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).

https://www.juniper.net/documentation/us/en/software/junos/routing-policy/bgp/topics/example/bgp-advertise-peer-as.html

Intermediate System to Intermediate System (IS-IS) is a link-state routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices. It operates in two levels, Level 1 (L1) and Level 2 (L2), and supports hierarchical routing within a multi-area network.

Let's analyze each statement to determine its correctness in the context of IS-IS multi-area networks.

1. **Statement A: Internal L1 PDUs are flooded to the local area's L2 routers.**

- This statement is correct. L1 PDUs (Protocol Data Units) are flooded within the L1 area and also to the L2 routers that are present in the same area. These L2 routers act as the boundary routers that connect the local L1 area to other L1 areas via L2.

2. **Statement B: External L2 PDUs are flooded to all L2 routers in other areas.**

- This statement is correct. L2 PDUs are flooded throughout the entire L2 backbone, which includes all L2 routers in different areas. This ensures that inter-area routing information is shared across the network.

3. **Statement C: Internal L1 PDUs are flooded to all L1 routers in other areas.**

- This statement is incorrect. Internal L1 PDUs are only flooded within the local L1 area. They do not cross L1 area boundaries; inter-area communication is handled by L2 routers.

4. **Statement D: Internal L1 PDUs are only flooded to the local area's L1 routers.**

- This statement is correct. Internal L1 PDUs are indeed only flooded within their local L1 area, and do not go beyond it.

5. **Statement E: External L2 PDUs are only flooded to the local area's L2 routers.**

- This statement is incorrect. External L2 PDUs are flooded to all L2 routers throughout the IS-IS network, not just to those in the local area. This allows L2 routers to maintain a complete map of the network's topology.

**Conclusion**:

Given the analysis, the correct answers are:

**A. Internal L1 PDUs are flooded to the local area's L2 routers.**

**B. External L2 PDUs are flooded to all L2 routers in other areas.**

**D. Internal L1 PDUs are only flooded to the local area's L1 routers.**

**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 of IS-IS in multi-area networks.

https://www.juniper.net/documentation/us/en/software/junos/traffic-mgmt-qfx/topics/concept/cos-qfx-series-default-scheduling-classification-understanding.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.

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

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."

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:

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.

BGP route target filtering can be configured on PE devices or on route reflectors (RRs). Configuring BGP route target filtering on RRs is more efficient and scalable, as it reduces the number of BGP sessions and updates between PE devices. To configure BGP route target filtering on RRs, the following steps are required:

Configure the family route-target statement under the BGP group or neighbor configuration on the RRs. This enables the exchange of the route-target address family between the RRs and their clients (PE devices). Configure the resolution rib bgp.l3vpn.0 resolution-ribs inet.0 statement under the routing-options configuration on the RRs. This enables the RRs to resolve next hops for VPN routes using the inet.0 routing table.

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.

Interprovider Option C for Layer 3 VPNs involves the use of Autonomous System Boundary Routers (ASBRs) to exchange labeled VPN-IPv4 routes between different Autonomous Systems (AS). This option requires BGP sessions between ASBRs, and the VPN routes are carried end-to-end using MPLS labels. Here’s a detailed analysis of the roles of different routers in this scenario:

1. **ASBR Routers**:

- ASBRs are responsible for exchanging VPN-IPv4 routes between different ASes.

- **A. ASBR routers maintain the internal routes from its own AS and the loopback addresses from the other AS PEs.**

- Correct. ASBRs maintain routes to internal destinations within their own AS, and they also need to know the loopback addresses of PEs in the other AS to set up the BGP sessions and MPLS tunnels.

2. **PE Routers**:

- PE routers are responsible for maintaining VPN routes and label information to forward VPN traffic correctly.

- **B. PE routers maintain the internal routes from its own AS, the loopback address from the other AS PEs, and the L3VPN routes.**

- Correct. PE routers need to maintain:

- Internal routes within their AS for routing.

- Loopback addresses of other AS PEs for establishing MPLS LSPs.

- L3VPN routes to provide end-to-end VPN connectivity.

3. **P Routers**:

- P routers are the core routers that do not participate in BGP VPN routing but forward labeled packets based on MPLS labels.

- **C. P routers only maintain the internal routes from their own AS.**

- Correct. P routers maintain the internal routing information to forward packets within the AS and use MPLS labels for forwarding VPN packets. They do not maintain VPN routes or routes from other ASes.

4. **Incorrect Statements**:

- **D. P routers maintain the internal routes from its own AS and the loopback address from the other AS PEs.**

- Incorrect. P routers do not need to maintain the loopback addresses of other AS PEs. They only maintain internal routing and MPLS label information.

- **E. ASBR routers maintain the internal routes from its own AS, the loopback address from the other AS PEs, and the L3VPN routes.**

- Incorrect. ASBR routers do not maintain L3VPN routes. They exchange labeled VPN-IPv4 routes with other ASBRs and forward them to PE routers.

**Conclusion**:

The correct answers are:

**A. ASBR routers maintain the internal routes from its own AS and the loopback addresses from the other AS PEs.**

**B. PE routers maintain the internal routes from its own AS, the loopback address from the other AS PEs, and the L3VPN routes.**

**C. P routers only maintain the internal routes from their own AS.**

**References**:

- Juniper Networks Documentation on Interprovider VPNs: [Interprovider VPN Configuration](https://www.juniper.net/documentation/en_US/junos/topics/topic-map/mpls-vpn-interprovider.html)

- MPLS and VPN Architectures, CCIP Edition by Ivan Pepelnjak and Jim Guichard