, which are limited to a maximum value of 63 per link (and a maximum path metric of 1023). This is encoded in the original IS-IS TLVs (Type 2 for LSPs).

Wide Metrics: Wide metrics (introduced in RFC 3784) allow for larger metric values (up to 16,777,215) and are encoded in extended TLVs (Type 22 for LSPs, Type 135 for IP reachability). Wide metrics are necessary for modern networks where higher metric values are needed for better path selection or when integrating with other protocols like OSPF.

Load Balancing in IS-IS: IS-IS supports equal-cost multi-path (ECMP) routing, meaning if multiple paths to a destination have the same total metric, traffic can be load-balanced across those paths. For load balancing to work, all routers in the path must agree on the metrics and the paths must be equal-cost.

TLVs and Extended Prefixes: The exhibit shows "IP extended prefix" entries, which are carried in TLV 135 (for IPv4) when wide metrics are enabled. "IP prefix" (without "extended") refers to the older TLV 128, which uses narrow metrics.

3. Analyzing the Metrics in the Exhibit

R1’s Output:

R1 advertises 10.100.34.0/24 and 10.100.13.0/24 with a metric of 63.

The presence of both "IP prefix" and "IP extended prefix" for the same prefix (10.100.34.0/24) suggests R1 is in transition mode. Transition mode means R1 supports both narrow and wide metrics to maintain compatibility with routers that may not support wide metrics.

R2’s Output:

R2 advertises 10.100.12.0/24 and 10.100.23.0/24 with a metric of 1000.

These are listed as "IP extended prefix," meaning R2 is using wide metrics (since a metric of 1000 exceeds the narrow metric limit of 63 per link).

4. Identifying the Problem

The issue is that load balancing isn’t working over the new router R3. Let’s evaluate why:

Metric Discrepancy: R1 is using a metric of 63 (which fits within narrow metrics), while R2 is using a metric of 1000 (which requires wide metrics). This suggests that R1 and R2 are operating with different metric styles:

R1 is likely in transition mode (supporting both narrow and wide metrics).

R2 is using wide metrics exclusively (since its metric of 1000 can only be advertised using wide metrics).

R3’s Role: Since R3 is newly added and load balancing isn’t working, we need to consider R3’s configuration. The exhibit doesn’t show R3’s IS-IS database, but the options suggest a problem with wide metrics on either R1 or R3.

Wide Metrics Requirement: For load balancing to work, all routers in the IS-IS domain must consistently use the same metric style (either all narrow or all wide). If R3 doesn’t have wide metrics enabled, it can only process narrow metrics (max 63 per link). This means:

R3 would ignore R2’s advertisements (metric 1000) because they use wide metrics, which R3 can’t process.

R3 would only process R1’s narrow metric advertisements (metric 63), leading to incomplete routing information and preventing load balancing.

5. Evaluating the Options

Let’s go through each option to confirm the correct answer:

A. R2 is missing internal routes for R1:

R2’s database shows prefixes like 10.100.12.0/24 and 10.100.23.0/24, but it doesn’t show R1’s prefixes. However, this is expected because the exhibit only shows a subset of the database. The problem is about load balancing, not missing routes entirely. R2 likely has routes from R1 but may not be able to use them for load balancing if metric styles don’t match. This option is incorrect.

B. R1 is missing internal routes for R2:

R1’s database shows its own prefixes (10.100.34.0/24, 10.100.13.0/24) but not R2’s prefixes. Again, this is likely because the exhibit is limited. R1 should have R2’s routes in its database (since they’re both in Level 2), but the issue is load balancing, not missing routes. This option is incorrect.

C. R3 does not have wide-metrics enabled:

If R3 doesn’t have wide metrics enabled, it can only process narrow metrics (max 63). R2’s prefixes with a metric of 1000 (using wide metrics) would be ignored by R3. This would result in R3 having an incomplete view of the network, preventing load balancing across paths that involve R2’s prefixes. This aligns with the problem described and is the most likely cause. This option is correct.

D. R1 does not have wide-metrics enabled:

R1’s output shows "IP extended prefix" entries, which are only advertised if wide metrics are enabled. R1 is likely in transition mode (advertising both narrow and wide metrics), so it does support wide metrics. This option is incorrect.

6. Why Load Balancing Fails

For load balancing to work in IS-IS, the total path metrics to a destination must be equal across multiple paths. If R3 doesn’t support wide metrics:

R3 ignores R2’s prefixes (metric 1000) because they use wide metrics.

R3 only sees R1’s prefixes (metric 63) and builds its routing table based on narrow metrics.

As a result, R3 doesn’t see equal-cost paths involving R2’s prefixes, and load balancing fails.

If R3 had wide metrics enabled, it would process both R1’s and R2’s prefixes, calculate the total path metrics, and potentially find equal-cost paths for load balancing.

7. Solution

To fix the issue, R3 needs to have wide metrics enabled. In Junos, this can be done with the following configuration:

set protocols isis level 2 wide-metrics-only

This ensures R3 uses wide metrics exclusively, matching R2’s configuration and allowing it to process R2’s prefixes with metrics like 1000. R1 is already in transition mode, so it will remain compatible.

In the exhibit, we see an internal BGP (iBGP) setup within AS 65512, and an external BGP (eBGP) connection between R3 and ISP-A (AS 65511). The questions focus on the behavior of BGP routes advertised from R3 to R1 within the same AS.

1. **BGP Next-Hop Attribute (Option A and D)**:

- In iBGP, the next-hop attribute is **not** changed when a route is advertised to another iBGP peer. This means that when R3 advertises a route to R1, it retains the original next-hop value as learned from the eBGP peer (ISP-A).

- Therefore, Option D is correct: "By default, the next-hop value for these routes is not changed by R3 before being sent to R1."

2. **BGP Attributes (Option B and C)**:

- BGP attributes such as local preference, AS-path, and others are crucial for BGP route selection. The local preference attribute is used within an AS to indicate the preferred path for outbound traffic.

- When R3 advertises BGP routes to R1, it includes the local preference value assigned to those routes. This value is not removed and is propagated within the iBGP mesh.

- Therefore, Option C is correct: "By default, the BGP local-preference value that is assigned on R3 is advertised to R1."

**References**:

- Juniper Networks documentation on BGP behavior provides detailed insights into the propagation of BGP attributes within iBGP and eBGP contexts. Specifically, the Junos OS documentation covers the default behavior of next-hop and local preference attributes in BGP configurations.

- Junos OS BGP Configuration Guide: [Junos OS BGP Configuration Guide](https://www.juniper.net/documentation/en_US/junos/topics/concept/bgp-overview.html)

- For a deeper understanding of BGP attributes and their default behaviors, the "Juniper Networks Day One: Exploring BGP" book is an excellent resource.

When configuring Class of Service (CoS) properties for output queues, we need to manage packet drops during periods of congestion. Juniper's CoS framework provides several tools to manage congestion, including drop profiles, buffer sizes, and scheduling mechanisms. Let’s break down each option and identify the correct one.

Evaluating the Answer Choices

? D. drop profile (Correct Answer)

Why?

A drop profile defines when packets should be dropped based on the queue fill level.

Random Early Detection (RED) or Tail Drop can be used to manage congestion by discarding lower-priority packets first.

Drop profiles are configured under the scheduler to determine how aggressive packet dropping should be during congestion.

Example Juniper Configuration:

schedulers {

best-effort {

drop-profile low-drop;

}

}

drop-profiles {

low-drop {

fill-level 80 drop-probability 50;

}

}

fill-level 80 ? When the queue reaches 80% full, packet drops begin.

drop-probability 50 ? There is a 50% chance of dropping packets once the threshold is reached.

???? Official Juniper Documentation Reference:

???? Junos Class of Service Configuration Guide

"A drop profile determines how packets are discarded based on the queue fill level, allowing control over congestion behavior."

Why the Other Options Are Incorrect?

? A. buffer size (Incorrect)

Why?

The buffer size determines how many packets the queue can store before congestion occurs.

A larger buffer can delay drops, but it does not actively control dropping behavior.

It affects latency rather than controlling packet drops.

? B. priority (Incorrect)

Why?

Priority controls which queue gets serviced first, not how drops are handled.

Higher priority queues are serviced before lower-priority queues, but this does not prevent congestion-related drops.

? C. shaping rate (Incorrect)

Why?

Shaping limits the maximum transmission rate of the queue.

While shaping helps reduce congestion, it does not control which packets get dropped during congestion.

Shaping is useful for traffic smoothing, but it does not actively drop packets based on queue fill levels.

Final Answer: ? D. drop profile

Controls packet drops based on queue congestion.

Defines RED (Random Early Detection) or Tail Drop mechanisms.

Directly influences drop probability as the queue fills up.

???? Official Juniper Reference:

"Drop profiles are used to manage congestion by determining when and how aggressively packets are dropped based on queue fill level."

EVPN is a solution that provides Ethernet multipoint services over MPLS networks. EVPN uses BGP to distribute endpoint provisioning information and set up pseudowires between PE devices. EVPN uses different route types to convey different information in the control plane. The following are the main EVPN route types:

Type 1 - Ethernet Auto-Discovery Route: This route type is used for network-wide messaging and discovery of other PE devices that are part of the same EVPN instance. It also carries information about the redundancy mode and load balancing algorithm of the PE devices.

Type 2 - MAC/IP Advertisement Route: This route type is used for MAC and IP address learning and advertisement between PE devices. It also carries information about the Ethernet segment identifier (ESI) and the label for forwarding traffic to the MAC or IP address.

Type 3 - Inclusive Multicast Ethernet Tag Route: This route type is used for broadcast, unknown unicast, and multicast (BUM) traffic forwarding. It also carries information about the multicast group and the label for forwarding BUM traffic.

Type 4 - Ethernet Segment Route: This route type is used for multihoming scenarios, where a CE device is connected to more than one PE device. It also carries information about the ESI and the designated forwarder (DF) election process.

In an L2VPN scenario, the provider network connects two customer edge (CE) devices across a Layer 2 virtual private network. Let's analyze how OSPF operates in this setup.

1. **OSPF Neighborship in L2VPN**:

- An L2VPN provides a Layer 2 connection between two sites, making it transparent to Layer 3 protocols like OSPF. This means the CEs can form OSPF adjacencies directly with each other as if they were on the same local network.

2. **OSPF Configuration on CEs and PEs**:

- **Statement A: OSPF neighborship is formed between the CEs and PEs**:

- Incorrect. In an L2VPN, the provider's network is transparent to the OSPF running on the CEs. OSPF neighborship forms directly between the CEs, not between the CEs and PEs.

- **Statement B: The CE and PE OSPF areas can be different**:

- Correct. Since OSPF adjacencies form directly between the CEs and not between CEs and PEs, the OSPF areas on the CEs and PEs can be different. The provider network acts as a transparent bridge, and OSPF doesn't see the PEs.

- **Statement C: The CE and PE OSPF areas must match**:

- Incorrect. As noted above, because the OSPF neighborship forms directly between the CEs, the OSPF areas on the CEs and PEs do not need to match.

- **Statement D: OSPF neighborship is formed between the two CEs**:

- Correct. The L2VPN makes the connection between the two CEs appear as a direct Layer 2 link, allowing them to form an OSPF adjacency directly.

**Conclusion**:

Given the above analysis, the correct statements are:

**B. The CE and PE OSPF areas can be different.**

**D. OSPF neighborship is formed between the two CEs.**

**References**:

- Juniper Networks Documentation on L2VPNs: [Configuring Layer 2 VPNs](https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/layer-2-vpns-configuring.html)

- OSPF Configuration Guide: [Junos OS OSPF Configuration](https://www.juniper.net/documentation/en_US/junos/topics/concept/ospf-routing-overview.html)

In the exhibit, the output of the `show route protocol bgp` command is shown for the prefix `172.16.20.4/30`. Let's analyze the provided BGP routing table to determine which statements are correct.

1. **AS Path Analysis**:

- The AS path for the route `172.16.20.4/30` is shown as `2 I`.

- This indicates that the route was learned from AS 2 and it is an internal (iBGP) route within the same AS.

2. **Multiple Paths**:

- The route has two next-hop IP addresses: `10.0.18.2` via interface `ge-1/0/4.0` and `10.0.19.2` via interface `ge-1/0/5.0`.

- This indicates that BGP multipath is configured, which allows multiple equal-cost paths to be used for load balancing.

- BGP multipath must be explicitly configured to use multiple paths for the same prefix.

3. **Multihop vs. Multipath**:

- **Multihop Configuration**: This is typically used for establishing BGP sessions with peers that are not directly connected. It is not related to load balancing.

- **Multipath Configuration**: This is used to enable load balancing across multiple paths for the same prefix, which is the case here.

**Conclusion**:

Given the above analysis:

- **C. This route is learned from the same AS number**: Correct. The AS path `2 I` indicates the route was learned from the same AS number (AS 2).

- **D. The multipath configuration is used for load balancing**: Correct. The presence of multiple next-hops indicates that BGP multipath is configured for load balancing.

Thus, the correct answers are:

**C. This route is learned from the same AS number.**

**D. The multipath configuration is used for load balancing.**

**References**:

- Junos OS BGP Multipath Documentation: [Junos OS BGP Multipath](https://www.juniper.net/documentation/en_US/junos/topics/topic-map/bgp-multipath.html)

- Junos OS BGP Configuration Guide: [Junos OS BGP Configuration](https://www.juniper.net/documentation/en_US/junos/topics/concept/bgp-routing-overview.html)

OSPF is an interior gateway protocol that uses link-state routing to exchange routing information among routers within a single autonomous system. OSPF prevents routing loops in multi-area networks by using two methods: area hierarchy and SPF algorithm. Area hierarchy is the concept of dividing a large OSPF network into smaller areas that are connected to a backbone area (area 0). This reduces the amount of routing information that each router has to store and process, and also limits the scope of link-state updates within each area. All areas are required to connect to area 0 either directly or through virtual links2. SPF algorithm is the method that OSPF uses to calculate the shortest path to each destination in the network based on link-state information. The SPF algorithm runs on each router and builds a shortest-path tree that represents the topology of the network from the router’s perspective. The SPF algorithm prunes looped paths within an area by choosing only one best path for each destination3.

Understanding the Problem in MPLS CoS Classification

How EXP Bits Are Used for CoS in MPLS

Traffic is sent from VPN-A Site-1 ? CE-1 ? PE-1 ? P-1 ? PE-2 ? CE-2.

The MPLS LSP (Label Switched Path) from PE-1 to PE-2 is expected to carry MPLS EXP bits, which are used for Class of Service (CoS) classification.

PE-2 should classify traffic based on EXP bits received in the MPLS label.

What Happens with PHP (Penultimate Hop Popping)?

By default, the penultimate router (P-1) pops the top MPLS label before sending the packet to PE-2.

Since the EXP bits are in the top MPLS label, they get removed along with the label.

This means that PE-2 no longer sees the correct EXP bits, leading to incorrect traffic classification.

Solution: Configure Explicit-Null on PE-2

Explicit Null (explicit-null) must be configured on PE-2 to ensure that P-1 does NOT remove the MPLS label.

Instead of removing the label, P-1 will send a label of 0 (for IPv4) or 2 (for IPv6) to PE-2.

This preserves the MPLS EXP bits, allowing PE-2 to classify the traffic correctly.

Evaluating the Answer Choices Again

? B. Configure the explicit-null statement on PE-2.

Correct, because:

PE-2 is the egress LSR, where Ultimate Hop Popping (UHP) must be enabled.

Configuring explicit-null ensures that P-1 does not remove the label, preserving the EXP bits for CoS classification at PE-2.

Configuration on PE-2:

set protocols mpls explicit-null

Juniper Documentation Reference:

"Explicit-null must be configured on the egress LSR to prevent PHP from removing the top MPLS label, thereby preserving the EXP bits."

? A. Configure the explicit-null statement on PE-1.

Incorrect, because:

Explicit-null must be configured on the egress LSR (PE-2), not the ingress LSR (PE-1).

PE-1 only labels the traffic but does not control PHP behavior on P-1.

? C. Configure VPN prefix mapping for the PE-1_to_PE-2 LSP.

Incorrect, because:

VPN prefix mapping is used for mapping VPN routes to LSPs but does not solve the EXP bit issue.

The problem here is label removal (PHP), not route mapping.

? D. Set a static CoS value for the PE-1_to-PE-2 LSP.

Incorrect, because:

This does not preserve the original EXP bits, it only applies a static CoS value.

It’s a workaround, not a fix.

Final Answer: ? B. Configure the explicit-null statement on PE-2.

Key Takeaways

Penultimate Hop Popping (PHP) removes the outer MPLS label at P-1, which also removes the EXP bits used for CoS classification.

To keep EXP bits intact, configure explicit-null on the egress PE (PE-2).

This forces P-1 to send a label (0 for IPv4, 2 for IPv6) to PE-2, preserving the EXP bits for CoS classification.

Official Juniper Documentation Reference

???? Juniper MPLS CoS and PHP Behavior Guide

"To retain CoS EXP bits at the egress LSR, configure explicit-null on the egress PE. This prevents PHP from stripping the MPLS label before reaching the final PE router."

https://www.juniper.net/documentation/us/en/software/junos/bgp/topics/topic-map/load-balancing-bgp-session.html

Intermediate System to Intermediate System (IS-IS) is a link-state routing protocol that supports Level 1 (L1), Level 2 (L2), or both (L1/L2) operations. The way IS-IS sends Hello (IIH) packets depends on whether the interface is point-to-point (P2P) or broadcast (LAN).

Evaluating the Answer Choices

? Option A: "If a point-to-point interface is in both L1 and L2, separate hello messages are sent for each level."

Incorrect!

On point-to-point (P2P) interfaces, only one combined Hello message is sent for both L1 and L2.

IS-IS P2P Hellos include both Level 1 and Level 2 TLVs in the same message.

IS-IS is an interior gateway protocol that uses link-state routing to exchange routing information among routers within a single autonomous system. IS-IS uses two types of addresses to identify routers and areas: system ID and area address. The system ID is a unique identifier for each router in an IS-IS domain. The system ID is 6 octets long and can be derived from the MAC address or manually configured. The area address is a variable-length identifier for each area in an IS-IS domain. The area address can be 1 to 13 octets long and is composed of high-order octets of the address. An IS-IS instance may be assigned multiple area addresses, which are considered synonymous. Multiple synonymous area addresses are useful when merging or splitting areas in the domain1. In this question, we have a network based on IS-IS with four routers (R1_1, R1_2, R2_1, and R2_2) belonging to area 0001. The area address for area 0001 is 49.0001. The NSEL byte for area 0001 is the last octet of the address, which is 01. The NSEL byte stands for Network Service Access Point Selector (NSAP Selector) and indicates the type of service requested from the network layer2. Therefore, the correct statement in this scenario is that the NSEL byte for area 0001 is 01.

The customer interface in an LDP-signaled pseudowire is the interface on the PE router that connects to the CE device. An LDP-signaled pseudowire is a type of Layer 2 circuit that uses LDP to establish a point-to-point connection between two PE routers over an MPLS network. The customer interface can have different encapsulation types depending on the type of traffic that is carried over the pseudowire. The encapsulation types are ethernet-ccc, vlan-ccc, extended-vlan-ccc, atm-ccc, frame-relay-ccc, ppp-ccc, cisco-hdlc-ccc, and tcc-ccc. Depending on the encapsulation type, the customer interface can accept or reject tagged or untagged frames in the data plane, and include or exclude VLAN tags in the control plane LDP advertisement. The following table summarizes the behavior of different encapsulation types:

The Dijkstra algorithm in IS-IS operates as follows:

Tree Database Initialization: The local router (root) is added to the tree database with a cost of 0.

Candidate Database Population: Neighbors of the root (from the LSDB) are placed into the candidate database with their associated costs.

Processing Nodes: The node with the lowest cost in the candidate database is moved to the tree database.

Neighbor Evaluation: For each neighbor of the newly added node (from the LSDB), if the neighbor is not already in the tree or candidate database, it is added to the candidate database. If it exists in the candidate with a higher cost, it is updated with the lower cost.

Termination: The algorithm stops when the candidate database is empty, ensuring all shortest paths are computed.

Analysis of Options:

A. Incorrect. The local router is placed directly into the tree database, not the candidate database.

B. Correct (with context). When a node is added to the tree database, its neighbors (existing in the LSDB) are evaluated. If these neighbors are not already in the tree or candidate database, they are added (not "moved") to the candidate database. The wording "moved" is technically inaccurate, but this option aligns closest with the process of populating the candidate database using LSDB entries during tree database processing.

C. Incorrect. Tuples (nodes) with the lowest cost are moved from the candidate database to the tree database, not from the tree to the LSDB. The LSDB remains static during SPF computation.

D. Incorrect. The algorithm stops when the candidate database is empty, not the tree database. The tree database grows as nodes are processed.