Pass the Juniper JNCIP-SP JN0-664 Questions and answers with CertsForce

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.

Understanding the Problem in MPLS CoS Classification

How EXP Bits Are Used for CoS in MPLS

Traffic is sent from VPN-A Site-1 → CE-1 → PE-1 → P-1 → PE-2 → CE-2.

The MPLS LSP (Label Switched Path) from PE-1 to PE-2 is expected to carry MPLS EXP bits, which are used for Class of Service (CoS) classification.

PE-2 should classify traffic based on EXP bits received in the MPLS label.

What Happens with PHP (Penultimate Hop Popping)?

By default, the penultimate router (P-1) pops the top MPLS label before sending the packet to PE-2.

Since the EXP bits are in the top MPLS label, they get removed along with the label.

This means that PE-2 no longer sees the correct EXP bits, leading to incorrect traffic classification.

Solution: Configure Explicit-Null on PE-2

Explicit Null (explicit-null) must be configured on PE-2 to ensure that P-1 does NOT remove the MPLS label.

Instead of removing the label, P-1 will send a label of 0 (for IPv4) or 2 (for IPv6) to PE-2.

This preserves the MPLS EXP bits, allowing PE-2 to classify the traffic correctly.

Evaluating the Answer Choices Again

✅ B. Configure the explicit-null statement on PE-2.

Correct, because:

PE-2 is the egress LSR, where Ultimate Hop Popping (UHP) must be enabled.

Configuring explicit-null ensures that P-1 does not remove the label, preserving the EXP bits for CoS classification at PE-2.

Configuration on PE-2:

set protocols mpls explicit-null

Juniper Documentation Reference:

"Explicit-null must be configured on the egress LSR to prevent PHP from removing the top MPLS label, thereby preserving the EXP bits."

❌ A. Configure the explicit-null statement on PE-1.

Incorrect, because:

Explicit-null must be configured on the egress LSR (PE-2), not the ingress LSR (PE-1).

PE-1 only labels the traffic but does not control PHP behavior on P-1.

❌ C. Configure VPN prefix mapping for the PE-1_to_PE-2 LSP.

Incorrect, because:

VPN prefix mapping is used for mapping VPN routes to LSPs but does not solve the EXP bit issue.

The problem here is label removal (PHP), not route mapping.

❌ D. Set a static CoS value for the PE-1_to-PE-2 LSP.

Incorrect, because:

This does not preserve the original EXP bits, it only applies a static CoS value.

It’s a workaround, not a fix.

Final Answer: ✅ B. Configure the explicit-null statement on PE-2.

Key Takeaways

Penultimate Hop Popping (PHP) removes the outer MPLS label at P-1, which also removes the EXP bits used for CoS classification.

To keep EXP bits intact, configure explicit-null on the egress PE (PE-2).

This forces P-1 to send a label (0 for IPv4, 2 for IPv6) to PE-2, preserving the EXP bits for CoS classification.

Official Juniper Documentation Reference

???? Juniper MPLS CoS and PHP Behavior Guide

"To retain CoS EXP bits at the egress LSR, configure explicit-null on the egress PE. This prevents PHP from stripping the MPLS label before reaching the final PE router."

Understanding the Exhibit and the Problem Statement

The diagram shows a Layer 3 VPN (L3VPN) setup where multiple customers (Customer A, B, and C) are connected across a service provider network using PE (Provider Edge) routers.

PE1 and PE2 exchange VPN routes for all customers using BGP/MPLS Layer 3 VPN (L3VPN) routing.

The question asks how to verify if PE2 is receiving VPN routes for Customer C.

Evaluating the Answer Choices

✅ B. show route table bgp.l3vpn.0 (Correct Answer)

Why?

This command shows all VPN routes stored in the BGP Layer 3 VPN table (bgp.l3vpn.0).

Since PE routers exchange VPN routes using MP-BGP, this table contains the VPN-IPv4 or VPN-IPv6 routes for all customers.

If PE2 is receiving routes from PE1 for Customer C, they will appear in bgp.l3vpn.0.

❌ A. show route table inet.0 (Incorrect)

Why?

The inet.0 table contains global unicast routes for the service provider's network.

VPN routes do not appear here because they are stored in VRF-specific tables.

This command won’t help verify VPN route exchange between PE1 and PE2.

❌ C. show route table customer-c.inet.0 (Incorrect)

Why?

The table customer-c.inet.0 represents the VRF routing table for Customer C on the local PE.

It only shows locally installed routes for Customer C but does not confirm if PE2 is receiving routes from PE1.

This command is useful to check local VPN routing but not BGP route propagation.

❌ D. show route summary (Incorrect)

Why?

This command only provides a summary of route counts per protocol (BGP, OSPF, etc.).

It does not display specific VPN routes.

It is useful for general troubleshooting but doesn’t confirm VPN route receipt.

Final Answer: ✅ show route table bgp.l3vpn.0 (Option B)

Official Juniper Documentation Reference

Junos MPLS VPNs Configuration Guide???? Juniper Documentation

"The show route table bgp.l3vpn.0 command displays all VPN-IPv4 routes learned via MP-BGP for Layer 3 VPNs."

https://www.juniper.net/documentation/us/en/software/junos/bgp/topics/topic-map/load-balancing-bgp-session.html

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: