350-501 Practice Test 45 — 15 Questions

Question 1

An engineer is troubleshooting an MPLS L3VPN where the customer edge routers can exchange routes with the provider edge routers, but the provider edge routers are not exchanging VPNv4 routes with each other. The engineer verifies that the iBGP session between the provider edge routers is established using the loopback interfaces and that the underlying IGP is converged. Which action must be taken to allow the provider edge routers to exchange VPNv4 routes?

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

Accepted Answer

Activate the VPNv4 address family under the BGP routing process on both provider edge routers.. For MPLS L3VPN, provider edge routers must exchange VPNv4 routes using iBGP with the VPNv4 address family activated. Without this, even with a functioning iBGP session and IGP, VPNv4 routes are not exchanged. Activating the VPNv4 address family under the BGP process on both PE routers enables the necessary route exchange, allowing customer routes to be propagated across the MPLS core.

Answer

Enable MPLS LDP on the interfaces connecting the provider edge routers to the core.

Answer

Configure the provider edge routers to use LDP for label distribution.

Answer

Configure route reflectors to reflect the VPNv4 routes between the provider edge routers.

Question 2

A network engineer is deploying MPLS L3VPN on Cisco IOS XR routers. The PE router must maintain separate VRF routing tables and advertise VPNv4 prefixes to remote PEs. Which protocol is required to carry VPNv4 routes between PE routers?

Accepted Answer

MP-BGP with the VPNv4 address family. MP-BGP with the VPNv4 address family is the protocol that exchanges VPNv4 routes between PE routers in an MPLS L3VPN. It carries the route distinguisher and route target extended communities that allow PEs to import and export routes into the correct VRF. The other protocols either do not support VPNv4 or serve different purposes, such as label distribution or traffic engineering.

Answer

LDP with the VPNv4 address family

Answer

OSPFv3 with the IPv4 address family

Answer

RSVP-TE with the VPNv4 address family

Question 3

An engineer is configuring an SR-MPLS TE policy on a Cisco IOS XR headend router. The policy uses a dynamic path computed by a PCE, and the headend must signal the policy using the PCEP protocol. Which statement correctly describes the role of the PCE in this scenario?

Accepted Answer

The PCE computes the end-to-end path and provides the segment list to the headend via PCEP.. In SR-MPLS TE with PCE, the PCE computes the end-to-end path and provides the segment list to the headend via PCEP. The headend then uses this segment list to instantiate the SR policy. The PCE does not distribute labels, signal RSVP-TE, or act as the headend. This architecture centralizes path computation and simplifies TE policy management.

Answer

The PCE establishes the RSVP-TE reservation for the policy along the computed path.

Answer

The PCE distributes MPLS labels for the prefix segments along the path.

Answer

The PCE acts as the headend and initiates the SR policy signaling to the tailend.

Question 4

A network administrator is configuring MPLS LDP on a Cisco IOS XE router. The router must establish LDP sessions with directly connected neighbors. Which command enables LDP on an interface?

Accepted Answer

mpls ip. To enable LDP on an interface in Cisco IOS XE, the mpls ip command is used under the interface configuration. This command activates MPLS forwarding and LDP on that interface, allowing the router to establish LDP sessions and exchange labels with directly connected neighbors. Other commands manage LDP parameters but do not enable it on the interface.

Answer

mpls label protocol ldp

Answer

mpls ldp router-id loopback0

Answer

mpls ldp autoconfig

Question 5

A service provider is using Cisco NSO (Network Services Orchestrator) to automate the deployment of MPLS L3VPN services. The network engineer needs to ensure that the NSO service model can dynamically allocate route distinguishers (RDs) and route targets (RTs) for each VPN instance. Which NSO component is responsible for generating these unique identifiers based on service input parameters?

Accepted Answer

The service model logic (e.g., YANG model and service callbacks). In Cisco NSO, the service model logic, defined by YANG models and service callbacks, is responsible for generating unique RDs and RTs. This logic uses input parameters to compute these identifiers, ensuring uniqueness and consistency. The NED handles device communication, the CDB stores data, and the Web UI provides a front end, but none of these generate the identifiers.

Answer

The NED (Network Element Driver)

Answer

The NSO Web UI

Answer

The CDB (Configuration Database)

Question 6

A service provider's MPLS core uses LDP for label distribution. The network operations team wants to migrate to Segment Routing (SR-MPLS) without disrupting existing LDP-based LSPs during the transition. On a Cisco IOS XR router, which configuration step allows both LDP and SR-MPLS to coexist and forward traffic simultaneously?

Accepted Answer

Configure the IGP to advertise Prefix-SIDs and enable LDP on the same interfaces, allowing both protocols to install labels in the LFIB.. To migrate from LDP to SR-MPLS without disruption, both protocols must run concurrently. Enabling SR-MPLS under the IGP (IS-IS or OSPF) and keeping LDP enabled on the same interfaces allows the router to maintain label bindings from both. The LFIB can then forward traffic using LDP labels or SR labels, depending on the incoming label, facilitating a gradual transition.

Answer

Disable LDP globally and rely on SR-MPLS to automatically translate LDP labels to Segment Routing labels.

Answer

Configure LDP to use the Segment Routing Global Block (SRGB) for label allocation, ensuring compatibility with SR-MPLS.

Answer

Enable SR-MPLS under the IGP and configure LDP to advertise labels only for prefixes that do not have a Prefix-SID.

Question 7

A service provider runs IS-IS as its IGP and is deploying Segment Routing MPLS. The network architect wants to ensure that MPLS forwarding entries for Prefix-SIDs are installed in the LFIB even before any SR policy traffic is sent. On a Cisco IOS XR router, which configuration step is required to enable SR-MPLS forwarding for the IS-IS domain?

Accepted Answer

Configure segment-routing mpls under the IS-IS address-family and enable mpls on the router.. In Cisco IOS XR, SR-MPLS is enabled by configuring segment-routing mpls under the IS-IS address-family and enabling MPLS on the router. This allows the router to advertise Prefix-SIDs as IS-IS extensions and to program the LFIB with the corresponding incoming labels. Without this configuration, the router will not participate in SR-MPLS forwarding, and any SR policies that rely on those SIDs will not function.

Answer

Enable MPLS LDP on all core-facing interfaces and redistribute IS-IS into LDP.

Answer

Configure RSVP-TE globally and enable traffic-engineering under IS-IS.

Answer

Enable BGP labeled-unicast and advertise the loopback prefixes with MPLS labels.

Question 8

A service provider is implementing MPLS Traffic Engineering (TE) with RSVP-TE in its core network. The network engineer wants to ensure that TE tunnels are established with the most efficient path and that they can be rerouted quickly in case of a link failure. Which protocol is used by RSVP-TE to distribute TE link attributes and topology information?

Accepted Answer

OSPF-TE or IS-IS TE. RSVP-TE relies on OSPF-TE or IS-IS TE to distribute TE link attributes and topology information. These IGP extensions flood TE information such as available bandwidth, administrative groups, and TE metric throughout the IGP area. The ingress router uses this information to run CSPF and compute the best path for a TE tunnel. RSVP-TE then signals the tunnel along that path. Without IGP TE extensions, the TE database would not be populated, and CSPF could not function.

Answer

PCEP

Answer

BGP-LS

Answer

RSVP-TE itself

Question 9

A service provider is designing a large-scale Layer 3 VPN service and wants to avoid running a separate routing protocol instance for every customer on the PE routers. The design team decides to use a single BGP autonomous system across the entire provider core and edge. Which architecture feature allows the PE routers to keep customer routes separate while sharing one BGP process?

Accepted Answer

MPLS Layer 3 VPN with per-VRF route targets and route distinguishers. MPLS L3VPN uses VRFs on PE routers, with route distinguishers making customer prefixes unique and route targets controlling which VRFs import which routes. This design lets a single BGP process carry many customers' routes while keeping their forwarding and routing tables separate, which is exactly what the provider needs instead of running one routing instance per customer.

Answer

OSPF multi-area design with a separate area per customer

Answer

BGP confederation with sub-autonomous systems per customer

Answer

IS-IS Level 2 backbone with Level 1 areas per customer

Question 10

A service provider runs IS-IS Level 2 as its IGP in a core of Cisco ASR 9000 routers running IOS XR. The provider wants to enable Segment Routing using MPLS with a Segment Routing Global Block (SRGB) of 16000-23999. Which configuration is required on each core router to enable SR-MPLS with the default behavior?

Accepted Answer

segment-routing mpls under the global configuration, plus an IGP segment-routing mpls address-family configuration under the IS-IS process.. To enable SR-MPLS in IOS XR, you must activate it globally with 'segment-routing mpls' and then enable it per address family under the IGP. The IGP then advertises prefix SIDs and the SRGB. Without the IGP-level configuration, the router will not generate or advertise SIDs for its prefixes, so MPLS forwarding based on SR will not work.

Answer

mpls traffic-eng tunnels under global configuration, plus 'mpls ldp' under the IS-IS process.

Answer

segment-routing srv6 under global configuration, plus 'segment-routing srv6' under the IS-IS process.

Answer

segment-routing mpls under global configuration, plus 'mpls ldp' under the IS-IS process.

Question 11

A network engineer is deploying Segment Routing over an MPLS network using OSPF as the IGP. The engineer wants to enable SR-MPLS and ensure that Prefix-SIDs are advertised. Which TWO steps are required to accomplish this? (Choose two.)

Accepted Answer

Configure the OSPF process with segment-routing mpls.. To deploy SR-MPLS with OSPF, you must enable segment-routing mpls globally on the router and also enable segment-routing mpls under the OSPF process. The global command activates SR-MPLS forwarding, while the OSPF command allows OSPF to advertise Prefix-SIDs in its LSAs. Together, these steps ensure that the router participates in SR-MPLS and that Prefix-SIDs are propagated to other routers, enabling end-to-end segment routing.

Answer

Configure a static MPLS label for each prefix.

Answer

Enable LDP on all interfaces.

Answer

Configure the Segment Routing Global Block (SRGB) to match the default range.

Question 12

A network engineer is deploying an SR-MPLS Traffic Engineering (SR-TE) policy on a Cisco IOS XR headend router. The policy uses a segment list with two SIDs: 16002 and 16005. The engineer wants to verify that the policy is correctly installed and that traffic is being forwarded along the intended path. Which two commands would provide the necessary verification? (Choose two.)

Accepted Answer

show segment-routing traffic-eng forwarding policy name <policy-name>. To verify an SR-TE policy on Cisco IOS XR, the engineer should use show segment-routing traffic-eng policy name to check the policy's operational state and active candidate path, and show segment-routing traffic-eng forwarding policy name to confirm the forwarding entries and label stack. These commands together provide a complete picture of the policy's control-plane and data-plane status, ensuring traffic is forwarded along the intended SID list.

Answer

show bgp ipv4 labeled-unicast

Answer

show mpls forwarding labels 16002 16005

Answer

show isis segment-routing prefix-sid

Question 13

A service provider is deploying MPLS L3VPN services and wants to use Segment Routing instead of LDP for label distribution. The network uses OSPF as the IGP. Which configuration step is required on each router to enable Segment Routing for MPLS with OSPF?

Accepted Answer

Configure segment-routing mpls under the global configuration and enable segment-routing under the OSPF process.. To enable SR-MPLS with OSPF, the router must have segment-routing mpls configured globally or under the IGP, and segment-routing enabled under the OSPF process. This activates the advertisement of Prefix-SIDs and allows MPLS forwarding based on SR. LDP is not required and should not be enabled if SR is the intended label distribution method. MPLS TE and SRLB are not prerequisites for basic SR-MPLS deployment.

Answer

Enable MPLS TE under the OSPF process and configure an SRGB.

Answer

Configure an SRLB and enable LDP synchronization with OSPF.

Answer

Enable MPLS LDP under the OSPF process and configure segment-routing mpls globally.

Question 14

A service provider is deploying MPLS L3VPN over Cisco IOS XR and must ensure the VPNv4 control plane scales across many PEs. The design team is reviewing how route targets and route distinguishers interact. Which two statements correctly describe their roles? (Choose two.)

Accepted Answer

The route target controls which VRFs import the VPNv4 route on the receiving PE.. The route distinguisher guarantees uniqueness of customer prefixes in the VPNv4 NLRI, while route targets act as extended communities that determine import into VRFs. Together they let a provider carry overlapping customer addressing and control which sites receive which routes, which is the foundation of scalable MPLS L3VPN deployments.

Answer

The route distinguisher must match the route target value for a VRF to import the route correctly.

Answer

A single VRF can import routes only from the exact route target it exports, with no additional import targets allowed.

Answer

The route target is prepended to the prefix to make it globally unique in the VPNv4 table.

Question 15

A service provider is migrating from LDP to Segment Routing in an IS-IS network. During the migration, the engineer notices that some routers are not advertising Prefix-SIDs for their loopback interfaces. Which of the following is the most likely cause?

Accepted Answer

The loopback interfaces are not enabled for IS-IS and segment-routing mpls.. During an LDP-to-SR migration, a common oversight is failing to enable IS-IS and segment-routing mpls on the loopback interfaces. Without IS-IS on the loopback, the router does not advertise a Prefix Reachability TLV for that prefix, so no Prefix-SID sub-TLV can be included. Additionally, the prefix-sid index must be configured under the IS-IS interface address-family. Ensuring these steps are completed on all routers will resolve the missing Prefix-SID advertisements.

Answer

The routers are not configured with a Segment Routing Global Block (SRGB).

Answer

The IS-IS router is configured as Level-1 only, and Prefix-SIDs are only advertised in Level-2.

Answer

The routers are using LDP and SR simultaneously, causing label conflicts.