350-501 Practice Test 29 — 15 Questions

Question 1

You are a network engineer at a service provider. Your network uses MPLS L3VPN with OSPF as the IGP and LDP for label distribution. A customer has two sites connected to different PEs (PE1 and PE2) in the same VPN. The customer's CE routers are running eBGP with the PEs. Recently, the customer reports that traffic between the two sites is intermittently dropping. Upon investigation, you find that the BGP session between PE1 and the CE at site A goes down briefly every few minutes. The logs on PE1 show BGP notifications with error code 'Hold Timer Expired'. The CE router at site A is a low-end device with limited CPU. What is the most likely cause and the best course of action?

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

Accepted Answer

Increase the BGP hold time on PE1 to 180 seconds.. The BGP session drops due to 'Hold Timer Expired' because the low-end CE router's CPU is overloaded, causing it to fail sending BGP keepalives within the default 90-second hold time. Increasing the hold time on PE1 to 180 seconds (option D) gives the CE more time to send keepalives, reducing false timeouts. This directly addresses the root cause—insufficient CPU to maintain timely keepalives—without changing routing protocols or suppressing routes.

Answer

Configure BGP route dampening on PE1 to suppress flapping routes.

Answer

Disable LDP on the link between PE1 and CE1.

Answer

Change the IGP from OSPF to IS-IS to reduce routing updates.

Question 2

An engineer is configuring MPLS on a Cisco router. Which label operation is performed on the ingress LSR when a packet enters the MPLS domain?

Accepted Answer

Push a new label. On the ingress LSR (the first router a packet hits when entering an MPLS domain), the router performs a FIB lookup on the destination IP, determines the appropriate LSP, and imposes (pushes) a new label onto the packet before forwarding it into the MPLS core. This is the fundamental 'label imposition' function of an ingress LER.

Answer

Pop the top label

Answer

No label operation

Answer

Swap the top label

Question 3

A service provider is deploying MPLS-TE with RSVP-TE in their core network. They notice that some LSPs are not being established due to resource contention. Which action would best address this issue without redesigning the entire traffic engineering deployment?

Accepted Answer

Enable preemption on RSVP-TE LSPs with appropriate priority levels.. Enabling preemption on RSVP-TE LSPs with appropriate setup and hold priorities allows higher-priority LSPs to tear down lower-priority LSPs to free up bandwidth, resolving resource contention without redesigning the entire TE deployment. This is the standard mechanism defined in RFC 3209 for managing bandwidth contention in MPLS-TE networks.

Answer

Increase the bandwidth of all core links.

Answer

Configure LSP path-option explicit paths with strict hops.

Answer

Disable RSVP-TE and use LDP for label distribution.

Question 4

In BGP route selection, which attribute is considered before LOCAL_PREF?

Accepted Answer

Weight. Cisco's BGP best-path algorithm evaluates Weight first (Cisco-proprietary, locally significant), then LOCAL_PREF, then locally originated routes, AS_PATH length, origin type, MED, and so on. Since Weight is evaluated before LOCAL_PREF, it is the correct answer to 'which attribute is considered before LOCAL_PREF.'

Answer

AS_PATH length

Answer

MED

Answer

LOCAL_PREF

Question 5

Which IS-IS TLV is used to carry IPv6 reachability information?

Accepted Answer

TLV 236. TLV 236 (IPv6 Reachability) carries IPv6 prefixes and associated metrics.

Answer

TLV 135

Answer

TLV 232

Answer

TLV 242

Question 6

A service provider is implementing BGP PIC (Prefix Independent Convergence) for its core network. Which mechanism does BGP PIC primarily rely on to achieve fast convergence?

Accepted Answer

Precomputed backup paths (BGP PIC edge). BGP PIC uses precomputed backup paths (e.g., via BGP PIC edge or BGP add-path) to switch traffic upon failure without waiting for BGP convergence.

Answer

BFD

Answer

Route reflectors

Answer

Communities

Question 7

In IS-IS, what is the result of configuring wide metrics on all interfaces?

Accepted Answer

Higher metric values are supported for traffic engineering. Wide metrics (24-bit or 32-bit) allow values up to 16,777,215 or higher, enabling TE and faster convergence with precise metric adjustments.

Answer

Only L2 routers can use wide metrics

Answer

Metrics are limited to 63

Answer

IS-IS TLVs are backward compatible with narrow metrics

Question 8

Which TWO statements about MPLS label switching are correct? (Choose two.)

Accepted Answer

The transit LSR performs label swapping.. Option A is correct because a transit (intermediate) LSR receives a labeled packet, looks up the incoming label in its LFIB, swaps it for the outgoing label, and forwards the packet toward the next hop. Option C is correct because the ingress LSR (ingress edge router) is responsible for imposing (pushing) an MPLS label onto the IP packet as it enters the MPLS domain, creating the label stack entry. Option B is incorrect because the CE (Customer Edge) router is outside the MPLS domain and receives plain IP packets, not labeled frames. Option D is incorrect because PHP is performed by the penultimate (second-to-last) LSR, which pops the label before forwarding to the egress LSR, not by the egress router itself. Option E is incorrect because the egress LSR typically removes the label (or receives an already-popped packet via PHP) and forwards based on the IP header, rather than performing label swapping.

Answer

The CE receives a frame with an MPLS label.

Answer

PHP (Penultimate Hop Popping) causes the egress router to pop the label.

Answer

The egress LSR performs label swapping before forwarding.

Question 9

Which THREE of the following are correct statements about EVPN-VXLAN in a data center fabric? (Select three.)

Accepted Answer

EVPN route type 3 is used for BUM traffic forwarding.. EVPN route type 3 (Inclusive Multicast Ethernet Tag Route) is used to advertise the VTEP addresses that participate in a given broadcast domain (EVI). This enables the underlay to forward BUM (Broadcast, Unknown unicast, Multicast) traffic by building a head-end replication list or by signaling multicast group membership, ensuring that BUM frames are delivered to all remote VTEPs in the same VXLAN segment.

Answer

VXLAN is a Layer 3 overlay that requires an IGP in the overlay.

Answer

VXLAN always requires IP multicast in the underlay for BUM traffic.

Question 10

In an OSPF network, an area is configured as a Not-So-Stubby Area (NSSA). Which statement about LSA types in this area is correct?

Accepted Answer

Type 7 LSAs are originated by ASBR within the NSSA and converted to Type 5 by the ABR.. In an NSSA, external routes are imported by an ASBR as Type 7 LSAs (NSSA External LSAs), which are flooded only within the NSSA. The ABR of the NSSA then translates those Type 7 LSAs into Type 5 LSAs (AS External LSAs) before flooding them into the backbone area, allowing the rest of the OSPF domain to learn the external routes. This translation is the defining behavior of NSSA.

Answer

Type 1 and Type 2 LSAs are not allowed in NSSA.

Answer

Type 4 LSAs are used to advertise the ABR to other areas.

Answer

Type 5 LSAs are flooded within the NSSA.

Question 11

A service provider wants to protect its core routers from CPU exhaustion caused by excessive ICMP traffic. Which control plane protection mechanism on IOS XR would be most appropriate to rate-limit ICMP packets destined to the router?

Accepted Answer

Implement CoPP by creating a class-map for ICMP and applying a police rate under a control-plane policy-map. CoPP (Control Plane Policing) on IOS XR allows the administrator to classify control-plane traffic (such as ICMP destined to the router) using a class-map and then apply a police rate in a policy-map attached to the control plane. This rate-limits ICMP without dropping all ICMP, protecting the route processor CPU from exhaustion while still permitting legitimate management traffic. It is the standard, granular mechanism for protecting the control plane on IOS XR.

Answer

Enable MPLS traffic engineering to reroute ICMP

Answer

Use BGP prefix filtering to block ICMP routes

Answer

Configure an ACL to deny ICMP on all interfaces

Question 12

An SP is troubleshooting an MPLS L2VPN VPLS network where MAC flapping is occurring between two PEs. Which mechanism in VPLS prevents loops and ensures that a broadcast frame from one PE is not reflected back to the originating PE?

Accepted Answer

Split horizon. In a VPLS environment, split horizon is the mechanism that prevents loops by ensuring that a broadcast, multicast, or unknown unicast frame received from a pseudowire (PW) is never forwarded back to any other PW on the same PE. This rule, defined in RFC 4761 and RFC 4762, stops the originating PE from receiving its own reflected frames, thus eliminating the MAC flapping that would occur if the frame were looped through the MPLS core.

Answer

Spanning Tree Protocol (STP)

Answer

Rapid Spanning Tree Protocol (RSTP)

Answer

MAC address aging

Question 13

Which TWO methods can RESTCONF use to create a new data resource? (Choose two.)

Accepted Answer

POST. RESTCONF can create a new data resource using POST (option A), which targets a collection or list resource and creates a new child resource, with the server assigning the resource identifier and returning 201 Created. RESTCONF can also create a new data resource using PUT (option C), which creates the resource at a client-specified URI if it does not already exist, returning 201 Created (or 204 No Content if it replaces an existing resource). PATCH (option B) is used to modify or partially update an existing resource, not to create a new one. DELETE (option D) removes an existing resource, and GET (option E) only retrieves data, so neither creates a resource.

Answer

PATCH

Answer

DELETE

Answer

GET

Question 14

A service provider wants to prevent IP spoofing attacks from customer edge devices connected to a PE router. The customer prefixes are known and asymmetric routing is not present. Which uRPF mode should be configured on the PE-CE interface?

Accepted Answer

Strict mode uRPF. Strict mode uRPF checks that the source IP of incoming packets is reachable via the same interface the packet arrived on. Since customer prefixes are known and asymmetric routing is not present, strict mode is appropriate and provides the strongest anti-spoofing protection. It ensures packets from customer prefixes only enter via the correct PE-CE interface.

Answer

No uRPF needed because BGP prefix filtering prevents spoofing

Answer

VRF-aware uRPF

Answer

Loose mode uRPF

Question 15

Which LISP feature allows a device to register its location to a mapping system so that another device can find it?

Accepted Answer

ETR. The Egress Tunnel Router (ETR) is the LISP device that registers its location (RLOC) to the mapping system (Map-Server) so that other devices can resolve the endpoint identifier (EID) to that RLOC. This registration process populates the mapping database, enabling remote Ingress Tunnel Routers (ITRs) to find and forward traffic to the ETR. Without ETR registration, the mapping system would have no location information for the EID.

Answer

Map-Resolver (MR)

Answer

Proxy ITR (PITR)

Answer

xTR