The highlighted text 10.20.30.40:1 represents the Route Distinguisher (RD). In BGP/MPLS IP VPNs, the RD is an 8-byte value prepended to a customer's non-unique IPv4 prefix (e.g., 192.168.1.0/24) to create a globally unique 12-byte address known as a VPN-IPv4 address. This process allows service providers to support multiple customers who may use overlapping private IP address spaces. The format shown, IP-address:number, is a standard representation for a Type 1 RD. The show router bgp routes vpn-ipv4 command on the Nokia 7750 SR displays these unique VPN-IPv4 prefixes, which are composed of the RD and the customer's original IPv4 prefix.

A. The LDP identifier is used to establish and maintain Label Distribution Protocol (LDP) sessions between routers and is not part of a BGP route prefix.

B. The Service identifier is used in the context of service transport tunnels (Service Distribution Paths - SDPs), not for distinguishing Layer 3 VPN routes.

C. The Route Target (RT) is a BGP extended community attribute that governs the import and export of routes into VRFs, but it is not prepended to the prefix itself.

E. The tunnel label is the MPLS label used for transport across the provider core. The BGP update carries a VPN label, but neither is represented by the RD.

1. Nokia 7750 SR OS Services Guide, Release 21.10.R1, "VPRN Services": In the section "VPN-IPv4 and VPN-IPv6 Address Formats", the guide states, "A VPN-IPv4 address is a 12-byte quantity, beginning with an 8-byte Route Distinguisher (RD) and ending with a 4-byte IPv4 address... The purpose of the RD is to allow the service provider to create distinct routes to the same IPv4 address prefix." This document confirms the structure and purpose of the RD as displayed in the command output.

2. IETF RFC 4364, "BGP/MPLS IP Virtual Private Networks (VPNs)": Section 4.2, "The VPN-IPv4 Address", defines a VPN-IPv4 address as a 12-octet quantity composed of an 8-octet Route Distinguisher (RD) and a 4-octet IPv4 address. It specifies that the RD's only purpose is to make IPv4 prefixes from different VPNs unique. The format IP-address:number corresponds to the Type 1 RD defined in this RFC.

3. Nokia 7750 SR OS Routing Protocols Guide, Release 21.10.R1, "BGP": The section on "Displaying BGP Routing Information" describes the output of commands like show router bgp routes. The format shown in the question, RD:prefix, is consistent with the command output examples for VPN-IPv4 routes provided in this guide.

In an MPLS VPN architecture, a P (Provider) router's role is to perform high-speed packet forwarding based on MPLS labels, a process known as label switching. When a customer packet traverses the provider core, it is encapsulated with at least one MPLS label (the transport label). The P router makes its forwarding decision solely based on this transport label, which identifies the path to the egress PE router. The P router never inspects the customer's IP packet header and is therefore completely unaware of the customer's IP routes. This design is fundamental to the scalability of MPLS VPNs, as it keeps the provider core free from carrying extensive customer routing information.

B. The ingress PE router must have a route to the customer's destination in the appropriate VRF to determine the BGP next-hop (egress PE) and assign the correct VPN label.

C. The egress PE router must have a route to the customer's destination in the appropriate VRF to perform the final IP lookup and forward the packet to the correct customer site.

D. This statement is incorrect because, as explained, both ingress and egress PE routers require specific customer routes to function, whereas P routers do not.

1. Nokia 7750 SR OS Services Architecture Guide, Release 22.10.R1, "VPRN Services" chapter, section "Packet Flow for a VPRN Service": This guide details the packet walkthrough. It states, "The P-router receives the packet and, based on the top label, it swaps the label and forwards the packet to the next-hop P-router or the egress PE-router." This confirms the P router operates only on the transport label, not the customer's destination IP address.

2. RFC 4364: BGP/MPLS IP Virtual Private Networks (VPNs), Section 4.1, "Forwarding": This foundational IETF standard specifies the architecture. It states, "The 'P-routers' (provider routers that are not PE-routers) are not aware of the VPN routes. They forward packets to the egress PE-router by switching labels. The ingress PE-router must know which label to use when sending a packet to an egress PE-router." (Page 7). This directly supports that P routers do not have customer routes.

3. Nokia Virtual Private Routed Network (VPRN) Configuration Example, Section "VPRN Overview": This document illustrates the roles, stating, "The P routers in the core of the provider's network are not aware of the VPN routes. When a P router receives a packet from a PE router, it uses the MPLS label to forward the packet to the proper PE router." This reinforces the concept of a customer-route-free core.

Inter-AS model C (RFC 4364 option C) only requires that each autonomous system have a transport MPLS tunnel of some sort (LDP, RSVP-TE, or SR). LDP itself is therefore optional.

What is mandatory is:

• inside each AS, VPN-IPv4/-IPv6 routes are carried by MP-iBGP (normally PE–RR or PE–PE);

• between ASBRs the labeled VPN routes are exchanged with MP-eBGP;

• on Nokia SR OS routers that act as ASBRs the command enable-inter-as-vpn must be configured to permit advertisement of VPN‐labeled NLRI across an external BGP session.

Thus, only statement B, which makes LDP compulsory, is false.

A. MP-iBGP is indeed needed within every AS for VPN-IPv4 route dissemination, so the statement is correct.

C. ASBRs must run MP-eBGP to exchange labeled VPN routes across the AS boundary; the statement is correct.

D. Nokia SR OS requires enable-inter-as-vpn on ASBRs for option C; the statement is correct.

1. Nokia SR OS 22.x VPRN Guide, “Inter-AS Option C,” section 9.4.2, pp. 604-606 – notes that intra-AS transport may be LDP or RSVP-TE and describes MP-iBGP & MP-eBGP requirements.

2. Nokia SR OS 22.x BGP Guide, “enable-inter-as-vpn command,” section 8.2.1, p. 331 – states the command is mandatory on ASBRs to advertise VPN-IPv4/6 NLRI over eBGP.

3. Rosen & Rekhter, RFC 4364, sec. 10 (Option C) – specifies only “some MPLS tunneling technology” inside each AS, not necessarily LDP.

4. MIT 6.829 Course Notes, “BGP/MPLS IP VPNs” (Fall 2020), pp. 12-13 – summarizes that any MPLS transport (LDP or RSVP) suffices for option C, highlighting MP-eBGP between ASBRs.

The configuration shown in the exhibit is both syntactically correct and complete for its intended purpose. In Nokia's SR OS, the as-override command can be configured at either the BGP group level or, for more granular control, at the BGP neighbor level. The exhibit correctly places the as-override command within the neighbor 10.1.1.1 context, which is a valid configuration hierarchy. This command instructs the PE router to replace the customer's AS number (65501) with its own AS number in BGP updates sent to that specific CE neighbor, effectively preventing the BGP loop-prevention mechanism from incorrectly dropping the routes.

B. 'as-override' should be configured under the BGP context.

This is incorrect. The as-override command is not a global BGP parameter; it is applied on a per-group or per-neighbor basis to control ASPATH modification for specific peers.

C. 'as-override' should be configured under the group context.

This is not mandatory. While as-override can be configured at the group level to apply to all neighbors within that group, configuring it at the neighbor level (as shown) is also valid and often preferred.

D. 'as-override' should not be part of this configuration.

This is incorrect. The question explicitly states the goal is to use the AS-override method to prevent BGP loops in a VPRN, which is the exact scenario this feature is designed for.

1. Nokia 7750 SR OS R20.10.R1 MPLS Guide, Section: VPRN BGP PE-CE Routing Protocol Support

Page 398, "AS Override": "To avoid this problem, the PE router can be configured to change the AS number of the customer site to its own AS number in the AS path of the BGP update. This is accomplished by configuring the as-override option on the PE router that is a neighbor of the CE router." This confirms the feature's purpose.

2. Nokia 7750 SR OS R20.10.R1 BGP Command Reference, Section: BGP Command Reference

Page 119, as-override: This section details the command syntax and context. It shows the command is available under config>router>bgp>group and config>router>bgp>group>neighbor. This confirms the configuration in the exhibit (neighbor level) is a valid context. The document states, "This command enables AS override on a per-group or per-neighbor basis."

The Carrier Supporting Carrier (CSC) VPRN is a service configured and managed by the backbone (or wholesale) service provider on its Provider Edge routers, known as CSC-PEs. These CSC-PEs connect to the customer carrier's routers, known as CSC-CEs. The CSC VPRN provides an MPLS-based transport service for the customer carrier, effectively acting as a "backbone-as-a-service." The customer carrier's CSC-CE router is a participant in the VPRN service provided by the backbone carrier; it does not host the CSC VPRN service configuration itself.

A. The primary business driver for the CSC model is to enable a customer carrier to offer services over a wide area without the capital expenditure of building its own MPLS core.

B. "Carrier's carrier" and "carrier serving carrier" are widely used industry synonyms that accurately describe the business relationship in a CSC model.

C. The customer carrier leverages the backbone transport provided by the CSC VPRN to connect its own points of presence, from which it delivers its own retail services (like Internet or VPNs) to its end-customers.

1. Nokia 7750 SR OS R21.10.R1 Layer 3 Services Guide: VPRN, Section: "Carrier Supporting Carrier (CSC)".

Reference for Correct Answer (D): The guide states, "A VPRN is created on the backbone provider's PE (CSC-PE) for the carrier customer." This explicitly places the CSC VPRN configuration on the backbone provider's equipment, not the customer carrier's.

Reference for Incorrect Option (A): The guide's introduction to CSC explains that it allows a carrier to "outsource its backbone" to another provider.

Reference for Incorrect Option (C): The guide describes the model: "The customer carrier can then use this backbone to offer its own services to its customers."

2. IETF RFC 4364: BGP/MPLS IP Virtual Private Networks (VPNs), Section 10: "Carrier's Carrier" VPNs.

Reference for Correct Answer (D) & Incorrect Option (B): This section, which provides the foundational standard for the "Carrier's Carrier" (CSC) model, describes the backbone carrier's routers as PEs and the customer carrier's routers as CEs. The VPN service is provided by the PEs to the CEs, confirming the configuration location and the terminology.

The statement is false. Route policies are a fundamental and mandatory component for controlling the exchange of routes between a customer's VRF (VPN Routing and Forwarding) instance and the provider's MP-BGP backbone. When BGP is used as the PE-CE protocol, the PE router learns standard IPv4 routes from the CE. An export policy must be applied on the PE to select which of these learned routes are redistributed into MP-BGP, converted into VPN-IPv4 prefixes, and advertised to other PE routers. This policy mechanism is critical for security, preventing route leaks, and applying specific attributes (like site-of-origin or other communities) to customer routes as they traverse the provider's core network. The use of BGP as the PE-CE protocol does not eliminate this requirement; in fact, it often necessitates more sophisticated policies to leverage BGP's rich attribute set.

The statement "True" is incorrect because it posits that using BGP as the PE-CE protocol obviates the need for route policies to control redistribution into MP-BGP. This is fundamentally wrong. The process of taking routes from a specific customer's routing domain and advertising them into a shared backbone always requires a policy control point to ensure proper route filtering and attribute manipulation, regardless of the PE-CE protocol in use.

1. Nokia 7750 SR OS Services Guide, Release 22.10.R1, Chapter: Virtual Private Routed Network Services, Section: VPRN BGP. The documentation explicitly describes the configuration process where export policies are applied to the VPRN BGP instance. It states, "Export policies are used for routes advertised to other BGP peers." This includes the MP-BGP peers in the core, confirming that policies are used to control the redistribution from the VRF's BGP table into the core MP-BGP advertisements. The configuration context config>service>vprn>bgp>export is the mechanism for applying this control.

2. RFC 4364 - BGP/MPLS IP Virtual Private Networks (VPNs), Section 4.3.2, "Exporting VPN-IPv4 Routes from VRFs to BGP." This foundational RFC directly addresses the process. It states: "The PE's decision to export a particular route from a particular VRF is a matter of policy." This confirms that a policy-based decision is an integral part of the architecture for deciding which routes learned from a CE are advertised to other PEs.

3. Nokia Data Center Fabric with SR Linux Cookbook, Section 3.3.2.2, "EVPN-VXLAN L3 service for inter-subnet forwarding." While discussing a different encapsulation (EVPN-VXLAN), the principle remains identical for L3VPN services. The guide shows the application of an export policy (from-ip-vrf-1) to the BGP instance within the VRF to control which routes are advertised into the EVPN fabric. This demonstrates the consistent requirement for export policies in Nokia's L3VPN service architecture. The configuration snippet shows: bgp { export-policy from-ip-vrf-1; } within the VRF context, reinforcing the necessity of policy control.

The statement is false. A fundamental feature of a VPRN (Virtual Private Routed Network) service, which is based on the IETF standard for BGP/MPLS IP VPNs (RFC 4364), is the ability to support different PE-CE (Provider Edge to Customer Edge) routing protocols at various sites belonging to the same customer VPRN.

The PE router acts as a boundary, performing route redistribution. It learns routes from a CE device using one protocol (e.g., OSPF at Site A), redistributes these routes into MP-BGP to be carried across the provider's MPLS core, and then at a remote PE router, redistributes them from MP-BGP into the local PE-CE protocol (which could be BGP, RIP, or even static routing at Site B). This architecture decouples the routing protocols used at each customer site, providing significant flexibility for both the service provider and the customer.

The "True" option is incorrect because it contradicts the core design and a primary advantage of MPLS L3VPN/VPRN services. Forcing a single PE-CE routing protocol across all sites would be a significant and unnecessary limitation, removing the flexibility that the MP-BGP-based architecture is designed to provide.

1. Nokia 7750 SR OS Services Guide, Release 22.10.R1, "VPRN services" chapter, Section: "PE-CE routing protocols". This section details the configuration of various routing protocols (BGP, OSPF, RIP, IS-IS) and static routing between PE and CE routers. The documentation describes configuring these protocols on a per-interface (per-site) basis within a single VPRN service, implicitly and explicitly demonstrating that different sites can use different protocols. For example, one sap can be in an OSPF area while another sap in the same VPRN service can have a BGP neighbor relationship configured.

2. Nokia Virtual Private Routed Network Service Student Guide, Course Number 4A0-106, Module 3: VPRN Routing. This official courseware explains the VPRN architecture. It explicitly states that the PE router can run different routing protocols with different CEs for the same VPRN. It details the process of route redistribution from a PE-CE protocol into MP-BGP and vice-versa, which is the mechanism that enables this flexibility.

3. Stallings, W. (2016). Foundations of Modern Networking: SDN, NFV, and Cloud Computing. Pearson Education, Inc. Chapter 11, Section 11.4 "MPLS Virtual Private Networks". While a general textbook, it is representative of university-level courseware. This chapter explains the architecture of RFC 4364 L3VPNs. It describes the role of the PE router in isolating the customer's routing from the provider's core and from other customer sites. It clarifies that the PE-CE protocol is a local matter for each site and that MP-BGP is used to stitch the sites together, allowing for heterogeneous PE-CE routing protocols.

In a Carrier Supporting Carrier (CSC) VPRN model, the ingress backbone provider edge router (PE1) receives packets from the customer carrier's edge router (CSC-CE1) that are already MPLS-labeled. CSC-CE1 applies the customer's own VPN label before forwarding the packet to PE1. PE1 then pushes two additional labels onto this already-labeled packet: a BGP/VPN label for the backbone VPRN to reach the egress PE (PE2) and a transport label (e.g., LDP) to traverse the backbone core. Therefore, the statement that PE1 pushes two labels onto the "IP packet" is false; it pushes them onto a labeled packet, resulting in a three-label stack.

A. CE1 forwards the data packet to PE1 via the VPRN 20 interface.

This statement is correct. The packet travels from CE1 to CSC-CE1, which then forwards it to PE1 over the access interface assigned to the backbone carrier's VPRN 20.

C. CSC-CE1 pops the LDP label and swaps the BGP label.

This statement is incorrect

The three standard models for implementing inter-AS VPRNs, as defined in RFC 4364, are Option A, Option B, and Option C. Options A, C, and D in the question correspond to these valid models. Option B describes the exchange of labeled IPv4 routes (BGP labeled-unicast address family) between route reflectors. This mechanism is used to establish transport MPLS Label Switched Paths (LSPs) across autonomous systems, not to exchange VPRN service routes. VPRN service connectivity requires the exchange of VPN-IPv4 routes (BGP vpn-ipv4 address family), which carry the necessary Route Distinguisher (RD) and Route Target (RT) information to maintain VPN separation and routing policies.

A. This describes Inter-AS VPRN Option B, a standard and widely used method where AS Border Routers (ASBRs) exchange VPN-IPv4 routes using MP-eBGP.

C. This describes Inter-AS VPRN Option A, also known as back-to-back VRF. It is a valid, albeit less scalable, implementation method.

D. This describes Inter-AS VPRN Option C, where Provider Edge (PE) routers in different autonomous systems establish a direct, multi-hop MP-eBGP session to exchange VPN-IPv4 routes.

1. Nokia 7750 SR OS Services Guide, Release 21.10.R1, "VPRN Services" chapter, Section: "Inter-AS VPRNs". This document explicitly details the three valid inter-AS VPRN models: Option A (Back-to-Back VRF at the ASBR), Option B (MP-eBGP between ASBRs for VPN-IPv4), and Option C (MP-eBGP between PEs for VPN-IPv4). This confirms that A, C, and D are viable options.

2. RFC 4364, "BGP/MPLS IP Virtual Private Networks (VPNs)", Section 10, "Multi-AS Backbones". This foundational RFC defines the three inter-AS VPN architectures.

Section 10a describes the VRF-to-VRF model (Option A/C).

Section 10b describes the MP-eBGP exchange of VPN-IPv4 routes between ASBRs (Option A/A).

Section 10c describes the multi-hop MP-eBGP between PEs in different ASes (Option D).

3. RFC 8277, "Using BGP to Bind MPLS Labels to Address Prefixes", Section 1, "Introduction". This RFC, which obsoletes RFC 3107, defines the BGP Labeled Unicast address family (AFI=1, SAFI=4). It clarifies that its purpose is to advertise an MPLS label for an IP prefix, which is a transport plane function for building LSPs, and is distinct from the VPN-IPv4 address family (AFI=1, SAFI=128) used for VPRN services. This supports why option B is not a method for implementing the VPRN service itself.

The statement is false because the eBGP session between a Carrier Supporting Carrier Provider Edge (CSC-PE) and a CSC Customer Edge (CSC-CE) does not exchange VPN-IPv4 routes. Instead, it exchanges standard IPv4 routes with an associated MPLS label (BGP-LU). The CSC-CE advertises its internal VPRN routes as labeled IPv4 prefixes to the CSC-PE. The CSC-PE then converts these labeled IPv4 routes into VPN-IPv4 routes for transport across the backbone provider's network. The correct configuration on the CSC-CE involves enabling label advertisement under the family ipv4 context, not using a family vpn-ipv4 context for this specific eBGP session.

B. This is a valid and often necessary configuration. If the customer carrier uses the same AS number at multiple sites, the BGP ASPATH loop detection mechanism on the CSC-PE would discard routes, requiring loop-detect off to be enabled.

C. This is the specific command required on a Nokia 7750 SR to enable CSC functionality for a VPRN service. It allows the VPRN to accept labeled IPv4 routes from the CSC-CE router.

D. This accurately describes the standard mechanism for route exchange between PE routers in an MPLS VPN backbone. The CSC-PEs (PE1 and PE2) use MP-iBGP with the family vpn-ipv4 address family to exchange the customer carrier's routes.

1. Nokia 7450 ESS, 7750 SR, 7950 XRS, and VSR Layer 3 Services Guide, Release 21.10.R1, Document Number: 3HE 17179 AAAD TQZZA.

Section: Carrier Supporting Carrier VPRN: "The CSC-PE router receives labeled IPv4 routes from the CSC-CE router and converts them into VPN-IPv4 routes. The VPN-IPv4 routes are then advertised over the backbone carrier’s network to the other CSC-PE routers." This confirms that the CSC-PE to CSC-CE link carries labeled IPv4, not VPN-IPv4, making statement A false.

Section: Carrier Supporting Carrier VPRN: "To configure a VPRN service as a CSC VPRN, the carrier-carrier-vpn command must be configured in the VPRN service." This confirms statement C is correct.

Section: Carrier Supporting Carrier VPRN: "To avoid BGP discarding routes with the same AS in the path, the loop-detect command can be configured on the CSC-PE to disable loop detection." This confirms statement B is correct.

The overall architecture described in the guide, consistent with RFC 4364, validates that CSC-PEs exchange routes among themselves using MP-iBGP with the vpn-ipv4 address family, confirming statement D.