350-501 Practice Questions 2 — 50 Questions

Question 1

In EVPN, what is the function of the Ethernet Segment Identifier (ESI)?

Accepted Answer

It identifies a multi-homed device or network segment.. The ESI identifies a multi-homed device or network, enabling all-active redundancy and fast convergence.

Answer

It is used for load balancing among PEs.

Answer

It uniquely identifies a customer MAC address.

Answer

It identifies a VXLAN tunnel.

Question 2

Which MEF Carrier Ethernet service type is used for a rooted-multipoint topology, where a single root site communicates with multiple leaf sites but leaf sites cannot communicate directly?

Accepted Answer

E-Tree. E-Tree (Ethernet Tree) is a rooted-multipoint service where the root can communicate with leaves, but leaves cannot communicate with each other.

Answer

E-Access

Answer

E-LAN

Answer

E-Line

Question 3

In IOS XR, what is the commit/rollback configuration model?

Accepted Answer

Configuration changes are staged and committed atomically; rollback is supported.. The commit/rollback model applies changes atomically and allows reverting to previous configurations if needed.

Answer

It requires a reboot to apply changes.

Answer

Changes are applied immediately and cannot be undone.

Answer

It only applies to interface configurations.

Question 4

Which THREE of the following are benefits of using EVPN over traditional VPLS for L2VPN services? (Select three.)

Accepted Answer

Integrated L2 and L3 services (e.g., IRB). EVPN offers BGP control plane, multi-homing with ESI, and integrated L2/L3 services.

Answer

Supports only point-to-point pseudowires

Answer

Eliminates the need for VPLS

Question 5

Which architecture feature of IOS XR ensures that a failure in one routing protocol process does not affect other processes on the same router?

Accepted Answer

Process separation with each protocol as a separate process. IOS XR's process separation model runs each routing protocol as an independent process with its own protected memory space. If one protocol process fails (e.g., OSPF crashes), the other processes (BGP, ISIS) continue running unaffected, providing fault isolation and preventing a single protocol failure from cascading across the router.

Answer

Admin plane vs default SDR

Answer

Commit/rollback configuration model

Answer

CPU and memory allocation per card

Question 6

Which MEF service type is used for multipoint-to-multipoint Ethernet connectivity, such as a corporate LAN extension across multiple sites?

Accepted Answer

E-LAN. E-LAN (Ethernet LAN) is the MEF service type that provides multipoint-to-multipoint Ethernet connectivity, allowing any site to communicate with any other site as if they were on the same LAN. It is the standard choice for corporate LAN extension across multiple locations. E-Line is point-to-point and E-Tree is rooted multipoint, so neither matches the multipoint-to-multipoint requirement.

Answer

E-Line

Answer

E-Tree

Answer

E-Access

Question 7

An SP uses Cisco ASR 9000 routers with IOS XR. Which statement correctly describes the commit/rollback configuration model in IOS XR?

Accepted Answer

Changes are staged in a candidate configuration and applied only after 'commit'; rollback to a previous commit is possible.. IOS XR uses a two-stage configuration model: changes are made in a candidate configuration and only take effect after the 'commit' command. The system maintains a rollback buffer of previous commits, allowing rollback to any of the last 10 committed configurations using 'rollback configuration' or 'load rollback changes'. This provides safe, transactional configuration changes.

Answer

Only 'commit replace' is supported; rollback requires manual re-entry.

Answer

The router automatically commits changes every 10 minutes.

Answer

Changes take effect immediately without commit; rollback is not supported.

Question 8

An SP network uses OSPF with RSVP-TE for traffic engineering. Which OSPF extension is required to advertise TE link parameters such as maximum reservable bandwidth?

Accepted Answer

OSPF Type 10 Opaque LSAs. OSPF TE extensions are defined in RFC 3630, which introduces the Opaque LSA (Type 9, 10, 11). Specifically, Type 10 Opaque LSAs carry TE information. The TE LSA is carried in an Opaque LSA of area scope (Type 10).

Answer

OSPF Type 7 LSAs

Answer

OSPF Type 11 Opaque LSAs

Answer

OSPF Type 5 LSAs

Question 9

A service provider wants to automate service provisioning using NSO. Which protocol is typically used by NSO to configure network devices?

Accepted Answer

NETCONF. Cisco NSO (Network Services Orchestrator) uses NETCONF as its primary southbound protocol to configure network devices, leveraging YANG data models for structured, transactional configuration. NETCONF's candidate/commit model aligns with NSO's intent-based automation approach.

Answer

CLI scripting

Answer

SNMP

Answer

RESTCONF

Question 10

An SP uses SR-PCE for traffic engineering. Which protocol does the SR-PCE use to communicate path computation requests and responses with path computation clients (PCCs)?

Accepted Answer

PCEP. PCEP (Path Computation Element Protocol) is used between PCCs (e.g., routers) and a PCE (e.g., SR-PCE) for path computation requests and responses.

Answer

BGP-LS

Answer

NETCONF

Answer

RESTCONF

Question 11

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 12

An engineer configures an RSVP-TE tunnel with a bandwidth of 100 Mbps and an explicit path that excludes a specific interface. The tunnel fails to come up. Which term describes the path computation method used by RSVP-TE to find a feasible path given these constraints?

Accepted Answer

CSPF (Constrained Shortest Path First). RSVP-TE uses Constrained Shortest Path First (CSPF) to compute a path that satisfies constraints such as bandwidth and explicit exclusions. CSPF is an extension of SPF that takes into account additional constraints like available bandwidth, link attributes, and administrative groups. Therefore, the path computation method is CSPF.

Answer

SPF (Shortest Path First)

Answer

Bellman-Ford

Answer

Dijkstra with ECMP

Question 13

In an MPLS L3VPN network, a customer has overlapping IP addresses between two VPNs. The provider edge routers are configured with VRF instances. Which method ensures that traffic from one VPN does not leak into the other when using MPLS labels?

Accepted Answer

Use distinct route targets for import and export per VRF.. Route targets (RTs) control the import and export of VPNv4 routes between VRFs on a PE router. By assigning distinct import and export RTs per VRF, the PE ensures that routes from one VPN are not installed into the routing table of another VPN, even if the customer uses overlapping IP addresses. MPLS labels are allocated per VRF forwarding instance, but the isolation is enforced by the RT-based filtering of BGP VPNv4 routes, not by label ranges or route distinguishers alone.

Answer

Assign different MPLS label ranges to each VRF.

Answer

Apply BGP community strings to filter routes.

Answer

Configure different route distinguishers for each VPN.

Question 14

An SP is deploying MPLS in the core/backbone to support L3VPN services. What is the purpose of MP-BGP in an MPLS VPN architecture?

Accepted Answer

To exchange customer VPN routes and VPN labels between PE routers. MP-BGP (Multiprotocol BGP) is used in MPLS L3VPNs to exchange VPN-IPv4 routes and their associated MPLS labels (VPN labels) between PE routers. It carries the route target extended communities that control import/export of routes into VRFs, enabling the PE routers to learn customer routes and the labels needed to forward traffic to the correct CE. Without MP-BGP, PEs would not have a scalable way to distribute customer routes across the provider core.

Answer

To forward data packets across the MPLS core

Answer

To establish IGP adjacencies between PE and CE routers

Answer

To distribute MPLS labels for LSPs

Question 15

A service provider is designing a new MPLS core network and wants to simplify traffic engineering without deploying a centralized controller. Which technology should be used to enable source-routed paths with explicit path control?

Accepted Answer

Segment Routing (SR-MPLS). Segment Routing (SR-MPLS) enables source-routed paths with explicit path control by encoding the path as a list of segment identifiers (SIDs) in the packet header, allowing the ingress router to steer traffic along a specific path without a centralized controller. This simplifies traffic engineering in an MPLS core network by eliminating the need for per-flow signaling and state maintenance on intermediate nodes, which is exactly what the question requires.

Answer

BGP-LU

Answer

LDP

Answer

RSVP-TE

Question 16

In the context of SR-PCE, which protocol does the PCE use to communicate with the path computation client (PCC) to set up SR-TE policies?

Accepted Answer

PCEP. PCEP (Path Computation Element Communication Protocol) is the standard protocol used between a PCE and a Path Computation Client (PCC) to request and receive path computations and to set up SR-TE policies. In SR-PCE deployments, the PCE uses PCEP to download the segment list to the head-end router, which instantiates the SR-TE policy.

Answer

NETCONF

Answer

OSPF

Answer

BGP-LS

Question 17

An engineer is deploying Segment Routing in an MPLS network. To ensure that routers can forward packets based on SR-MPLS labels without requiring LDP, which requirement must be met?

Accepted Answer

IS-IS or OSPF must have the SR extension enabled.. Segment Routing (SR) can operate without LDP by encoding the forwarding path as a list of MPLS labels derived from the IGP. For routers to allocate and advertise these SR-MPLS labels, the IGP (IS-IS or OSPF) must have the Segment Routing extension enabled. This allows the IGP to distribute prefix-SIDs (labels) that routers use to forward packets, eliminating the need for LDP.

Answer

MPLS LDP must be enabled on all interfaces.

Answer

All routers must run BGP-LU.

Answer

The IGP must have the overload bit set.

Question 18

A service provider is designing a core network with IS-IS as the IGP. To support MPLS traffic engineering, which IS-IS extensions are required?

Accepted Answer

IS-IS TE extensions (RFC 5305). IS-IS TE extensions (RFC 5305) are required to carry MPLS Traffic Engineering information, such as link bandwidth, administrative groups, and TE metric, in Type-Length-Value (TLV) fields within IS-IS Link State PDUs. These extensions enable the IGP to flood TE attributes that MPLS TE uses for constraint-based path computation, such as with RSVP-TE or Segment Routing.

Answer

IS-IS multi-topology (MT)

Answer

IS-IS wide metrics

Answer

IS-IS L1/L2 routing

Question 19

A service provider is designing an OSPF network with multiple areas. Which TWO area types allow external routes (Type 5 LSAs) to be injected into the area? (Choose two.)

Accepted Answer

Standard area. Option B (Standard area) is correct because a standard OSPF area accepts all LSA types, including Type 5 external LSAs originated by ASBRs, so external routes can be injected and flooded throughout it. Option D (Backbone area, Area 0) is correct because the backbone is a normal (non-stub) area that permits Type 5 LSAs; in fact, external routes are typically injected at an ASBR and propagated across Area 0 to other areas. The unmarked options do not belong: an NSSA (A) blocks Type 5 LSAs and instead uses Type 7 LSAs that are translated to Type 5 only at the ABR, a stub area (C) blocks Type 5 LSAs entirely, and a totally stubby area (E) blocks Type 5 LSAs as well as Type 3 and Type 4 LSAs, relying solely on a default route.

Answer

NSSA

Answer

Stub area

Answer

Totally stubby area

Question 20

An engineer is troubleshooting an SR-MPLS TE tunnel that is not coming up. The headend router is configured with an explicit candidate path that uses a segment list containing SIDs 16010, 16020, and 16030. The show segment-routing traffic-eng policy command indicates that the policy is down. Which of the following is the most likely cause?

Accepted Answer

One or more of the SIDs in the segment list are not reachable in the IGP, so the path cannot be resolved.. An SR-TE policy with an explicit segment list requires all SIDs in the list to be reachable via the IGP (or BGP-LS). If any SID is missing or unreachable, the headend cannot resolve the path, and the policy remains down. Troubleshooting should focus on verifying that each SID in the segment list is advertised and reachable, using show isis segment-routing prefix-sid or similar commands. Once the missing SID is resolved, the policy should come up.

Answer

The headend router does not have a BGP session with the other routers to learn the SIDs.

Answer

The policy is configured with a color that does not match any BGP color community.

Answer

The SRGB is not configured on the headend router.