350-501 Practice Test 28 — 15 Questions

Question 1

A service provider has deployed QoS on its MPLS backbone to offer different classes of service. The provider notices that EF (Expedited Forwarding) traffic sometimes experiences high jitter during congestion. Following recommended practices, which action is MOST likely to reduce jitter for EF traffic?

Accepted Answer

Use priority queuing (LLQ) for EF traffic with a policer. Option B is correct because priority queuing ensures EF traffic is served before other queues, reducing jitter. Option A is wrong because shaping reduces bandwidth but does not eliminate jitter if congestion exists. Option C is wrong because WRED is for congestion avoidance and can drop EF packets if not excluded. Option D is wrong because changing DSCP does not improve jitter directly; proper queuing is key.

Answer

Apply traffic shaping on all EF packets at the edge

Answer

Change the DSCP value of EF traffic to AF41 to reduce drops

Answer

Apply weighted random early detection (WRED) on the EF queue

Question 2

A multicast service provider is designing an IPv4 PIM Sparse Mode (PIM-SM) network with multiple RPs. The network must support Anycast RP using MSDP. Which THREE statements are correct regarding this design?

Accepted Answer

Anycast RP allows multiple RPs to share the same IP address, providing redundancy and load balancing. Options A, C, and E are correct. Option A: MSDP is used to exchange active source information between anycast RPs. Option C: Anycast RP provides load balancing and redundancy automatically. Option E: Source-specific multicast (SSM) can be used with PIM-SM and Anycast RP if IGMPv3 is supported. Option B is wrong: Anycast RP works with both static and dynamic RP assignments, but Auto-RP is not required. Option D is wrong: MSDP is not needed for SSM because SSM uses (S,G) channels and does not rely on an RP.

Answer

Auto-RP must be used to distribute the RP information to all routers

Answer

MSDP is only required if the network uses Source-Specific Multicast (SSM)

Question 3

A service provider is troubleshooting an MPLS L3VPN where customers behind CE-A cannot reach CE-B. The PE routers are Cisco ASR 9000 series. On PE-A, the show cef vrf CUSTOMER prefix 10.1.1.0/24 command displays 'punt' as the forwarding path. What is the most likely cause?

Accepted Answer

The IP prefix is not resolved via an MPLS label in the LFIB. Option C is correct because when VRF routes are punted to the CPU, it typically indicates that the label for the next hop is missing or that the FIB resolution failed, often due to missing MPLS label binding. Option A is wrong because BGP VPNv4 routes are not required for local VRF forwarding. Option B is wrong because MTU would cause drop, not punt. Option D is wrong because CE-PE routing is usually static or BGP, not OSPF in most designs.

Answer

The MPLS MTU on the interface is too small

Answer

The VRF is missing the route-target import statement

Answer

The CE is not running OSPF with the PE

Question 4

A service provider operates an MPLS-TE network using RSVP-TE with a full mesh of tunnels between core routers. The network uses OSPF as IGP with traffic engineering extensions. Recently, a new headend router PE-New was added and configured with several TE tunnels to remote destinations. However, some tunnels repeatedly go down after a few minutes and show in the 'down' state with the error 'Tunnel path option 0: no path to destination (TEDB lookup failed)'. The TE tunnels that remain up are those to destinations that are directly connected to the same OSPF area. The network has multiple OSPF areas (area 0, 1, 2) with inter-area routes redistributed. The operator suspects the issue is related to the TEDB (Traffic Engineering Database) not having complete information. Which action is MOST likely to resolve the issue?

Accepted Answer

Change the IGP to IS-IS with wide metrics and enable MPLS-TE on all IS-IS levels. Option D is correct: In multi-area OSPF, the TEDB is not flooded across area boundaries by default. To have TE information for inter-area destinations, IS-IS is preferred; but if OSPF is used, enabling 'flooding' of TE LSAs across areas requires configuring 'mpls traffic-eng inter-area' or similar (on Cisco IOS XR, the command is 'mpls traffic-eng inter-area [level-2]' but for OSPF, it's 'mpls traffic-eng area X' with redistribution; however, the simplest fix is to use a single area or IS-IS. Option A: Static routes do not provide TE information. Option B: Adding IP routes does not populate the TEDB. Option C: Increasing hold-priority does not fix missing TEDB entries.

Answer

Increase the tunnel hold-priority and setup-priority values

Answer

Add the remote loopbacks to OSPF using network statements in area 0

Answer

Configure static routes for the remote destinations on PE-New

Question 5

Based on the exhibit, why is the route 10.10.10.0/24 from remote PE not installed in the VRF Customer-A on PE1?

Accepted Answer

The VRF imports route-target 200:200, but the route has RT 100:100. Option B is correct: The route has RT 100:100, but VRF Customer-A imports RT 200:200. Thus, the route is not imported. Option A is wrong because the localpref is 100, which is default and not an issue. Option C is wrong because the RD mismatch is between VRFs, but RD does not affect import; RT does. Option D is wrong because label allocation is working fine (vpn-label:24000).

Answer

The localpref is too low (100)

Answer

The MPLS label allocation failed on the remote PE

Answer

The route distinguisher on the VRF (100:1) does not match the remote RD

Question 6

Which TWO actions are required to enable model-driven telemetry on a Cisco IOS XR router?

Accepted Answer

Configure a subscription that refers to a sensor group and a destination group.. To enable model-driven telemetry, you must configure a destination group (receiver) and a subscription that ties the sensor path to the destination. Option A is correct because a destination group defines where telemetry data is sent. Option D is correct because a subscription binds the sensor path and destination. Option B is wrong because the sensor path is configured in a sensor group, not the subscription. Option C is wrong because an ACL is not required for telemetry. Option E is wrong because NETCONF is not mandatory; telemetry can use gRPC or other protocols.

Answer

Apply an access-list to allow telemetry traffic from the router.

Answer

Enable NETCONF on the router for telemetry to function.

Answer

Configure the subscription with the sensor path directly.

Question 7

A service provider needs to offer L3VPN services to multiple customers, each with overlapping IP addresses. The provider plans to use MPLS VPNs with VRFs. Which statement is TRUE regarding the configuration of VRFs on the PE routers?

Accepted Answer

Route targets (RTs) are used to control the import and export of VPNv4 routes between VRFs. Option D is correct because route targets (RTs) are used to control the import/export of VPNv4 routes between PE routers, ensuring proper segregation and connectivity. Option A is wrong because RD must match within a VPN to avoid route ambiguity, but it can differ if RTs are properly configured; however, the question asks for TRUE statement. Option B is wrong because MPLS label allocation per VRF is automatic. Option C is wrong because OSPF is not required; any IGP or BGP can be used.

Answer

The VRF must run OSPF to exchange routes with the CE router

Answer

MPLS labels are assigned per prefix inside a VRF by manual configuration

Answer

Each VRF must have a unique route distinguisher across the entire provider network

Question 8

A network architect is designing a Segment Routing (SR) network for a service provider with a requirement for fast convergence upon node failure. The network uses IS-IS as the IGP with segment routing extensions. Which design choice BEST ensures that the network can achieve sub-50ms convergence without relying on any signaling protocol beyond the IGP?

Accepted Answer

Enable TI-LFA (Topology Independent Loop-Free Alternate) on all ISIS interfaces. Option A is correct because TI-LFA (Topology Independent Loop-Free Alternate) calculates backup paths based solely on IGP and does not require additional signaling. Option B is wrong because RSVP-TE requires TE tunnels and signaling. Option C is wrong because LDP is a separate signaling protocol. Option D is wrong because BFD can be used for fast detection but does not provide a backup path itself; it must be combined with a protection mechanism.

Answer

Deploy BFD with static routes for fast detection

Answer

Configure RSVP-TE with Fast Reroute on all label-switched paths

Answer

Use LDP with LDP FRR and path protection

Question 9

A service provider operates a large MPLS network with Segment Routing (SR) and BGP-LS enabled on all routers. They have deployed a centralized Path Computation Element (PCE) to compute SR-TE policies for optimal traffic engineering. The PCE is configured to receive the network topology via BGP-LS from a route reflector (RR). Recently, the PCE has been unable to compute paths for certain destinations, and logs show that the topology database is missing some links and nodes. The engineer verifies that all routers have BGP-LS configured and are peering with the RR. The RR's BGP table shows the BGP-LS NLRI received from all routers. However, the PCE sees only a subset of the topology. Which action should the engineer take to resolve the issue?

Accepted Answer

Configure the RR to send BGP-LS routes to the PCE. Verify that the RR has a BGP session with the PCE in the address-family link-state.. The PCE is not receiving the full topology because the RR is not sending BGP-LS routes to the PCE. The most likely cause is that the RR is not configured to advertise BGP-LS to the PCE. Option B is correct: the engineer should add the PCE as a BGP neighbor on the RR and ensure that the address-family link-state is activated. Option A is wrong because the BGP-LS sessions from routers to the RR are already working. Option C is wrong because the PCE itself likely has BGP-LS configured; the issue is the path before the PCE. Option D is wrong because the problem is not about policy filtering on the PCE; missing nodes/links indicate incomplete topology, not excessive data.

Answer

Check the IGP (OSPF/IS-IS) configuration on the routers. BGP-LS relies on IGP to obtain link-state information, and if IGP does not have full visibility, BGP-LS will not either.

Answer

Apply a prefix-list on the PCE to filter out unwanted BGP-LS prefixes, as the PCE may be overwhelmed.

Answer

Verify that the PCE itself has a BGP-LS adjacency to each router, bypassing the RR.

Question 10

Refer to the exhibit. An engineer configured a telemetry subscription to push interface state data to a collector. The subscription shows 'State: Invalid'. What is the most likely cause?

Accepted Answer

The collector at 192.168.1.1:57500 is not reachable or the service is down.. The 'State: Invalid' with error 'Connection refused' indicates that the receiver (collector) is not accepting the connection. The most likely fix is to ensure the collector is up and listening on the specified port. Option C correctly identifies this. Option A is wrong because the xpath syntax is correct for the model. Option B is wrong because encoding kvgpb is valid. Option D is wrong because the periodic update policy is correctly configured.

Answer

The encoding 'encode-kvgpb' is not supported; must use 'encode-json'.

Answer

The xpath filter is malformed; it should be /interfaces/interface/state.

Answer

The periodic update interval of 500 ms is too fast causing subscription failure.

Question 11

Which TWO actions are required to implement QoS trust boundaries correctly in a service provider network?

Accepted Answer

Configure classification at the edge. Option A is correct because trust should be set on access ports to accept marking from trusted sources. Option C is correct because classification must be performed at the network edge to enforce policy. Option B is incorrect because auto QoS is not a mandatory requirement. Option D is incorrect because MPLS EXP bits are used for core marking, not trust boundary. Option E is incorrect because shaping is applied at customer premises, not related to trust boundary.

Answer

Enable auto QoS on all interfaces

Answer

Apply shaping at customer premises

Answer

Use MPLS EXP bits for core marking

Question 12

A service provider is experiencing congestion on a core link connecting two P routers. The customer traffic is classified into three classes: voice (low latency), video (low loss), and data (best effort). The current configuration uses DSCP-based classification at the PE ingress, but many customers are remarking DSCP values to gain better service, bypassing the provider's QoS policy. The provider wants to enforce a strict trust boundary at the PE and re-mark all traffic according to a per-customer contract. Additionally, the provider must offer per-customer bandwidth guarantees, ensuring that one customer's traffic does not starve another customer's traffic on the congested core link. The solution must be scalable to hundreds of customers. What should the designer recommend?

Accepted Answer

Implement hierarchical QoS on the PE egress to customer-facing interfaces, with parent-level shaping per customer and child-level policing per class, and set a trust boundary to mark all traffic based on the customer contract at ingress.. Option A is correct because hierarchical QoS allows per-customer shaping at the parent level to enforce per-customer bandwidth limits, and per-class policing at the child level to enforce per-class contracts. This provides the required trust boundary and per-customer guarantees. Option B is wrong because auto-qos does not provide per-customer granularity and relies on trust which is already broken. Option C is wrong because MPLS TE tunnels reserve bandwidth but do not solve the trust boundary issue; also resetting EXP is complex. Option D is wrong because a flat policy on core does not allow per-customer differentiation and cannot enforce per-customer guarantees.

Answer

Apply a flat QoS policy on the core interfaces that polices each DSCP value to a fixed rate; trust is not needed because the core enforces its own limits.

Answer

Use auto-qos on all PE interfaces and rely on CoS trust; the core uses MPLS EXP derived from CoS to ensure proper queuing.

Answer

Configure MPLS Traffic Engineering tunnels on the core with bandwidth reservation per customer class; use EXP-null to preserve markings end-to-end.

Question 13

Based on the exhibit, which statement is true about the applied QoS policy?

Accepted Answer

Video traffic is being dropped at a rate of 5 kbps.. Option B is correct because the video class is exceeding its police rate (CIR 20 kbps) with an exceed rate of 5 kbps, indicating that 5 kbps of video traffic is being dropped. Option A is false because the voice class has no drops. Option C is false because the default class matches any traffic. Option D is false because no priority or LLQ is configured, only policing.

Answer

The default class matches only best-effort traffic.

Answer

The policy uses low-latency queuing for voice.

Answer

Voice traffic is experiencing drops due to policer.

Question 14

An SP engineer is designing a BGP-based MPLS L3VPN service. The PE routers are fully meshed via iBGP for VPNv4 routes, and an RR is deployed to reduce sessions. The engineer notices that all PE routers are receiving duplicate routes from different PEs, causing suboptimal path selection. Which BGP feature should be enabled to ensure proper load balancing across multiple equal-cost paths?

Accepted Answer

Enable BGP multipath on the PE routers. Option A is correct. BGP multipath allows the router to install multiple equal-cost paths into the routing table, load balancing traffic. Option B is wrong because BGP additional paths allows advertising more than one path per prefix but does not directly install them. Option C is wrong because BGP best path selection is a process, not a feature to alter outcomes. Option D is wrong because BGP route reflectors do not influence multipath behavior.

Answer

Enable BGP additional paths on the RR

Answer

Deploy a second RR to reduce the number of iBGP sessions

Answer

Configure the BGP best path selection algorithm to ignore interior cost

Question 15

Which TWO tasks are required when implementing segment routing in an MPLS network?

Accepted Answer

Configure the IGP (OSPF or IS-IS) with segment routing extensions. Options A and B are correct. Configuring the IGP (OSPF or IS-IS) for segment routing is essential to advertise prefix-SIDs and adjacency-SIDs. Enabling MPLS on interfaces ensures MPLS forwarding. Option C is not required because LDP is replaced by segment routing. Option D is not required as RSVP-TE is a different traffic engineering mechanism. Option E is part of the MPLS forwarding plane setup but not unique to segment routing; it is a prerequisite.

Answer

Configure RSVP-TE to establish LSPs

Answer

Enable CEF on all routers

Answer

Enable LDP on all routers and interfaces