350-501 Practice Exam 2 — 20 Questions

Question 1

A service provider is deploying a new MPLS core with Segment Routing and requires fast convergence upon link failure. They plan to use TI-LFA (Topology Independent Loop-Free Alternate). What is a prerequisite for TI-LFA to provide protection against any single link failure?

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

Accepted Answer

IGP must be a link-state protocol with complete topology information (OSPF or IS-IS).. TI-LFA relies on the IGP having a complete view of the network topology to compute a post-convergence path that avoids the failed link. OSPF and IS-IS are link-state protocols that flood link-state advertisements (LSAs) or link-state packets (LSPs) to provide this full topology database, which is essential for TI-LFA to calculate a loop-free backup path for any single link failure.

Answer

BGP-LU must be enabled for label distribution.

Answer

LDP must be enabled on all interfaces.

Answer

RSVP-TE must be configured with FRR.

Question 2

What is the role of Network Services Orchestrator (NSO) in service provider automation?

Accepted Answer

To orchestrate service provisioning across multiple devices. NSO provides service lifecycle management by modeling services with YANG and configuring devices via NETCONF.

Answer

To encrypt data plane traffic

Answer

To provide real-time traffic monitoring

Answer

To replace the routing protocol decision process

Question 3

In a carrier network using VRF-lite for customer isolation, which issue arises if route-target values are not correctly configured?

Accepted Answer

Customer routes leak into other VRFs.. In VRF-lite, route-target (RT) values control the import and export of routes between VRFs. If RTs are misconfigured, a VRF may import routes intended for another VRF, causing customer routes to leak across VRFs. This violates isolation because the VRF table incorrectly accepts routes based on mismatched RT import statements.

Answer

Customer routes are not installed in the global table.

Answer

BGP sessions between PEs flap.

Answer

The PE-CE routing protocol fails.

Question 4

Which TWO statements correctly describe the DiffServ model? (Choose two.)

Accepted Answer

DSCP uses 6 bits, providing 64 possible values. The DiffServ model uses the 6-bit Differentiated Services Code Point (DSCP) field in the IP header to mark packets, providing 2^6 = 64 possible values (0-63). This allows for up to 64 distinct per-hop behaviors (PHBs), enabling scalable QoS classification beyond the limited 8 classes of the older IP Precedence model.

Answer

AF classes use DSCP values 0-7

Answer

Best Effort is marked with DSCP 46

Answer

DiffServ uses a 3-bit field for QoS marking

Question 5

An operator is deploying EVPN-VPWS over an MPLS network. Which component uniquely identifies a multihomed Ethernet segment in EVPN?

Accepted Answer

Ethernet Segment Identifier (ESI). The Ethernet Segment Identifier (ESI) is a unique identifier for a multihomed segment. It is used for DF election, aliasing, and split-horizon.

Answer

Ethernet Tag ID

Answer

Route Distinguisher (RD)

Answer

MPLS Label

Question 6

An SP detects a volumetric DDoS attack targeting a customer network. The SP uses Cisco's S/RTBH technique to drop attack traffic. Which action is performed by the edge routers upon receiving a BGP route with a specific community?

Accepted Answer

The edge router installs a /32 route pointing to a discard interface. S/RTBH works by advertising a /32 prefix (the victim's IP) with a special BGP community (e.g., NO_EXPORT) and a next-hop of the discard interface (e.g., Null0). Edge routers install the route pointing to Null0, dropping traffic to that IP.

Answer

The edge router applies a BGP FlowSpec rule to rate-limit the traffic

Answer

The edge router forwards the attack traffic to a scrubbing center

Answer

The edge router sends ICMP unreachable messages to the attacker

Question 7

In an MPLS-based service provider core network, the aggregation layer typically connects to the core backbone. Which technology is primarily used at the aggregation layer to scale Ethernet services for many customers while preserving VLAN separation?

Accepted Answer

Q-in-Q (IEEE 802.1ad). Q-in-Q (802.1ad) allows service providers to stack a customer VLAN tag on top of a service VLAN tag, enabling scalable customer separation in metro Ethernet networks.

Answer

IEEE 802.1ah (MAC-in-MAC)

Answer

IEEE 802.3ad (Link Aggregation)

Answer

IEEE 802.1x (Port-based Network Access Control)

Question 8

Which Cisco automation tool uses service models in YANG and communicates via NETCONF to orchestrate device configurations and lifecycle management?

Accepted Answer

Cisco Network Services Orchestrator (NSO). NSO (Network Services Orchestrator) uses YANG models and NETCONF for configuration management. Ansible, Cisco DNA Center, and Chef are other tools but NSO is specifically for service orchestration.

Answer

Ansible

Answer

Chef

Answer

Cisco DNA Center

Question 9

An engineer is configuring MPLS VPN and needs to ensure that customer traffic is automatically marked with a specific QoS policy based on the VPN. Which method should be used to propagate QoS markings across the MPLS network?

Accepted Answer

Use MPLS EXP bits to mark traffic at the ingress PE and map to QoS at egress. In an MPLS VPN environment, QoS markings must be preserved across the MPLS backbone. MPLS EXP (Experimental) bits are the standard mechanism to carry QoS information within the MPLS label stack. At the ingress PE, customer traffic is classified and marked with the appropriate EXP bits based on the VPN or other criteria. The egress PE then uses these EXP bits to map traffic to the correct QoS policy, ensuring end-to-end QoS treatment.

Answer

Use 802.1p CoS on the CE-PE link and preserve it across the MPLS backbone

Answer

Use IP ToS bits to mark traffic and rely on MPLS to preserve them

Answer

Set DSCP at the ingress PE and preserve it across the MPLS backbone

Question 10

In IS-IS, which type of LSP is used to advertise prefix information for IPv4 and IPv6 in a modern SP network that supports traffic engineering?

Accepted Answer

TLV 135 (Extended IP Reachability). TLV 135 for IPv4 (extended IP reachability) and TLV 236 for IPv6 (IPv6 reachability) are used in IS-IS for prefix advertisement.

Answer

TLV 2 (IS Neighbors)

Answer

TLV 1 (Area Addresses)

Answer

TLV 22 (IS Extended Neighbors)

Question 11

Drag and drop the steps to configure a Layer 3 interface on a Cisco switch (SVI) into the correct order.

Accepted Answer

Create the VLAN, then enter interface VLAN mode, then assign IP address, then enable the interface.. SVI configuration requires creating the VLAN, then the interface VLAN, assigning an IP, and enabling it.

Answer

Enter interface VLAN mode, then assign IP address, then enable the interface, then create the VLAN.

Answer

Create the VLAN, then assign IP address, then enter interface VLAN mode, then enable the interface.

Answer

Create the VLAN, then enable the interface, then enter interface VLAN mode, then assign IP address.

Question 12

A service provider wants to provide Layer 3 VPN services to customers using MPLS. Which technology is used to distribute VPNv4 routes between PE routers?

Accepted Answer

MP-BGP. MP-BGP is used to carry VPNv4 routes between PE routers. OSPF, EIGRP, and IS-IS are IGPs and do not carry VPNv4 routes.

Answer

IS-IS

Answer

OSPF

Answer

EIGRP

Question 13

In a multicast environment, a PE router is not receiving multicast traffic from a source behind a CE. The PIM neighbors are established. The RP is reachable. What is the most likely issue?

Accepted Answer

The PE does not have the multicast VRF enabled. The most likely issue is that the PE router does not have the multicast VRF enabled. In a multicast VPN (MVPN) environment, even if PIM neighbors are established and the RP is reachable, the PE must have multicast routing explicitly enabled within the VRF using the 'ip multicast-routing vrf <name>' command. Without this, the PE will not process multicast traffic for that VRF, causing it to drop or ignore multicast packets from the source behind the CE.

Answer

The CE is not sending IGMP joins

Answer

The source address is not in the VRF

Answer

The RP is not configured in the VRF

Question 14

An SP network is running OSPFv3 for IPv6 and needs to support traffic engineering extensions for RSVP-TE. Which three statements about OSPFv3 TE extensions are true?

Accepted Answer

OSPFv3 TE uses TLV 1 (Router Address) in Opaque LSAs.. OSPFv3 uses Opaque LSA types 9, 10, and 11 for TE, with Type 10 carrying TE information link-state. OSPFv3 TE requires the OSPFv3 process to be configured for TE.

Answer

OSPFv3 TE requires the use of wide metrics.

Answer

OSPFv3 TE only supports point-to-point links.

Question 15

A service provider is implementing Multicast VPN (MVPN) in an MPLS backbone. They use Rosen GRE (draft-rosen) for multicast transport. What is the primary limitation of Rosen GRE compared to mLDP-based MVPN?

Accepted Answer

All multicast traffic is sent to every PE in the VPN, even if no receiver exists.. In Rosen GRE (draft-rosen) MVPN, a default MDT (Multicast Distribution Tree) is built using a shared PIM-SM group across all PEs in the VPN. All multicast traffic from any source in the VPN is encapsulated in GRE and sent to this default MDT group, meaning every PE receives the traffic regardless of whether it has any local receivers. This is the primary limitation because it wastes bandwidth and processing on PEs that do not need the traffic, unlike mLDP-based MVPN which builds trees only to PEs with receivers.

Answer

It requires full-mesh of GRE tunnels between all PEs.

Answer

It cannot use Source-Specific Multicast (SSM).

Answer

It does not support Protocol Independent Multicast (PIM).

Question 16

Refer to the exhibit. An engineer pushes this XML configuration via NETCONF, but the device rejects it with an error stating 'invalid policy-map structure'. What is the problem?

Accepted Answer

The class name must be defined in a class-map before being referenced in the policy-map. The error 'invalid policy-map structure' occurs because the policy-map references a class name that has not been defined in a class-map beforehand. In Cisco IOS, a class-map must exist before it can be used inside a policy-map; otherwise, the device rejects the configuration as structurally invalid.

Answer

The configuration is missing the class-default class

Answer

The YANG model does not support the 'police' keyword

Answer

The priority and police commands cannot coexist in the same class

Question 17

Which TWO are characteristics of Segment Routing?

Accepted Answer

Source routing capability. Segment Routing (SR) is a source-routing paradigm where the ingress node encodes a list of segments (instructions) into the packet header, guiding the packet through the network without intermediate routers needing to maintain per-flow state. Option B is correct because SR inherently provides source routing capability: the source node determines the path by stacking segment identifiers (SIDs), and transit nodes simply forward based on the top SID.

Answer

Uses only IPv6 data plane

Answer

Requires RSVP-like signaling for path setup

Answer

Requires a centralized SDN controller

Question 18

An SP engineer notices that BGP routes from a CE are not being installed in the VRF routing table, although the BGP session is established. The VRF configuration includes route-target import 100:1. The CE is sending routes with RT 100:1. What is the most likely cause?

Accepted Answer

The route is not matching any import map. Even though the BGP session is up and the CE is sending routes with the correct route-target (RT 100:1), the VRF may have an import map applied. An import map filters which received VPN routes are actually installed into the VRF routing table. If the route does not match the conditions defined in that import map, it will be accepted by BGP but not installed, which explains the symptom.

Answer

The BGP route is suppressed due to route update delay

Answer

The VRF name does not match

Answer

The RD is not configured

Question 19

Match each service provider architecture term to its description.

Accepted Answer

Provider Edge (PE) router: Connects to customer edge routers and provides MPLS VPN services.. The correct matches are: PE connects to customer edge, P is MPLS core, ASBR connects to other ASes. Common confusions: mistaking P for CE or confusing Route Reflector with ASBR.

Answer

Customer Edge (CE) router: Core router that forwards packets based on MPLS labels without customer awareness.

Answer

Route Reflector: Router that exchanges routing information with another autonomous system.

Question 20

You are a network engineer at a service provider that offers L3VPN services. A customer complains that their VPN traffic is intermittently dropping. The network uses OSPF as the IGP, LDP for label distribution, and MP-BGP for VPNv4 route exchange. The PE routers are connected to two core routers (P1 and P2) in a redundant design. Upon investigation, you find that the BGP next-hop for some VPN prefixes is set to the loopback interface of the remote PE, but the route to that loopback is sometimes unreachable due to a flapping link between P1 and P2. The IGP converges, but the BGP prefixes take longer to recover. What is the most likely cause of the intermittent drops?

Accepted Answer

BGP next-hop tracking is not enabled, causing BGP to rely on IGP convergence only. The intermittent drops occur because BGP next-hop tracking (NHT) is not enabled. Without NHT, BGP relies solely on IGP convergence to detect reachability changes for the BGP next-hop (the remote PE loopback). When the link between P1 and P2 flaps, OSPF converges quickly, but BGP does not immediately re-evaluate its next-hop reachability; it waits for the next BGP scan interval (default 60 seconds) or until a BGP update is received. This delay causes the VPN prefixes to remain installed with an unreachable next-hop, leading to traffic drops until BGP eventually withdraws or re-advertises the routes.

Answer

The OSPF network type is incorrectly set to point-to-point

Answer

The BGP timers are set too low, causing premature route withdrawal

Answer

The LDP session between the PEs is down