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

BGP route reflectors are BGP routers that are allowed to ignore the IBGP loop avoidance rule and advertise IBGP learned routes to other IBGP peers under specific conditions. BGP route reflectors can reduce the number of IBGP sessions and updates in a network by eliminating the need for a full mesh of IBGP peers. BGP route reflectors can have three types of peerings:

EBGP neighbor: A BGP router that belongs to a different autonomous system (AS) than the route reflector.

IBGP client neighbor: An IBGP router that receives reflected routes from the route reflector. A client does not need to peer with other clients or non-clients.

IBGP non-client neighbor: An IBGP router that does not receive reflected routes from the route reflector. A non-client needs to peer with other non-clients and the route reflector.

In the exhibit, we can see that RR1 and RR2 are route reflectors in the same AS with reachability redundancy. They have two types of peerings: EBGP neighbors (R1 and R4) and IBGP client neighbors (Client1, Client2, and Client3). RR1 and RR2 are also peered with each other as IBGP non-client neighbors.

The explicit-null statement enables the PE router to send an MPLS label with a value of 0 (explicit null) instead of an IP header for packets destined to the VPN customer sites. This allows the penultimate hop router (the router before the egress PE router) to preserve the EXP bits of the MPLS label and pass them to the egress PE router. The egress PE router can then use these EXP bits to classify the traffic according to the CoS policy2. In this example, PE-1 should configure the explicit-null statement under [edit protocols mpls label-switched-path PE-1_to_PE-2] hierarchy level.

https://www.juniper.net/documentation/us/en/software/junos/cos-security-devices/topics/concept/cos-transmission-scheduling-security-overview.html

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 Guide](https://www.juniper.net/documentation/en_US/junos/topics/concept/ospf-design-overview.html)

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

amplification attacks are a type of distributed denial-of-service (DDoS) attack that exploit the characteristics of certain protocols to amplify the traffic sent to a victim. For example, an attacker can send a small DNS query with a spoofed source IP address to a DNS server, which will reply with a much larger response to the victim. This way, the attacker can generate a large amount of traffic with minimal resources.

One of the methods to protect against amplification attacks is destination-based Remote Triggered Black Hole (RTBH) filtering. This technique allows a network operator to drop traffic destined to a specific IP address or prefix at the edge of the network, thus preventing it from reaching the victim and consuming bandwidth and resources. RTBH filtering can be implemented using BGP to propagate a special route with a next hop of 192.0.2.1 (a reserved address) to the edge routers. Any traffic matching this route will be discarded by the edge routers.

The problem in this scenario is that CE-1 is configured with an incorrect peer AS number for its BGP session with PE-1. The CLI output shows that CE-1 is using AS 65531 as its local AS number and AS 65530 as its peer AS number. However, PE-1 is using AS 65530 as its local AS number and AS 65531 as its peer AS number. This causes a mismatch in the BGP OPEN messages and prevents the BGP session from being established. To solve this problem, CE-1 should configure its peer AS number as 65530 under [edit protocols bgp group external] hierarchy level.

In the provided exhibit, PE-1 and PE-2 are configured with LDP-signaled pseudowires to provide Layer 2 connectivity between CE-1 and CE-2. The issue is that there is no connectivity between CE-1 and CE-2. Let’s analyze the potential causes for this fault.

1. **LDP-Signaled Pseudowire (L2 Circuit) Configuration**:

- Pseudowires in MPLS networks use LDP (Label Distribution Protocol) to signal the virtual circuit (VC) labels between PE routers.

- For successful connectivity, the VC ID (Virtual Circuit Identifier) and LSPs (Label Switched Paths) between the PE routers must be correctly configured and operational.

2. **Analysis of the Exhibit**:

- The output shows the status of the L2 circuit connection on PE-1.

- The status (St) for the interface is `rmt Dn`, which indicates that the remote site (PE-2) is down or unreachable.

3. **Potential Causes**:

- **A. The VC IDs are mismatched**:

- Correct. If the VC IDs configured on PE-1 and PE-2 do not match, the L2 circuit cannot be established. Mismatched VC IDs prevent the pseudowire from forming correctly.

- **B. There is no LSP configured from PE-1 to PE-2**:

- Correct. LSPs are required for MPLS forwarding. If there is no LSP from PE-1 to PE-2, the LDP session cannot establish a path for the pseudowire. This results in the pseudowire being down.

- **C. Interface ge-0/0/0 on PE-1 is down**:

- Incorrect. The interface status is shown as `Up`, meaning the physical interface is operational.

- **D. There is no LSP configured from PE-2 to PE-1**:

- While this might seem like a potential issue, the specific problem of the remote site being down (`rmt Dn`) typically relates more directly to the forward path from PE-1 to PE-2 (i.e., no LSP from PE-1 to PE-2). Hence, the more accurate immediate cause is covered in Option B.

**Conclusion**:

Given the analysis, the correct statements describing potential causes for the fault are:

**A. The VC IDs are mismatched.**

**B. There is no LSP configured from PE-1 to PE-2.**

**References**:

- Juniper Networks Documentation on L2 Circuits: [Configuring Layer 2 Circuits](https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/layer-2-circuits-configuring.html)

- MPLS Configuration Guide: [Juniper MPLS Configuration](https://www.juniper.net/documentation/en_US/junos/topics/topic-map/mpls-overview.html)

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.

the default behavior for iBGP is to propagate EBGP-learned prefixes without changing the next-hop. This can cause issues if the next-hop is not reachable via the IGP. One solution is to use the next-hop self command on R4, which will change the next-hop attribute to its own loopback address. This way, R5 can reach the next-hop via the IGP and install the route in its routing table.

Another solution is to add the external interface prefix (120.0.4.16/30) to the IGP routing tables of R4 and R5. This will also make the next-hop reachable via the IGP and allow R5 to use the route. According to 2, this is a possible workaround for a pure IP network, but it may not work well for an MPLS network.

The reason why the route is being hidden is that R5 cannot reach the BGP next hop 10.0.0.1, which is the address of R1. R5 does not have a route to 10.0.0.0/24 in its routing table, and neither does R4. Therefore, R5 cannot resolve the BGP next hop and marks the route as hidden.

There are two solutions that will solve this problem:

Option A: On R4, create a policy to change the BGP next hop to itself and apply it to IBGP as an export policy. This way, R5 will receive the route with a next hop of 172.16.1.2, which is reachable via the IGP. This solution is also known as next-hop-self1.

Option B: Add the external interface prefix to the IGP routing tables. This way, R4 and R5 will learn a route to 10.0.0.0/24 via the IGP and be able to resolve the BGP next hop. This solution is also known as recursive lookup2.

Option C is not correct because adding the internal interface prefix to the BGP routing tables will not help R5 reach the BGP next hop 10.0.0.1.

Option D is not correct because changing the BGP next hop to 172.16.1.1 on R4 will not help R5 either, since R5 does not have a route to 172.16.1.1 in its routing table.

References: 1: Configuring Next-Hop-Self for IBGP Peers 2: Understanding Recursive Lookup

Intermediate System to Intermediate System (IS-IS) is a link-state routing protocol used to move information efficiently within a computer network. It uses a series of Protocol Data Units (PDUs) to manage the network's topology and ensure consistency across all routers in the network. Specifically, Link State PDUs (LSPs), Complete Sequence Number PDUs (CSNPs), and Partial Sequence Number PDUs (PSNPs) play crucial roles in this process.

1. **PSNPs (Partial Sequence Number PDUs)**:

- **Acknowledge a received LSP**: PSNPs are used to acknowledge the receipt of LSPs. When a router receives an LSP, it sends a PSNP back to the sender to confirm that the LSP has been received.

- **Request a missing LSP**: PSNPs are also used to request missing LSPs. If a router identifies a missing LSP based on sequence numbers, it can send a PSNP to request the specific LSP from its neighbors.

2. **CSNPs (Complete Sequence Number PDUs)**:

- **Summarize LSPs**: CSNPs are used to summarize all the LSPs known to a router. They are typically sent at regular intervals to provide a complete list of LSPs in a database. They are not used to acknowledge or request specific LSPs but provide an overview of all LSPs for database synchronization.

Based on this understanding, let's evaluate the statements:

- **A. PSNPs are used to acknowledge a received LSP.**

- Correct. PSNPs serve the purpose of acknowledging LSPs received from other routers.

- **B. CSNPs are used to acknowledge a received LSP.**

- Incorrect. CSNPs are not used for acknowledging LSPs; they are used to provide a summary of all LSPs.

- **C. CSNPs are used to request a missing LSP.**

- Incorrect. CSNPs are not used to request missing LSPs; this is the role of PSNPs.

- **D. PSNPs are used to request a missing LSP.**

- Correct. PSNPs are used to request specific missing LSPs when a router detects that it is missing information.

**Conclusion**:

The correct statements about IS-IS are:

**A. PSNPs are used to acknowledge a received LSP.**

**D. PSNPs are used to request a missing LSP.**

**References**:

- Juniper Networks Documentation on IS-IS: [IS-IS Overview](https://www.juniper.net/documentation/en_US/junos/topics/concept/is-is-routing-overview.html)

- RFC 1195, Use of OSI IS-IS for Routing in TCP/IP and Dual Environments: [RFC 1195](https://tools.ietf.org/html/rfc1195) which details the operation and use of IS-IS, including the roles of PSNPs and CSNPs.

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.

The correct order of packet flow through configurable components in the Junos OS CoS features is as follows:

Behavior Aggregate Classifier: This component uses a single field in a packet header to classify traffic into different forwarding classes and loss priorities based on predefined or user-defined values.

Input Policer: This component applies rate-limiting and marking actions to incoming traffic based on the forwarding class and loss priority assigned by the classifier.

Multifield Classifier: This component uses multiple fields in a packet header to classify traffic into different forwarding classes and loss priorities based on user-defined values and filters.

Forwarding Policy Options: This component applies actions such as load balancing, filtering, or routing to traffic based on the forwarding class and loss priority assigned by the classifier.

Fabric Scheduler: This component schedules traffic across the switch fabric based on the forwarding class and loss priority assigned by the classifier.

Output Policer: This component applies rate-limiting and marking actions to outgoing traffic based on the forwarding class and loss priority assigned by the classifier.

Scheduler/Shaper/RED: This component schedules, shapes, and drops traffic at the egress interface based on the forwarding class and loss priority assigned by the classifier.

Rewrite Marker: This component rewrites the code-point bits of packets leaving an interface based on the forwarding class and loss priority assigned by the classifier.

The OSPF cost is carried in the LSAs that are exchanged within an OSPF area. When a router calculates the cost to a destination it uses the cost of the exit interface of each router in the path to the destination.

The auto-export parameter is a routing option that allows a routing instance to share routes with other routing instances or the master routing table. The auto-export parameter automatically exports routes from one routing instance to another based on the route target communities attached to the routes. In this scenario, the 10.101.1.0/24 route will be shared if the auto-export parameter is configured under [edit routing-options] hierarchy level.

Layer 2 VPNs don’t support fragmentation in the provider network. It is critical that the provider network supports the largest frame that the CE devices can generate after the MPLS and virtual routing and forwarding (VRF) labels are added by the PE devices. This example leaves the CE devices at the default 1500-byte maximum transmission unit (MTU) while configuring the provider core to support a 4000 byte MTU. This configuration avoids discards by ensuring the CE devices cannot exceed the MTU in the provider’s network.

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

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. To create a sham link, you need to configure the local and remote addresses of the PE routers under the [edit protocols ospf area area-id] hierarchy level1.

BGP Layer 2 VPNs use BGP to distribute endpoint provisioning information and set up pseudowires between PE devices. BGP uses the Layer 2 VPN (L2VPN) Routing Information Base (RIB) to store endpoint provisioning information, which is updated each time any Layer 2 virtual forwarding instance (VFI) is configured. The prefix and path information is stored in the L2VPN database, which allows BGP to make decisions about the best path.

In BGP Layer 2 VPNs, each site has a unique site ID that identifies it within a VFI. The site ID can be manually configured or automatically assigned by the PE device. By default, the site ID is automatically assigned based on the order that you add the interfaces to the site configuration. The first interface added to a site configuration has a site ID of 1, the second interface added has a site ID of 2, and so on.

Option D is correct because by default on PE-2, the remote site IDs are automatically assigned based on the order that you add the interfaces to the site configuration. Option A is not correct because by default on PE-2, the site’s local ID is automatically assigned a value of 0 and does not need to be configured to match the total number of attached sites. Option B is not correct because you do not need to create a unique Layer 2 VPN routing instance for each site on the PE-2 device. You can create one routing instance for all sites within a VFI. Option C is not correct because you do not need to use separate physical interfaces to connect PE-2 to each site. You can use subinterfaces or service instances on a single physical interface.

Type 1: When Type value is 1, the Administrator field is 4-bytes and Assigned Number field is 2-bytes. The Administrator field should be set to the IP address (public IP addresses should be used). The Assigned Number field contains a number from a numbering space that is administered by the enterprise to which the IP address has been assigned by the appropriate authority.