350-501 Practice Exam 51 — 20 Questions

Question 1

Refer to the exhibit. CE1 is unable to ping the loopback of a remote CE. Which configuration change is required on PE1 to enable communication?

Accepted Answer

Apply the VRF to the interface facing CE and ensure BGP VRF neighbor is activated. The configuration is missing the VRF forwarding statement on the interface facing CE1 and the BGP VRF neighbor activation under the VRF address-family. Without applying the VRF to the interface, PE1 cannot associate the CE-facing interface with the correct VRF, and without activating the BGP neighbor under the VRF address-family, PE1 will not establish a BGP session with CE1 or exchange routes for that VRF. This prevents CE1 from learning the remote CE's loopback route.

Answer

Add `send-community extended` to the VRF BGP neighbor

Answer

Change the VPNv4 neighbor to point to 192.168.1.2

Answer

Set the CE neighbor remote-as to 65000

Question 2

Based on the exhibit, which OSPF network type is configured on this interface?

Accepted Answer

Point-to-Point. The OSPF network type is determined by the interface configuration. The 'ip ospf network point-to-point' command explicitly sets the interface to point-to-point mode, which eliminates the need for DR/BDR election and uses multicast hello packets (224.0.0.5) to neighbors. This is the correct answer because the exhibit shows this exact configuration.

Answer

Non-Broadcast

Answer

Broadcast

Answer

Point-to-Multipoint

Question 3

An SP is designing a MPLS L3VPN service with BGP as the PE-CE routing protocol. They want to ensure that the CE router does not become a transit router between two sites. Which BGP feature should be configured on the PE to prevent the CE from advertising routes received from one site to another?

Accepted Answer

Site of Origin (SoO). The Site of Origin (SoO) BGP community is the correct feature because it prevents routing loops in MPLS L3VPN environments where the same CE router connects to multiple sites. When configured on the PE, SoO tags routes received from a CE with a unique identifier for that site; if the PE receives a route with the same SoO value from another site, it discards the route, ensuring the CE does not become a transit router between sites.

Answer

allowas-in

Answer

disable-connected-check

Answer

as-override

Question 4

A service provider is implementing Segment Routing over IPv6 (SRv6) in their core. They want to provide path isolation for different services using SRv6 SIDs. Which SID structure is used to encode both the locator and the function?

Accepted Answer

The SID is divided into locator and function (and optionally arguments).. In SRv6, the SID is encoded as a 128-bit IPv6 address that is explicitly divided into three parts: the Locator (identifies the node or prefix), the Function (identifies the specific behavior or instruction), and optionally Arguments (additional parameters). This structure allows path isolation by assigning unique locator/function combinations per service, enabling per-service steering without additional label stacks.

Answer

The SID uses a separate label stack for function encoding.

Answer

The SID is an IPv6 address without any encoding.

Answer

The SID consists of a prefix only.

Question 5

A service provider is migrating from LDP to Segment Routing in its MPLS core. The team has enabled IS-IS as the IGP and configured segment routing under the IS-IS process on all core routers. However, after the migration, some LSPs are not being signaled correctly and traffic is blackholing. Which action should be taken to ensure seamless interworking between LDP and SR during the migration?

Accepted Answer

Configure 'segment-routing mpls sr-prefer' under the IS-IS process.. The 'segment-routing mpls sr-prefer' command under IS-IS configures the router to prefer SR labels for forwarding while maintaining LDP forwarding entries for fallback. This ensures seamless interworking between LDP and SR during migration, preventing blackholing when some routers are not fully SR-capable. Option B is incorrect because changing the IGP to OSPF is unnecessary and disruptive; IS-IS can support both LDP and SR simultaneously. Option C is incorrect because removing LDP from all routers would break LDP signaling for any router not yet migrated to SR, causing traffic loss. Option D is incorrect because 'mpls ldp igp sync' synchronizes LDP with IGP convergence, not SR-LDP interworking, and does not address label preference during coexistence.

Answer

Change the IGP to OSPF with segment routing enabled.

Answer

Remove the 'mpls ldp' configuration from all routers that have SR enabled.

Answer

Enable 'mpls ldp igp sync' on the IGP interfaces.

Question 6

In Segment Routing, what is the role of the 'Prefix-SID'?

Accepted Answer

It identifies a prefix in the IGP. A Prefix-SID is a Segment Routing identifier bound to an IGP prefix. It allows forwarding along the shortest path computed by the IGP to that prefix. Therefore, it identifies a prefix in the IGP, not a specific adjacency or node.

Answer

It identifies a specific adjacency

Answer

It is used for service chaining

Answer

It identifies a specific node

Question 7

Which three mechanisms are used within the DiffServ model to provide QoS in an SP network? (Choose three.)

Accepted Answer

Classification and marking at the edge using DSCP. In the DiffServ model, QoS is implemented by classifying and marking packets at the network edge using the DSCP field in the IP header. This marking determines how each packet is treated within the core network, enabling scalable, class-based service differentiation without per-flow state.

Answer

End-to-end path computation via PCEP

Answer

IntServ resource reservation via RSVP

Question 8

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 9

Refer to the exhibit. Which prerequisite for MPLS L3VPN is missing on PE1?

Accepted Answer

VRF is not defined. The exhibit shows that PE1 has an interface configured with an IP address and MPLS enabled, but no VRF definition is present. For MPLS L3VPN, a VRF must be defined on the PE router to separate customer routing tables and to associate the customer-facing interface with that VRF. Without the VRF, the PE cannot import/export VPN routes or maintain per-VPN routing instances, which is a fundamental prerequisite for L3VPN operation.

Answer

MPLS is not enabled on the interface

Answer

LDP is not enabled globally

Answer

OSPF is not redistributed into BGP

Question 10

A service provider is designing a Layer 2 MPLS VPN to connect multiple customer sites. Which technology should be used to provide end-to-end Ethernet service with MAC address transparency and support for multiple customer VLANs?

Accepted Answer

VPLS (Virtual Private LAN Service). VPLS (Virtual Private LAN Service) is the correct choice because it emulates an Ethernet bridge across an MPLS core, providing MAC address transparency and supporting multiple customer VLANs through a multipoint-to-multipoint topology. Unlike point-to-point services, VPLS learns MAC addresses and forwards frames based on them, enabling end-to-end Ethernet service across distributed sites.

Answer

MPLS Traffic Engineering (MPLS TE)

Answer

VPWS (Virtual Private Wire Service)

Answer

RSVP-TE signaling

Question 11

Which BGP address family must be used to exchange VPNv4 routes between PE routers in an MPLS L3VPN?

Accepted Answer

VPNv4 unicast. In an MPLS L3VPN, VPNv4 routes carry both the IPv4 prefix and the Route Distinguisher (RD) to ensure uniqueness across overlapping customer address spaces. The VPNv4 unicast address family (address-family ipv4 vpn) is the mandatory BGP address family used between PE routers to exchange these VPNv4 routes, enabling MPLS-based VPN reachability.

Answer

RT constraint

Answer

IPv4 unicast

Answer

L2VPN VPLS

Question 12

Which SRv6 endpoint behavior is used for L3VPN decapsulation and IP forwarding to the destination in a VRF?

Accepted Answer

End.DT4. End.DT4 is the correct SRv6 endpoint behavior for L3VPN decapsulation and IP forwarding to the destination in a VRF. It removes the outer IPv6 header and SRH, then performs a lookup in the specified VRF IPv4 routing table to forward the inner IPv4 packet. This behavior is defined in RFC 8986 and is the direct SRv6 equivalent of the MPLS-based per-VRF label allocation for L3VPN.

Answer

End.DT2U

Answer

End

Answer

End.X

Question 13

Which TWO BGP attributes are considered during the best path selection process and can be used to influence outbound traffic from an AS? (Choose two.)

Accepted Answer

LOCAL_PREF. Weight (Cisco proprietary) and LOCAL_PREF are the first two criteria in BGP path selection. They influence outbound traffic by choosing the preferred path for routes learned from multiple sources.

Answer

NEXT_HOP

Answer

MED

Answer

AS_PATH

Question 14

A network administrator is designing an MPLS TE tunnel for a service provider backbone. The goal is to provide bandwidth protection for voice traffic. Which MPLS TE feature should be configured?

Accepted Answer

Fast Reroute (FRR). MPLS TE Fast Reroute (FRR) provides sub-50 ms protection against link or node failures by pre-computing and pre-signaling a backup tunnel that bypasses the protected element. This meets the requirement for bandwidth protection of voice traffic, which is sensitive to packet loss and delay during network convergence.

Answer

Autoroute announce

Answer

Path-option

Answer

Bandwidth protection

Question 15

An engineer is configuring an MPLS Traffic Engineering tunnel using RSVP-TE. The tunnel is intended to carry voice traffic with strict delay requirements. Which RSVP-TE object should be used to enforce a maximum delay path?

Accepted Answer

Use the 'path-option' with 'metric delay' under the tunnel. The 'path-option with metric delay' command instructs RSVP-TE to compute a path that minimizes cumulative delay rather than IGP cost or hop count. This directly enforces a maximum delay constraint for voice traffic by selecting the path with the lowest measured or configured delay metric across the MPLS network.

Answer

Enable 'record-route' on the tunnel

Answer

Configure 'fast-reroute' protection

Answer

Specify an explicit-path (ERO) manually

Question 16

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 17

Which TWO statements correctly describe RSVP-TE reservation styles? (Choose two.)

Accepted Answer

Shared Explicit (SE) allows multiple senders to share a reservation but with explicit filters.. Shared Explicit (SE) allows sharing among senders with different selectors, and Fixed Filter (FF) creates distinct reservations per sender.

Answer

Shared Explicit (SE) does not allow any sharing.

Answer

Fixed Filter (FF) is the only style used in MPLS-TE.

Answer

Wildcard Filter (WF) is commonly used in MPLS-TE.

Question 18

A Cisco XR router is configured to stream telemetry via gRPC with TLS. The collector can connect but receives empty data. The telemetry configuration is as follows: sensor-group with 'openconfig-interfaces' paths. What is the likely cause?

Accepted Answer

The sensor-group path is not supported by the device. The collector can establish a gRPC-TLS connection but receives empty data because the sensor-group path 'openconfig-interfaces' is not supported on the Cisco XR router. Cisco IOS XR supports OpenConfig paths only if the corresponding YANG models are loaded and the device implements those models; if the path is unsupported, the router will accept the subscription but send no telemetry data for that path. This is a common misconfiguration when using gRPC telemetry with OpenConfig models on platforms that may not fully support all OpenConfig paths.

Answer

The router's CPU is overloaded

Answer

The collector does not support TLS

Answer

The telemetry interval is too short

Answer

The destination group is missing the 'encoding' configuration

Question 19

An SP engineer is configuring IS-IS on a router and specifies a NET address of 49.0001.1921.6800.1001.00. What does the '00' at the end represent?

Accepted Answer

SEL (NSAP Selector). The last two octets of an IS-IS NET address represent the SEL (NSAP Selector), which is always 00 for a router.

Answer

AFI

Answer

System ID

Answer

Area ID

Question 20

Which THREE QoS mechanisms can be used for traffic policing?

Accepted Answer

dual-rate three-color. Traffic policing uses token bucket mechanisms to measure and enforce traffic rates. Dual-rate three-color (RFC 2698) uses two token buckets (CIR/PIR) to mark packets as green, yellow, or red, allowing separate policing of committed and peak rates. Single-rate two-color (RFC 2697) uses one token bucket to mark packets as either conforming (green) or exceeding (red). Both are standard policing implementations.

Answer

WRED

Answer

shaping