350-501 Architecture Practice Test 7 — 15 Questions

Question 1

A service provider is deploying a metro Ethernet service using MPLS. The customer requires a point-to-point Ethernet Virtual Circuit (EVC) that preserves VLAN tags between two sites. The service provider wants to use a scalable MPLS-based solution that supports MAC address learning and avoids full-mesh pseudowires. Which technology should be used?

Choose an answer to begin — your selection is scored in the full session.

Accepted Answer

EVPN-MPLS with a VLAN-based service interface. EVPN-MPLS with a VLAN-based service interface provides a scalable Ethernet service over MPLS, using BGP to distribute MAC addresses and avoiding full-mesh pseudowires. It can preserve VLAN tags between sites and supports the required point-to-point EVC. The other options either rely on full-mesh pseudowires, lack MAC learning, or do not scale as required.

Answer

EVPN-VPWS with a single Ethernet segment per EVI

Answer

VPLS with a full mesh of pseudowires between all PEs

Answer

VPWS with a point-to-point pseudowire between the two PEs

Question 2

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 3

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 4

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 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

Answer

The tunnel interface is down

Answer

MPLS LDP is not configured

Question 6

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 an MPLS L3VPN, each customer (or VPN) is mapped to a separate VRF on the PE router, which isolates customer routing tables and allows overlapping IP address spaces to coexist. Option E is correct because customer IP prefixes are distributed between PE routers as VPNv4 routes via MP-BGP (multiprotocol BGP) using route targets and route distinguishers to maintain VPN membership. Option A is incorrect because MPLS L3VPN traffic is not forwarded by label switching alone; PE routers perform IP lookup within the VRF and impose labels, and the first hop may be IP-based. Option C is incorrect because LDP is not strictly required in the core; label distribution can be provided by RSVP-TE, SR (Segment Routing), or other mechanisms. Option D is incorrect because P routers only maintain the global IGP routing table and forward labeled packets; they do not hold per-VPN routing tables, which reside on PE routers.

Answer

All traffic is forwarded using MPLS label switching only

Answer

LDP is required for label distribution in the core

Answer

The P routers maintain a full routing table for each VPN

Question 7

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 removes the need to run LDP: SR distributes segment identifiers (SIDs) via IGP extensions (IS-IS or OSPF) or BGP, so label distribution no longer depends on the separate LDP protocol. Option D is correct because SR encodes the path as an ordered list of SIDs in the packet header, letting the ingress node steer traffic through a predetermined path — a form of source routing. Option A is wrong because segment routing does not change BGP peering requirements; BGP-LU or BGP-based SID distribution may still be used. Option C is wrong because SR provides traffic engineering via SR-TE policies without requiring RSVP-TE, which is one of its key advantages. Option E is wrong because SR still relies on an IGP (IS-IS or OSPF) to advertise prefix SIDs and topology.

Answer

Reduces the number of BGP peers required

Answer

Requires RSVP-TE for traffic engineering

Answer

Eliminates the need for IGP

Question 8

Which THREE of the following are key components of an MPLS L3VPN architecture? (Choose three.)

Accepted Answer

VRF on PE routers. Option B is correct because a VRF (Virtual Routing and Forwarding) instance on PE routers is the fundamental construct that isolates each customer's routing table and allows overlapping address spaces in an MPLS L3VPN. Option C is correct because LDP (Label Distribution Protocol) is the standard mechanism used to distribute labels for the transport (core) LSPs between P and PE routers, enabling label switching of VPN traffic across the provider backbone. Option E is correct because MP-BGP (Multiprotocol BGP) with the VPNv4 address family is the protocol used to exchange customer VPN routes between PE routers, carrying the route distinguisher and route target extended communities. Option A is not a key component of a basic L3VPN architecture; RSVP-TE is an optional traffic-engineering mechanism for MPLS, not required for VPN operation. Option D is incorrect because P routers only switch labeled packets and do not maintain customer VPN routing tables — that is the role of the PE routers.

Answer

RSVP-TE for traffic engineering

Answer

P routers with full VPN routing tables

Question 9

Refer to the exhibit. A network engineer configures a BGP route-map to set communities on routes advertised to a neighbor. After applying the configuration, the engineer checks the BGP table on the neighbor router and does not see the communities. What is the most likely reason?

Accepted Answer

The neighbor is missing the 'send-community' command. C is correct because BGP communities are not sent to a neighbor by default. Even if a route-map sets the community values correctly, the neighbor will not receive them unless the 'send-community' command is configured under the neighbor statement. This command enables the advertisement of the community attribute in BGP updates.

Answer

The prefix-list does not match the exact prefix

Answer

The community values are not in the format 'AA:NN'

Answer

The route-map needs to be applied inbound

Question 10

Refer to the exhibit. An engineer configures MPLS LDP on a router. The router has two interfaces with IP addresses 10.0.0.1/30 and 10.0.0.5/30. The engineer notices that LDP sessions are not established. The OSPF neighbor adjacencies are up. What is the most likely cause?

Accepted Answer

LDP is not enabled on the interfaces. D is correct because MPLS LDP requires explicit interface-level activation under the `mpls ip` command. Even if OSPF adjacencies are up and the global LDP process is configured, LDP will not form sessions on interfaces where `mpls ip` is missing. The engineer likely enabled LDP globally but forgot to enable it on the specific interfaces, which is a common oversight.

Answer

The OSPF network statements do not cover the interfaces correctly

Answer

The MPLS MTU is set to 1500, which is too low

Answer

The label range is too small

Question 11

A service provider is redesigning its MPLS core to support both L3VPN and L2VPN services. They want to minimize the number of labels in the forwarding table while maintaining per-VRF label allocation. Which label allocation mode should be used for the L3VPN service?

Accepted Answer

Per-VRF label allocation. Per-VRF label allocation assigns a single MPLS label per VRF, meaning all routes within a VRF share the same label. This minimizes the number of labels in the forwarding table compared to per-prefix allocation, while still maintaining per-VRF isolation for L3VPN services. It is the recommended mode when the goal is to reduce label consumption without sacrificing VPN separation.

Answer

Per-CE label allocation

Answer

Per-prefix label allocation

Answer

Per-instance label allocation

Question 12

A service provider is deploying Segment Routing over IPv6 (SRv6) and needs to ensure that the forwarding plane can support SRv6 encapsulated packets. Which hardware capability is most critical for SRv6 at the line rate?

Accepted Answer

IPv6 extension header processing. SRv6 (Segment Routing over IPv6) encapsulates packets with an IPv6 header containing a Segment Routing Header (SRH), which is a type of IPv6 extension header. For line-rate forwarding, the hardware must natively process IPv6 extension headers in the forwarding plane without punting to the CPU, as software processing would cause performance degradation. Option C is correct because this capability is the most critical for SRv6 at line rate.

Answer

MPLS label swap capability

Answer

VXLAN tunnel termination

Answer

NAT64 translation support

Question 13

A network engineer is troubleshooting an OSPF issue in a service provider network. The network uses OSPFv2 with multiple areas. Routers in area 0 are able to ping each other, but routers in area 1 cannot reach routers in area 2. All routers are OSPF-enabled and have correct network type configurations. What is the most likely cause?

Accepted Answer

Missing or misconfigured ABRs between area 1 and area 0, or area 2 and area 0. In OSPFv2, inter-area communication must traverse area 0 (the backbone). Routers in area 1 and area 2 can only reach each other if Area Border Routers (ABRs) exist between each non-backbone area and area 0, and those ABRs are correctly configured and have full adjacencies. Since area 0 routers can ping each other but area 1 and area 2 cannot, the most likely cause is a missing or misconfigured ABR on either side, preventing the exchange of Type 3 summary LSAs between the areas.

Answer

Mismatched OSPF area IDs on routers in area 1 and area 2

Answer

Lack of BGP peering between ABRs

Answer

OSPF network type mismatch preventing adjacency formation

Question 14

A service provider is implementing MPLS-TE with FRR (Fast Reroute) using one-to-one backup tunnels. After activation, they notice that the number of labels in the forwarding table has increased significantly. What is the most likely reason for this increase?

Accepted Answer

One-to-one backup creates a unique backup tunnel for each protected LSP. One-to-one backup tunnels in MPLS-TE FRR create a dedicated backup LSP for each protected primary LSP. This means every primary LSP gets its own unique set of labels for the backup path, leading to a significant increase in the number of labels in the forwarding table. In contrast, facility backup shares a single bypass tunnel among multiple protected LSPs, which conserves labels.

Answer

RSVP-TE signaling is not properly aggregating labels

Answer

Penultimate Hop Popping (PHP) is disabled

Answer

Facility backup is being used instead of one-to-one

Question 15

Which TWO statements about MPLS label operations in a service provider core are correct? (Choose two.)

Accepted Answer

The ingress router pushes a label stack onto the IP packet. Option A is correct because the ingress LSR (ingress PE) is responsible for imposing (pushing) the MPLS label stack onto the incoming IP packet, encapsulating it for transport across the MPLS core. Option D is correct because the penultimate hop router (PHP) typically pops the top label before forwarding the packet to the egress router, a behavior known as penultimate hop popping (PHP), which offloads the final label lookup from the egress LSR. Option B is incorrect because the egress router normally receives a packet with a single label (or an implicit null label), not two labels by default. Option C is incorrect because the penultimate hop's default behavior is to pop the top label, not swap it; label swapping occurs at intermediate transit LSRs. Option E is incorrect because PHP is enabled by default in MPLS networks, not disabled.

Answer

The egress router receives an MPLS packet with two labels by default

Answer

The penultimate hop performs label swap for the top label

Answer

The penultimate hop is disabled by default in MPLS networks