Juniper JN0-664 Service Provider Professional (JNCIP-SP) Exam Practice Test

PIM sparse mode (PIM-SM) is a multicast routing protocol that uses a pull model to deliver multicast traffic. In PIM-SM, routers with receivers send join messages to their upstream neighbors toward a rendezvous point (RP) or a source-specific tree (SPT). The RP or SPT acts as the root of a shared distribution tree for a multicast group. Traffic is only forwarded to routers that request to join the distribution tree by sending join messages. PIM-SM does not flood traffic to all routers or prune routers without receivers, as PIM dense mode does.

The vrf-target statement is used in routing-instances to define route-target communities for VPN route import and export policies.

vrf-target export → Controls which route targets (RTs) are added to advertised routes (used when sending routes to remote PEs).

vrf-target import → Controls which VPN routes are accepted into the VRF (used when receiving routes from remote PEs).

The exhibit shows the configuration of a VRF (Virtual Routing and Forwarding) instance on a Juniper PE router. Let's break down the key components:

VRF Configuration (VPN-A)

The instance type is VRF, meaning this is an L3VPN (Layer 3 VPN).

The routing instance contains a static route (10.1.0.0/16 next-hop 10.1.0.1).

The interface ge-0/0/2.0 is assigned to the VRF.

Route Distinguisher (RD): 172.17.20.1:1

VRF-Export Policy: vpn-a-export

VRF-Target: target:65512:1 (This defines which routes will be imported into the VRF).

VRF Export Policy (vpn-a-export)

The vpn-a-export policy adds two BGP communities (route targets) to exported VPN routes:

community add vpn-a-target;

community add vpn-m-target;

accept;

The vpn-a-target community corresponds to target:65512:1.

The vpn-m-target community corresponds to target:65512:2.

Policy-Options (Community Definitions)

community vpn-a-target members target:65512:1;

community vpn-m-target members target:65512:2;

This confirms that routes exported from this VRF will have BOTH target:65512:1 and target:65512:2.

Evaluating the Answer Choices

✅ Option A: "VPN routes are exported with the target:65512:1 and target:65512:2 route targets."

The vpn-a-export policy explicitly adds both vpn-a-target (65512:1) and vpn-m-target (65512:2) to exported routes.

This is correct. ✅

❌ Option B: "You cannot use the vrf-target and vrf-export statements in the same VRF."

This is incorrect.

Juniper allows the use of both vrf-target and vrf-export in the same VRF:

vrf-target is used for importing routes.

vrf-export defines export policies (which can add additional route targets).

This is incorrect. ❌

❌ Option C: "VPN routes with the target:65512:1 and target:65512:2 route targets are imported."

The vrf-target target:65512:1; statement only controls importing routes.

The import policy does not include target:65512:2, so routes tagged with target:65512:2 alone would not be imported into this VRF.

This is incorrect. ❌

❌ Option D: "VPN routes are exported with only the target:65512:1 route target."

The export policy (vpn-a-export) clearly adds both 65512:1 and 65512:2.

This is incorrect. ❌

Final Answer:

✅ A. VPN routes are exported with the target:65512:1 and target:65512:2 route targets.

Verification from Juniper Documentation

Juniper MPLS L3VPN Configuration Guide confirms that vrf-target is used for importing, while vrf-export can be used for exporting multiple route targets.

Juniper Routing Policy Documentation states that export policies can add multiple BGP communities (route targets).

RFC 4364 (BGP/MPLS IP VPNs) defines the use of route targets for VPN route control.

In IS-IS (Intermediate System to Intermediate System) routing, each router is identified by a unique ISO (International Organization for Standardization) address, also known as a Network Entity Title (NET). The NET consists of three parts:

1. **Area Identifier**: Indicates the area to which the router belongs.

2. **System Identifier**: Uniquely identifies the router within the area.

3. **NSAP Selector (NSEL)**: Typically set to 00 for a router, indicating the Network Service Access Point.

The format of the ISO address is `49.XXXX.YYYY.YYYY.ZZZZ.ZZZZ.00`, where:

- `49` is the AFI (Authority and Format Identifier) indicating a private address.

- `XXXX` is the Area Identifier.

- `YYYY.YYYY.YYYY` is the System Identifier.

- `ZZZZ.ZZZZ` is the NSAP Selector.

Given the address `49.0001.00a0.c96b.c490.00`:

- **Area Identifier**: `49.0001`

- **System Identifier**: `00a0.c96b.c490`

- **NSAP Selector**: `00`

**Explanation**:

- **A. 00a0.c96b.c490 is the system identifier**:

- Correct. The System Identifier in an ISO address is a 48-bit (6-byte) field used to uniquely identify the router. In this address, `00a0.c96b.c490` is the correct 6-byte System Identifier.

- **B. 0001.00a0.c96b.c490 is the system identifier**:

- Incorrect. This includes the Area Identifier as part of the System Identifier, which is not correct.

- **C. c96b.c490 is the system identifier**:

- Incorrect. This is only part of the System Identifier. The full System Identifier must be 6 bytes long.

- **D. c490 is the system identifier**:

- Incorrect. This is an incomplete and incorrect part of the System Identifier.

**Conclusion**:

The correct part of the address that represents the System Identifier is:

**A. 00a0.c96b.c490 is the system identifier.**

**References**:

- Juniper Networks Documentation on IS-IS: [IS-IS

- ISO/IEC 10589, the IS-IS routing protocol standard.

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.

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, the PE-to-CE protocol used is OSPF, and there is an OSPF neighborship between CE-1 and CE-2 within the same Area 0. Let's analyze the default OSPF routing behavior in this setup to determine the correct statement.

1. **OSPF Neighborship**:

- CE-1 and CE-2 have an OSPF neighborship directly within Area 0.

- OSPF prefers intra-area routes over inter-area and external routes.

2. **Default Routing Behavior**:

- Since CE-1 and CE-2 are directly connected through an OSPF link within the same area, OSPF will prefer this direct intra-area path over any other paths learned via the PE routers and the L3VPN.

- This is because intra-area routes have a lower metric compared to inter-area or external routes.

3. **Metric Considerations**:

- By default, OSPF will route traffic between Site-1 and Site-2 through the direct link between CE-1 and CE-2, unless the link's metric is artificially increased to make it less preferable.

- There is no need to adjust metrics for the CE-1 to PE-1 link to prefer the CE-1 to CE-2 path, as OSPF already prefers direct intra-area paths.

**Conclusion**:

Given the default behavior of OSPF and the topology shown in the exhibit, the correct statement is:

**B. Hosts at Site-1 will reach hosts at Site-2 through the CE-1 and CE-2 link by default.**

**References**:

- OSPF Design Guide: [Juniper Networks OSPF Design

- Juniper Networks Technical Documentation on OSPF: [Junos OS OSPF Configuration

Route reflection is a BGP feature that allows a router to reflect routes learned from one IBGP peer to another IBGP peer, without requiring a full-mesh IBGP topology. Route reflectors do not change any existing BGP attributes by default when advertising routes, unless explicitly configured to do so. A BGP peer does not require any configuration changes to become a route reflector client, only the route reflector needs to be configured with the client parameter under [edit protocols bgp group group-name neighbor neighbor-address] hierarchy level.

Option A (Correct):

Complete Sequence Number PDUs (CSNPs) are periodically flooded by the Designated Intermediate System (DIS) on multi-access networks (e.g., Ethernet).

This ensures all routers on the segment synchronize their Link-State Databases (LSDBs).

To allow Site 1 to access three VLANs that are located in Site 2 and Site 3, you need to configure three L2VPN routing instances on PE-1, one for each VLAN. Each L2VPN routing instance will have a different VLAN ID and a different VNI for VXLAN encapsulation. Each L2VPN routing instance will also have a different vrf-target export value to identify which VPN routes belong to which VLAN. This way, PE-1 can forward traffic from Site 1 to Site 2 and Site 3 based on the VLAN tags and VNIs.

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

- Junos OS BGP Configuration Guide: [Junos OS BGP

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."

To transport IPv6 traffic over an IPv4-based MPLS network using BGP, you need to configure two address families: family inet6 labeled-unicast and family inet6 unicast. The former is used to exchange IPv6 routes with MPLS labels between PE routers, and the latter is used to exchange IPv6 routes without labels between PE and CE routers. The mpis ipv6-tunneling command enables the PE routers to encapsulate the IPv6 packets with an MPLS label stack and an IPv4 header before sending them over the MPLS network.

The output is from the show l2vpn connections command on a Juniper router. This command is used to verify the status of Layer 2 VPN (L2VPN) pseudowires between Provider Edge (PE) routers.

Breakdown of Key Information:

Instance: vpn-A

This is the L2VPN instance being monitored.

Connection Status (St)

The connection status is "Up", meaning the pseudowire is operational.

Local Site: CE1-2 (2)

The PE router is attached to a single local site (CE1-2).

Uptime & Connection Flaps

The output shows the last time the connection was up:

Time last up: Apr 11 14:35:27 2020

The "# Up trans" value is 1, meaning this connection has been established once and has not flapped since it was initiated.

VLAN ID Mismatch Check

The legend includes "VM – VLAN ID mismatch", but this status is not present in the connection output.

This means there is NO VLAN ID mismatch.

Flow Labels

The Flow Label Transmit is No, and the Flow Label Receive is No.

This means the PE router does NOT have the capability to pop flow labels.

Analyzing the Exhibit and Understanding the Issue

The exhibit shows BGP configurations on CE-1 and PE-1, which are connected via EBGP.

CE-1 (Customer Edge)

Uses AS 64511 and establishes an EBGP session with PE-1 (AS 65550).

Configured to export 10 static routes (192.168.1.0/24 - 192.168.10.0/24) using the static-to-bgp policy.

PE-1 (Provider Edge)

Uses AS 65550 and is peering with CE-1 (AS 64511).

Configured with a prefix-limit of 5 on received routes from CE-1.

Teardown enabled, meaning if more than 5 prefixes are received, the BGP session is shut down.

Identifying the Problem

CE-1 is correctly configured with peer AS 65550, so Option B ("CE-1 is configured with an incorrect peer AS") is incorrect ❌.

CE-1 is advertising exactly 10 static routes (as per policy).

PE-1 has a prefix-limit maximum 5 with teardown enabled.

This means that when CE-1 advertises more than 5 prefixes, PE-1 shuts down the BGP session.

BGP moves to the "Active" state, indicating that the session has been disrupted and PE-1 is trying to re-establish the connection.

CE-1 is reachable since the session was initially established before the limit was exceeded, so Option D ("CE-1 is unreachable") is incorrect ❌.

CE-1 is not advertising its entire routing table, only the static prefixes listed in the policy, so Option A ("CE-1 is advertising its entire routing table") is incorrect ❌.

Correct Answer

✅ C. The prefix limit has been reached on PE-1

Verification from Juniper Documentation

Juniper BGP Prefix Limit Documentation confirms that exceeding the prefix limit with teardown causes the BGP session to go into "Active" state.

Juniper Troubleshooting Guide for BGP Peering Issues states that when a BGP session reaches the prefix limit and has teardown enabled, the session is terminated.

EVPN is a technology that connects L2 network segments separated by an L3 network using a virtual Layer 2 network overlay over the Layer 3 network. EVPN uses BGP as its control protocol to exchange different types of routes for different purposes. Type 2 routes are used to advertise MAC address and IP address pairs learned using ARP snooping from the local CE devices. Type 3 routes are used to join a multicast tree to flood traffic such as broadcast, unknown unicast, and multicast (BUM) traffic.

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.

When a Junos device receives a packet on an interface with transmission scheduling, traffic is placed into different queues based on Class of Service (CoS) policies. If a queue exceeds its allocated bandwidth, Junos has default behaviors for handling excess traffic.

Key Junos Behaviors for Transmission Scheduling

Queues Can Borrow Bandwidth from Other Queues ✅

If a queue has excess traffic, it can use bandwidth from underutilized queues, as long as bandwidth is available.

Reference from Juniper Documentation:

"By default, each queue can exceed the assigned bandwidth if additional bandwidth is available from other queues."

Queues Have Credit-Based Tracking ✅

A queue that stays within its allocated bandwidth is considered to have positive bandwidth credit.

A queue that exceeds its allocation is considered to have negative bandwidth credit.

Reference from Juniper Documentation:

"A queue receiving traffic in excess of its bandwidth allocation is considered to have negative bandwidth credit."

Evaluating the Answer Choices

✅ B. The exceeding queue will be considered to have negative bandwidth credit.

Correct, because when a queue exceeds its allocated bandwidth, Junos assigns it negative bandwidth credit.

This means the queue is in debt and must recover before it can transmit additional packets.

✅ C. The excess traffic will use bandwidth available from other queues.

Correct, because Junos allows excess traffic to borrow bandwidth from underutilized queues by default.

If a forwarding class does not use its allocated bandwidth, other queues can borrow the unused bandwidth.

Why the Other Answers Are Incorrect?

❌ A. The excess traffic will be discarded.

Incorrect, because Junos does not immediately discard excess traffic unless the queue cannot borrow bandwidth.

By default, Junos allows bandwidth sharing, and only if no bandwidth is available does it drop packets.

❌ D. The exceeding queue will be considered to have positive bandwidth credit.

Incorrect, because when a queue exceeds its assigned bandwidth, it gets negative bandwidth credit, not positive credit.

Verified Juniper Official Reference

???? Junos CoS Transmission Scheduling Overview

"By default, each queue can exceed the assigned bandwidth if additional bandwidth is available from other queues."

"A queue receiving traffic in excess of its bandwidth allocation is considered to have negative bandwidth credit."

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.

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.

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.