350-501 Architecture — 10 Questions

Question 1

A service provider is designing a new MPLS L3VPN service. The customer requires that their VPN traffic be isolated from other customers and that the provider edge routers maintain separate routing tables for each VPN. Which architectural component is essential for this separation?

Accepted Answer

VRF (Virtual Routing and Forwarding). VRF (Virtual Routing and Forwarding) is the essential architectural component that enables a provider edge router to maintain separate, isolated routing tables for each VPN customer. Each VRF instance contains its own routing table, CEF (Cisco Express Forwarding) table, and associated interfaces, ensuring that traffic from one customer is never forwarded using another customer's routing information. This per-VPN isolation is fundamental to MPLS L3VPN services as defined in RFC 4364.

Answer

MPLS label stacking

Answer

VLAN tagging on the customer-facing interfaces

Answer

BGP route reflectors

Question 2

An operator notices that a new MPLS-TE tunnel is not being established. The tunnel configuration includes a dynamic path option and a bandwidth of 100 Mbps. The network uses RSVP-TE with CSPF. The link-state database shows sufficient reservable bandwidth on all links along the calculated path. What is the most likely cause of the tunnel establishment failure?

Accepted Answer

RSVP-TE is not enabled on the transit interfaces. RSVP-TE must be explicitly enabled on every interface that will participate in MPLS-TE label-switched path (LSP) signaling. Even if the link-state database shows sufficient reservable bandwidth, without RSVP-TE enabled on transit interfaces, the PATH messages cannot be processed, and the tunnel will fail to establish. This is a common misconfiguration when deploying MPLS-TE.

Answer

The path option is misconfigured with a strict explicit path

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MPLS LDP is not enabled on the core routers

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The tunnel source interface is not configured with an IP address

Question 3

Which network architecture model separates the control plane and data plane in a way that allows for centralized control and distributed forwarding?

Accepted Answer

SDN architecture. Software-Defined Networking (SDN) architecture explicitly separates the control plane from the data plane, centralizing network intelligence in a controller while leaving packet forwarding to distributed switches and routers. This decoupling enables programmatic, centralized control of the network, with the controller (e.g., OpenDaylight, Cisco APIC) making forwarding decisions and pushing flow entries to devices via protocols like OpenFlow or OpFlex. The result is a logically centralized brain with physically distributed forwarding, which is the defining characteristic of SDN.

Answer

MPLS architecture

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VPN architecture

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QoS architecture

Question 4

A service provider is troubleshooting a BGP route advertisement issue. Routes from a customer are not being advertised to the upstream provider. The PE router is configured with 'neighbor 10.0.0.1 route-map RMAP out'. The route-map RMAP permits the customer prefix. However, the BGP table on the PE shows the prefix as valid but not advertised. What is a likely cause?

Accepted Answer

The next-hop is not reachable from the upstream provider. The BGP table shows the prefix as valid but not advertised, which indicates that BGP has the route but is not sending it to the upstream neighbor. A common cause is that the next-hop for the customer prefix is not reachable from the PE router via the interface used to reach the upstream provider. BGP will not advertise a route if the next-hop is not reachable in the routing table (unless 'neighbor x.x.x.x next-hop-self' is configured), because the upstream router would be unable to forward traffic to that next-hop.

Answer

The BGP session to the upstream provider is flapping

Answer

The prefix is not in the global routing table

Answer

The route-map is applied inbound instead of outbound

Question 5

In a carrier's network, MPLS-TE tunnels are used to steer traffic away from congested links. The operator configures a tunnel with a bandwidth of 200 Mbps and a dynamic path. The CSPF computation shows a path with sufficient bandwidth, but the tunnel fails to come up. The RSVP neighbor is established. What is the most likely cause?

Accepted Answer

The path message is rejected due to resource reservation failure. The tunnel fails to come up because the CSPF-computed path cannot reserve the requested 200 Mbps bandwidth. Even though the path has sufficient bandwidth on paper, the RSVP resource reservation process may fail due to a lack of available bandwidth at the exact time of reservation, or due to a mismatch in bandwidth pool configuration (e.g., global vs. subpool). Since the RSVP neighbor is established, the issue is specifically with the reservation step, not with reachability or LDP.

Answer

The tunnel destination is not reachable via IGP

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The tunnel interface is down

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MPLS LDP is not configured

Question 6

A service provider wants to offer Layer 2 VPN services using MPLS. Which technology should be used to transport Ethernet frames across the MPLS core?

Accepted Answer

Pseudowire. Pseudowire (A) is the correct technology because it provides a point-to-point Layer 2 circuit over an MPLS core, allowing Ethernet frames to be encapsulated and transported transparently. This is defined in RFC 4448 (Ethernet over MPLS) and enables service providers to offer E-Line services. Pseudowire uses MPLS labels to forward frames across the core without requiring the core routers to participate in the customer's Layer 2 control plane.

Answer

LDP

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VPLS

Answer

L3VPN

Question 7

An engineer is designing an MPLS L3VPN solution and must ensure that the provider edge (PE) routers can handle routing updates for multiple customers without interfering with each other. Which mechanism should be used on the PE routers?

Accepted Answer

VRF-Lite. VRF-Lite (Virtual Routing and Forwarding) is the correct mechanism because it allows a single PE router to maintain multiple separate routing tables (VRFs), each dedicated to a different customer. This ensures that routing updates for one customer are isolated from another, preventing interference. VRF-Lite achieves this without MPLS, using only IP forwarding and per-VRF routing instances.

Answer

Route distinguishers

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Route reflectors

Answer

MPLS-TE

Question 8

A network operator is deploying segment routing in an MPLS network. They want to use a centralized controller to compute paths based on traffic demand and network constraints. Which architecture is being used?

Accepted Answer

Segment Routing Path Computation Element (SR-PCE). The scenario describes a centralized controller computing paths based on traffic demand and network constraints, which is the definition of a Path Computation Element (PCE) architecture. In Segment Routing, the SR-PCE (Segment Routing Path Computation Element) is the centralized controller that calculates optimal paths using traffic engineering constraints and then communicates the path information (via PCEP) to the headend router. This is distinct from distributed control plane approaches like LDP or SR-BE, and from SR-TE which is the overall traffic engineering mechanism but not the specific centralized controller architecture.

Answer

LDP-based MPLS

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Segment Routing Traffic Engineering (SR-TE)

Answer

Segment Routing Best Effort (SR-BE)

Question 9

Which TWO of the following are characteristics of MPLS L3VPN architecture? (Choose two.)

Accepted Answer

Each customer has a separate VRF on the PE router. Option B is correct because in MPLS L3VPN, each customer is assigned a separate Virtual Routing and Forwarding (VRF) instance on the Provider Edge (PE) router. This VRF maintains a unique routing table and forwarding table per customer, ensuring complete isolation between different VPN customers on the same PE.

Answer

All traffic is forwarded using MPLS label switching only

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LDP is required for label distribution in the core

Answer

The P routers maintain a full routing table for each VPN

Question 10

Which TWO of the following are advantages of segment routing over traditional LDP-based MPLS? (Choose two.)

Accepted Answer

Eliminates the need for the LDP protocol. Option B is correct because segment routing (SR-MPLS) eliminates the need for the LDP protocol by encoding MPLS labels directly in the IGP (e.g., OSPF or IS-IS) using extensions like RFC 8665 and RFC 8667. This simplifies the control plane by removing an entire protocol (LDP) and its associated state, reducing operational complexity and convergence time.

Answer

Reduces the number of BGP peers required

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Requires RSVP-TE for traffic engineering

Answer

Eliminates the need for IGP