JN0-664 Service Provider Professional (JNCIP-SP) Questions and Answers

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.

BGP add-path is a feature that allows the advertisement of multiple paths through the same peering session for the same prefix without the new paths implicitly replacing any previous paths. This behavior promotes path diversity and reduces multi-exit discriminator (MED) oscillations. BGP add-path is implemented by adding a path identifier to each path in the NLRI. The path identifier can be considered as something similar to a route distinguisher in VPNs, except that a path ID can apply to any address family. Path IDs are unique to a peering session and are generated for each network3. In this question, we have a route reflector (RR) that receives two routes for the same prefix (192.168.0.0/16) from an EBGP neighbor. By default, the RR will only advertise its best path to its clients (R1 and R2). However, we want R1 and R2 to receive both routes from the RR. To achieve this, we need to configure BGP add-path on the RR and enable it to send multiple paths for the same prefix to its clients.

https://www.juniper.net/documentation/us/en/software/nce/feature-guide-virtual-private-lan-service/topics/task/vpls-ldp-signaling-solutions.html

https://www.juniper.net/documentation/us/en/software/junos/vpn-l2/topics/concept/vpns-configuring-vpls-routing-instances.html#id-11510150__id-11568648

The issue arises because R3’s group-range (224.1.1.0/28) is more specific than R2’s group-range (224.1.1.0/24). In PIM bootstrap (BSR), the RP with the longest prefix (most specific group-range) is preferred, regardless of priority. Even though R3 has a higher bootstrap priority (210 vs. R2’s 200), its more specific /28 group-range takes precedence for groups within 224.1.1.0/28.

Why Option D is Correct:

To force R4 to use R2 for all joins, R2’s group-range must be more specific than R3’s. For example:

If R2’s group-range is changed to 224.1.1.0/28 (same as R3) but with a higher priority, R2 would win (priority is compared only when group-ranges are equal).

If R2’s group-range is changed to 224.1.1.0/29 (more specific than /28), it will override R3’s /28 for groups in the /29 range.

The key is prefix specificity, which overrides priority in BSR elections.

Why Other Options Are Incorrect:

A. Change bootstrap priority on R2 to be higher than R3:

Priority is evaluated only when group-ranges are identical. Since R3’s group-range (/28) is more specific than R2’s (/24), R3 will still win for groups in 224.1.1.0/28, even if R2’s priority is higher.

B. Change the default route in inet.2 on R4:

RPF routes (inet.2) determine how traffic reaches the RP, but they do not influence RP election logic (BSR priority/group-range).

C. Change R2’s local address to be higher than R3’s:

The RP address is a tiebreaker only if priorities and group-ranges are equal. Since R3’s group-range is more specific, this change has no impact.

Key Takeaways:

BSR RP Election Order:

Longest group prefix (most specific).

Highest priority (if prefixes are equal).

Highest RP address (if prefixes and priorities are equal).

To override R3, R2 must advertise a more specific group-range (e.g., /28 or smaller) to ensure it is selected for the desired multicast groups.

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.

Junos platforms use different mechanisms to evaluate incoming traffic for CoS purposes, such as:

Behavior aggregate classifiers: These classifiers use a single field in a packet header to classify traffic into different forwarding classes and loss priorities based on predefined or user-defined values.

Fixed classifiers: These classifiers use a fixed field in a packet header to classify traffic into different forwarding classes and loss priorities based on predefined values.

Multifield classifiers: These classifiers use multiple fields in a packet header to classify traffic into different forwarding classes and loss priorities based on user-defined values and filters.

Rewrite rules and traffic shapers are not used to evaluate incoming traffic for CoS purposes, but rather to modify or shape outgoing traffic based on CoS policies.

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.

This is an EVPN Type-2 route, also called a MAC/IP advertisement route, that is used to advertise host IP and MAC address information to other VTEPs in an EVPN network. The route type field in the EVPN NLRI has a value of 2, indicating a Type-2 route. The device advertising this route into EVPN is 192.168.101.5, which is the IP address of the VTEP that learned the host information from the local CE device. This IP address is carried in the MPLS label field of the route as part of the VXLAN encapsulation.

class-of-service (CoS) is a feature that allows you to prioritize and manage network traffic based on various criteria, such as application type, user group, or packet loss priority. CoS uses different components to classify, mark, queue, schedule, shape, and drop traffic according to the configured policies.

One of the components of CoS is drop profiles, which define how packets are dropped when a queue is congested. Drop profiles use random early detection (RED) algorithm to drop packets randomly before the queue is full, which helps to avoid global synchronization and improve network performance. Drop profiles can be discrete or interpolated. A discrete drop profile maps a specific fill level of a queue to a specific drop probability. An interpolated drop profile maps a range of fill levels of a queue to a range of drop probabilities and interpolates the values in between.

In the exhibit, we can see that the class-of-service configuration shows an interpolated drop profile with two fill levels (50 and 75) and two drop probabilities (20 and 60). Based on this configuration, we can infer the following statements:

The drop probability jumps immediately from 20% to 60% when the queue level reaches 75% full. This is not correct because the drop profile is interpolated, not discrete. This means that the drop probability gradually increases from 20% to 60% as the queue level increases from 50% full to 75% full. The drop probability for any fill level between 50% and 75% can be calculated by using linear interpolation formula.

The drop probability gradually increases from 20% to 60% as the queue level increases from 50% full to 75% full. This is correct because the drop profile is interpolated and uses linear interpolation formula to calculate the drop probability for any fill level between 50% and 75%. For example, if the fill level is 60%, the drop probability is 28%, which is calculated by using the formula: (60 - 50) / (75 - 50) * (60 - 20) + 20 = 28.

To use this drop profile, you reference it in a scheduler. This is correct because a scheduler is a component of CoS that determines how packets are dequeued from different queues and transmitted on an interface. A scheduler can reference a drop profile by using the random-detect statement under the [edit class-of-service schedulers] hierarchy level. For example: scheduler test { transmit-rate percent 10; buffer-size percent 10; random-detect test-profile; }

To use this drop profile, you apply it directly to an interface. This is not correct because a drop profile cannot be applied directly to an interface. A drop profile can only be referenced by a scheduler, which can be applied to an interface by using the scheduler-map statement under the [edit class-of-service interfaces] hierarchy level. For example: interfaces ge-0/0/0 { unit 0 { scheduler-map test-map; } }

https://www.juniper.net/documentation/us/en/software/junos/bgp/topics/ref/statement/multihop-edit-protocols-bgp.html

To reduce the impact of suboptimal routing in OSPF and OSPFv3 after devices reboot, you can use the overload feature to prevent a router from being used as a transit router for a specified period of time. This allows the router to stabilize its routing table before forwarding traffic for other routers. To enable the overload feature, you need to do the following:

For OSPF, configure the overload statement under [edit protocols ospf] hierarchy level. You can also specify a timeout value in seconds to indicate how long the router should remain in overload state after it boots up. For example, set protocols ospf overload timeout 900 means that the router will be in overload state for 15 minutes after it boots up.

For OSPFv3, configure the overload statement under [edit protocols ospf3] hierarchy level. You can also specify a realm (ipv4-unicast or ipv6-unicast) and a timeout value in seconds to indicate how long the router should remain in overload state after it boots up for each realm. For example, set protocols ospf3 realm ipv4-unicast overload timeout 900 means that the router will be in overload state for 15 minutes after it boots up for IPv4 unicast routing.

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).

Bootstrap messages are PIM messages that are used to distribute rendezvous point (RP) information dynamically during an RP election. Bootstrap messages are sent by bootstrap routers (BSRs), which are routers that are elected to perform the RP discovery function for a PIM sparse-mode domain. Bootstrap messages contain information about candidate RPs and their multicast groups, as well as BSR priority and hash mask length. Bootstrap messages are forwarded to all routers within a PIM sparse-mode domain using hop-by-hop flooding.

Analyzing the Exhibit

The diagram represents BGP peering between:

AS 65512 (Enterprise Network)

AS 65511 (ISP-A)

R3 and R4 are peering with ISP-A using EBGP.

R1, R2, R3, and R4 are peering within AS 65512 using IBGP.

Understanding BGP Route Behavior

Option A: "By default, the next-hop value for these routes is not changed by ISP-A before being sent to R3." ❌

Incorrect!

EBGP behavior: When a BGP route is advertised via EBGP, the next-hop IP is changed to the router's own IP by default.

Since ISP-A is advertising routes via EBGP to R3, the next-hop is changed to ISP-A’s IP.

Thus, this statement is incorrect.

Option B: "The BGP local-preference value that is used by ISP-A is not advertised to R3." ✅

Correct!

BGP Local Preference (LOCAL_PREF) is an IBGP-only attribute.

Local Preference is NOT shared over EBGP because it is used within an AS to influence route selection.

ISP-A will not send LOCAL_PREF to R3, as R3 is in a different AS.

Thus, this statement is correct.

Option C: "All BGP attribute values must be removed before receiving the routes." ❌

Incorrect!

BGP does not remove all attributes when advertising routes. Some attributes are modified (e.g., next-hop, AS-PATH), but others (like MED, community) may be preserved.

Thus, this statement is incorrect.

Option D: "The next-hop value for these routes is changed by ISP-A before being sent to R3." ✅

Correct!

As per default EBGP behavior, the next-hop is changed when a route is advertised to an EBGP peer.

This means ISP-A changes the next-hop to its own IP before sending it to R3.

Thus, this statement is correct.

Final Answer:

✅ B. The BGP local-preference value that is used by ISP-A is not advertised to R3.

✅ D. The next-hop value for these routes is changed by ISP-A before being sent to R3.

Verification from Juniper Documentation:

Juniper BGP Configuration Guide confirms that LOCAL_PREF is not advertised over EBGP.

RFC 4271 (BGP-4) specifies that next-hop is changed by default when advertising routes via EBGP.

https://www.juniper.net/documentation/us/en/software/junos/sampling-forwarding-monitoring/bgp/topics/concept/bgp-multipath-unequal-understanding.html

Option A (Correct):

In IS-IS, the Designated Intermediate System (DIS) is elected based on the highest configured priority (as defined in Junos OS).

If priorities are equal, the router with the highest MAC address becomes the DIS.

A priority value of 10 will always override a lower priority (e.g., 1).

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.

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.