Free Practice Questions for Juniper JN0-664 Exam

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.

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)

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.

https://www.juniper.net/documentation/us/en/software/junos/ospf/topics/topic-map/configuring-ospfv2-sham-links.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)

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.

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; } }

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.