350-501 Architecture Practice Test 18 — 15 Questions

Question 1

A service provider is deploying a large-scale MPLS L3VPN service. The network architect wants to reduce the number of BGP sessions on each PE router while ensuring that all PEs can exchange VPNv4 routes. Which BGP scaling technique should be used?

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

Accepted Answer

Route reflectors. Route reflectors are the standard BGP scaling technique for large MPLS L3VPN networks. They allow a few dedicated routers to reflect VPNv4 routes to all PEs, removing the need for a full IBGP mesh. This reduces the number of BGP sessions on each PE while maintaining full route distribution.

Answer

BGP confederations

Answer

BGP maximum-prefix limiting

Answer

BGP route dampening

Question 2

A service provider offers a Layer 3 VPN to an enterprise. The customer edge router at one site sends the same prefix to two different PE routers, and the provider wants the PE to accept only the path with the higher BGP local preference while still receiving both copies. Which BGP behavior should the provider rely on at the receiving PE?

Accepted Answer

BGP selects the best path using the decision process, and local preference is compared before the AS path length.. The BGP decision process weighs local preference ahead of AS path length, so assigning a higher local preference to one received copy deterministically selects it. This gives the provider an explicit, controllable knob to prefer one customer edge path while still accepting both advertisements into the BGP table.

Answer

BGP compares MED before local preference, so the customer must set a lower MED on the preferred path.

Answer

BGP will install both paths in the RIB and load-share between them by default for the same prefix.

Answer

BGP will prefer the path with the shortest AS path, and local preference is only used for iBGP-learned routes.

Question 3

A service provider runs an IOS XR core where a single route processor (RP) failure must not interrupt forwarding or routing protocol adjacencies on the line cards. The design team wants to confirm which IOS XR architecture component is responsible for continuing to forward traffic and maintaining protocol state when the active RP fails and the standby RP takes over. Which component provides this capability?

Accepted Answer

Non-stop forwarding (NSF) coordinated by the standby RP, which maintains synchronization with the active RP so line cards keep forwarding during switchover.. In IOS XR, RP redundancy works together with non-stop forwarding: the standby RP is kept synchronized, and line cards retain forwarding state during switchover, so packets keep flowing and routing protocol adjacencies survive. The mechanism is built into the RP/line-card architecture rather than residing on a line-card ASIC, SysDB, or RPME. That is what allows a single RP failure to be transparent to the forwarding plane in this design.

Answer

The System Database (SysDB) on the standby RP, which owns all configuration and protocol state and resumes control of the line cards after failover.

Answer

The distributed Route Processor Management Engine (RPME) on each line card, which independently runs BGP and OSPF and forwards traffic using locally built RIBs.

Answer

The fabric management ASIC on each line card, which caches routing protocol hellos and re-advertises them if the active RP stops responding.

Question 4

A service provider runs an MPLS L3VPN over an SR-MPLS core. The operator wants to ensure that customer traffic follows a specific path that avoids a congested link, without using RSVP-TE. Which mechanism should be used to steer the traffic?

Accepted Answer

Segment Routing Traffic Engineering (SR-TE) with an explicit segment list. SR-TE uses a segment list to define an explicit path. The headend router pushes the list onto the MPLS label stack, and each segment directs the packet along the desired path. This allows the operator to avoid congested links without RSVP-TE, satisfying the design constraint.

Answer

MPLS LDP with IGP shortest path

Answer

BGP Link State (BGP-LS) with a centralized controller

Answer

RSVP-TE with explicit route objects

Question 5

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 6

An engineer is designing an MPLS L3VPN where the provider edge routers must exchange VPNv4 prefixes. The customer requires that the PE routers use a single BGP session to carry both IPv4 unicast and VPNv4 routes, while ensuring that the IPv4 unicast routes are not imported into any VRF. Which BGP address family configuration on IOS XR accomplishes this?

Accepted Answer

Activate both the IPv4 unicast and VPNv4 unicast address families under the same neighbor, and rely on the fact that IPv4 unicast routes have no route targets so they are never imported into a VRF.. Multiprotocol BGP lets one neighbor session activate several address families, such as IPv4 unicast and VPNv4 unicast. VPNv4 prefixes carry route targets that drive VRF import, while IPv4 unicast prefixes carry none, so they remain in the global table and are never imported into a VRF. Activating both families under the same neighbor is the correct way to meet the single-session and isolation requirements.

Answer

Activate the VPNv4 unicast address family under the neighbor and configure the neighbor with the 'send-community-ebgp' command to keep IPv4 unicast routes separate.

Answer

Configure the IPv4 unicast and VPNv4 unicast address families under the same neighbor, and apply a route-policy that sets the route distinguisher (RD) on the IPv4 unicast routes.

Answer

Activate only the VPNv4 unicast address family under the neighbor and carry IPv4 unicast prefixes inside VPNv4 using a default VRF RD.

Question 7

A service provider is using Cisco IOS XR and wants to implement a routing policy that sets the BGP community attribute on routes learned from a specific neighbor. The policy should also prepend the AS path three times for those routes. Which configuration correctly applies this policy?

Accepted Answer

Create a route-policy that sets the community and prepends the AS path, then apply it as an inbound policy on the neighbor.. In IOS XR, route-policies are used to manipulate BGP attributes. To modify routes learned from a neighbor, apply the policy inbound. A single route-policy can set the community and prepend the AS path. Applying it outbound would affect routes sent to the neighbor, not received from it.

Answer

Create a route-policy that sets the community and prepends the AS path, then apply it as an outbound policy on the neighbor.

Answer

Use a route-policy to set the community, and use a separate route-policy to prepend the AS path, then apply both inbound.

Answer

Use a route-policy to set the community, and use the 'as-path prepend' command directly under the neighbor configuration.

Question 8

A service provider is implementing MPLS L3VPN on Cisco IOS XR routers. The network engineer must configure the PE routers to support overlapping customer address spaces and control route distribution between VRFs. Which two statements about the MPLS L3VPN control plane are correct? (Choose two.)

Accepted Answer

Route distinguishers are used to make customer prefixes unique within the provider's BGP table.. In MPLS L3VPN, route distinguishers make customer prefixes unique in the provider's BGP table, while route targets act as extended communities to control import and export between VRFs. These two mechanisms work together to support overlapping address spaces and flexible route distribution.

Answer

VPNv4 prefixes are exchanged between PEs using EBGP sessions only.

Answer

Route distinguishers are extended BGP communities used to control import and export of routes between VRFs.

Answer

Route targets are used to make customer prefixes unique within the provider's BGP table.

Question 9

A service provider wants to synchronize clocks across its core routers so that timing accuracy supports one-way delay measurements and strict SLA reporting. The engineer must choose a synchronization technology that delivers sub-microsecond accuracy and is designed for packet networks. Which technology should be selected?

Accepted Answer

Precision Time Protocol (PTP) operating as IEEE 1588v2 with hardware timestamping.. PTP as defined by IEEE 1588v2, combined with hardware timestamping, is designed to deliver sub-microsecond synchronization over packet networks. That accuracy is what one-way delay measurement and strict SLA reporting require, whereas NTP and SNTP only reach millisecond precision and GPS alone does not distribute time to the rest of the core.

Answer

NTP with symmetric key authentication configured on all core routers.

Answer

SNTP configured in unicast client mode on every router.

Answer

GPS receivers installed only on the route reflectors, with no time distribution to other routers.

Question 10

A service provider is migrating its core to Segment Routing over an MPLS data plane using IS-IS. The design requires that SR-enabled routers advertise their globally unique SID and that the IGP computes shortest-path forwarding to each prefix-SID without additional signaling. Which TWO statements about this deployment are correct? (Choose two.)

Accepted Answer

The MPLS forwarding entry for a prefix-SID is derived from the IGP shortest path, so no RSVP-TE or LDP session is needed to establish the label binding.. Segment Routing over MPLS with IS-IS relies on the IGP to advertise Prefix-SIDs in prefix reachability sub-TLVs and to compute shortest-path forwarding entries from those SIDs. No LDP or RSVP-TE session is needed to create the label bindings, and standard IP prefix advertisement continues alongside the SR extensions. Prefix-SIDs serve ordinary shortest-path forwarding as well as being building blocks for SR policies.

Answer

IS-IS must be extended with a new TLV type for SR that replaces the standard prefix reachability TLV, disabling normal IP prefix advertisement.

Answer

Segment Routing requires LDP to distribute the SID-to-prefix mappings, because IS-IS alone cannot carry label information.

Answer

Prefix-SIDs are only usable for traffic engineering; they cannot be used for plain IGP-based shortest-path forwarding.

Question 11

A service provider is using Cisco IOS XR routers in its core. The network operations team requires that configuration changes be validated before committing to the running configuration, and that they be able to roll back to a previous configuration state if a change causes issues. Which IOS XR feature should be used to meet these requirements?

Accepted Answer

Two-stage configuration with commit and rollback. The two-stage configuration model in IOS XR allows administrators to make changes in a target configuration, validate them, and then commit. If issues arise, rollback can revert to a previous committed configuration. This provides the required safety and recovery. Other options are either subsets of this model or do not provide the full validation and rollback functionality.

Answer

Configuration session with exclusive lock

Answer

Configuration replace with load commit

Answer

Candidate configuration with automatic commit

Question 12

A service provider wants to provide a Layer 2 VPN service that transparently carries customer Ethernet frames, including customer VLAN tags, across an MPLS core. The provider wants to use a point-to-point pseudowire between two PE routers. Which technology should be deployed to meet this requirement?

Accepted Answer

Ethernet over MPLS point-to-point pseudowire using AToM (Any Transport over MPLS) with LDP signaling.. A point-to-point Layer 2 VPN that transparently carries Ethernet frames and customer VLAN tags across an MPLS core is implemented with an Ethernet pseudowire using AToM, signaled by LDP between the two PEs. VPLS is multipoint, L3VPN terminates Layer 2, and RSVP-TE tunnels alone do not emulate the Ethernet service. AToM matches the point-to-point requirement.

Answer

MPLS L3VPN using VRFs and MP-BGP VPNv4 route exchange between the PE routers.

Answer

Ethernet over MPLS using RSVP-TE signaled transport tunnels with no pseudowire emulation.

Answer

Virtual Private LAN Service (VPLS) using a full mesh of pseudowires between all PEs.

Question 13

A service provider uses BGP on IOS XR routers to exchange routes with customers. An engineer notices that a customer is advertising a prefix with an unexpected AS_PATH that includes the provider's own AS number, which the provider's inbound policy does not filter. The provider wants to prevent its own AS from appearing in routes learned from customers without disrupting legitimate customer routes. Which BGP feature should be applied on the customer-facing sessions?

Accepted Answer

Apply an inbound route-policy that matches the provider's own AS in the AS_PATH using an as-path-set and drops those routes.. To stop customers from advertising routes that contain the provider's own AS number, the provider applies an inbound route-policy that matches its AS in the AS_PATH and drops those updates. This is targeted filtering at the customer edge. Enforce-first-as only checks the leading AS, route refresh does not validate paths, and bestpath as-path ignore affects local selection rather than rejecting the routes.

Answer

Enable BGP route refresh capability on the customer-facing neighbors so that invalid AS_PATHs are automatically corrected.

Answer

Configure 'bgp enforce-first-as' on the customer-facing neighbors to reject updates where the first AS in the AS_PATH is not the sending peer's AS.

Answer

Configure the 'bgp bestpath as-path ignore' command on the customer-facing neighbors to disregard the AS_PATH during best-path selection.

Question 14

A service provider runs Cisco IOS XR on its core routers and wants to ensure high availability by separating control plane and data plane processes. The SP is concerned about a routing protocol process failure affecting packet forwarding. Which IOS XR architectural feature best addresses this concern?

Accepted Answer

Microkernel-based modular architecture with independent process spaces. The microkernel-based modular architecture of IOS XR ensures that each routing protocol runs as an independent process with its own memory space. If one process fails, it does not affect other processes or the forwarding plane, which continues to forward packets based on the last known good forwarding state. This design provides high availability and fault isolation, directly addressing the SP's concern.

Answer

Distributed Route Processor (DRP) architecture with separate control plane and forwarding plane

Answer

Nonstop Forwarding (NSF) with Stateful Switchover (SSO) between dual route processors

Answer

Multi-Protocol Label Switching (MPLS) with Traffic Engineering (TE) tunnels

Question 15

A service provider is deploying a large-scale MPLS L3VPN service. The network architect wants to reduce the number of BGP sessions on each PE router while ensuring that all PEs can exchange VPNv4 routes. Which BGP scaling technique should be used?

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

Accepted Answer

Process Isolation with Memory Protection. Cisco IOS XR uses a microkernel architecture with process isolation, where each process runs in its own memory space. This prevents a software defect in one process, such as a routing protocol, from affecting other processes or causing a router reload. Other features like NSR and modular packages provide different benefits but do not provide this level of fault containment.

Answer

Modular Software Packages

Answer

Preemptive Multitasking

Answer

Non-Stop Routing (NSR)