350-501 Architecture — 10 Questions

Question 1

An engineer needs to ensure that a core router can forward MPLS packets without checking the IP header. Which forwarding mechanism should be enabled?

Accepted Answer

CEF switching. Cisco Express Forwarding (CEF) is the required switching mechanism for MPLS forwarding. When MPLS is enabled on a router, CEF uses the Label Forwarding Information Base (LFIB) to forward packets based solely on the top label, without inspecting the IP header. This allows core routers (P routers) to perform label switching without IP lookups. Penultimate Hop Popping (PHP) is an optimization that occurs at the penultimate router, but it is not the mechanism that enables label-based forwarding; it relies on CEF. Therefore, enabling CEF switching is necessary for a core router to forward MPLS packets without checking the IP header.

Answer

Link Fragmentation and Interleaving (LFI)

Answer

Per-packet load balancing

Answer

Penultimate Hop Popping (PHP)

Question 2

Which TWO of the following are benefits of using EVPN over traditional VPLS for L2VPN services?

Accepted Answer

EVPN uses BGP for control plane to advertise MAC addresses.. EVPN uses BGP for its control plane to advertise MAC addresses, whereas VPLS relies on data-plane flooding. Option C is correct: EVPN supports active-active multi-homing with load balancing, unlike VPLS which typically uses active/standby.

Answer

EVPN uses a single MAC address per Ethernet segment.

Answer

EVPN requires a full mesh of pseudowires between all PEs.

Answer

EVPN only supports L2 extension, not L3.

Question 3

Which TWO of the following are disadvantages of using VPLS for L2VPN services compared to EVPN?

Accepted Answer

MAC address learning via flooding across all PEs.. VPLS disadvantages compared to EVPN include: D) MAC address learning via flooding across all PEs, which consumes bandwidth and is inefficient; E) Limited to active/standby multi-homing in most implementations, whereas EVPN supports active/active multi-homing for better load balancing and redundancy.

Answer

Provides better scalability than EVPN.

Answer

Requires BGP control plane for signaling.

Answer

Supports only point-to-point service, not multipoint.

Question 4

A service provider wants to deploy network slicing for 5G services. Which THREE of the following are typical requirements for network slicing?

Accepted Answer

End-to-end orchestration and lifecycle management.. Network slicing for 5G requires end-to-end orchestration and lifecycle management (A) to coordinate across domains, dedicated core network functions per slice (C) such as UPF and SMF to provide customized services, and resource isolation (D) in compute, network, and storage to guarantee performance and security. Option B is incorrect because slices do not need separate BGP AS numbers; they can use VRFs or other mechanisms within a shared AS. Option E is incorrect because slices typically share physical infrastructure; dedicated fibers and routers per slice are not required and would be cost-prohibitive.

Answer

Each slice must use a different BGP autonomous system number.

Answer

Dedicated physical fibers and routers per slice.

Question 5

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 6

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

Answer

MPLS LDP is not enabled on the core routers

Answer

The tunnel source interface is not configured with an IP address

Question 7

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

Answer

VPN architecture

Answer

QoS architecture

Question 8

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 9

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

Answer

The tunnel interface is down

Answer

MPLS LDP is not configured

Question 10

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

Answer

VPLS

Answer

L3VPN