350-501 Architecture Practice Test 3 — 15 Questions

Question 1

A service provider is designing a new MPLS core network and needs to choose a routing protocol that supports traffic engineering and fast convergence. Which protocol should be used?

Accepted Answer

IS-IS. IS-IS is the correct choice because it natively supports MPLS Traffic Engineering (TE) via IS-IS TE extensions (RFC 5305) and offers fast convergence through mechanisms like IS-IS Fast Flooding and LSP throttling. Unlike OSPF, IS-IS operates directly over Layer 2, making it more scalable and robust in large service provider core networks, and it avoids the need for a separate flooding domain for TE.

Answer

RIP

Answer

EIGRP

Answer

OSPF

Question 2

An engineer is troubleshooting a BGP route propagation issue. The customer is receiving a default route from upstream provider, but the route is not being installed in the routing table. The BGP table shows the route is valid but not best. What is the most likely cause?

Accepted Answer

A static default route with lower administrative distance exists. A static default route with a lower administrative distance (e.g., 1) will be preferred over a BGP default route (AD 20 for eBGP). Even though the BGP route is valid in the BGP table, it is not installed in the routing table because the static route is more trustworthy. This is a common scenario where the routing table already has a better path, preventing the BGP route from becoming 'best' and being installed.

Answer

The next-hop is unreachable

Answer

The route is filtered by an inbound route-map

Answer

The BGP session is not established

Question 3

A network engineer is designing a Layer 2 VPN using EVPN with MPLS data plane. The customer requires all-hub multipoint connectivity with optimal multicast replication. Which EVPN route type is used to advertise multicast group membership?

Accepted Answer

Route Type 6 (Selective Multicast Ethernet Tag). Route Type 6 (Selective Multicast Ethernet Tag) is used in EVPN to advertise multicast group membership for selective multicast replication. This allows the provider edge (PE) routers to build optimal multicast trees for specific groups, enabling efficient all-hub multipoint connectivity with EVPN-MPLS data plane.

Answer

Route Type 3 (Inclusive Multicast Ethernet Tag)

Answer

Route Type 2 (MAC/IP Advertisement)

Answer

Route Type 4 (Ethernet Segment)

Question 4

An engineer is configuring QoS on a service provider edge router to prioritize real-time traffic. Which queuing mechanism provides the lowest latency and jitter for voice traffic?

Accepted Answer

LLQ. LLQ (Low Latency Queuing) is the correct choice because it combines a strict priority queue with CBWFQ, ensuring that voice traffic is always served before any other queue. This strict priority scheduling minimizes both latency and jitter for real-time traffic, which is critical for voice quality.

Answer

FIFO

Answer

CBWFQ

Answer

WRED

Question 5

A service provider is implementing MPLS TE to optimize bandwidth utilization. The engineer notices that tunnels are not using the explicitly configured path. What is the most likely reason?

Accepted Answer

The explicit path contains a link that is down. When an MPLS TE tunnel is configured with an explicit path, the headend router uses RSVP to signal the path and verify that all links in the path are operational. If any link in the explicit path is down, RSVP signaling fails for that path, and the tunnel may fall back to dynamic path computation or remain down, rather than using the explicitly configured path. This is the most common reason for a tunnel not using its explicit path.

Answer

RSVP is not enabled on the headend

Answer

The tunnel destination is not reachable via IGP

Answer

The tunnel is configured with 'autoroute announce'

Question 6

A network engineer needs to design a multicast solution for IPTV distribution across a service provider network. Which protocol is best suited for building a shared distribution tree where the RP is dynamically elected?

Accepted Answer

PIM-SM with BSR. PIM-SM with BSR (Bootstrap Router) is the correct choice because it provides a dynamic, automated mechanism for electing an RP (Rendezvous Point) within a PIM-SM domain. BSR uses a distributed election process where candidate BSRs exchange bootstrap messages to elect a single BSR, which then collects candidate-RP advertisements and distributes the RP-set to all routers, enabling shared distribution trees without manual RP configuration.

Answer

PIM-DM

Answer

PIM-SM with Auto-RP

Answer

MSDP

Question 7

A service provider is deploying Segment Routing in the MPLS core using IS-IS. Which extension is required in IS-IS to advertise the prefix-SID?

Accepted Answer

Sub-TLV 3 (Prefix-SID). In IS-IS, the Prefix-SID is advertised using Sub-TLV 3, which is carried within TLV 135 (Extended IP Reachability). This sub-TLV contains the SID value and flags, enabling Segment Routing in the MPLS core. Without Sub-TLV 3, the prefix-SID cannot be signaled, making it the required extension.

Answer

TLV 22 (Extended IS Reachability)

Answer

TLV 242 (Router Capability)

Answer

TLV 135 (Extended IP Reachability)

Question 8

Drag and drop the steps to configure OSPF on a Cisco IOS router into the correct order.

Accepted Answer

Enter global configuration mode, then enable the OSPF process with 'router ospf' command, then advertise networks under the OSPF process using 'network' command.. OSPF configuration requires entering global config mode, enabling the OSPF process, and then advertising networks under the OSPF process.

Answer

Advertise networks using 'network' command, then enter global config mode, then enable OSPF process.

Answer

Enable OSPF process, then advertise networks, then enter global config mode.

Answer

Enter global configuration mode, then advertise networks, then enable OSPF process.

Question 9

Drag and drop the steps to configure EIGRP on a Cisco router into the correct order.

Accepted Answer

Router(config)# router eigrp 100
Router(config-router)# network 10.0.0.0
Router(config-router)# no auto-summary. EIGRP configuration requires entering the EIGRP process, advertising networks, and optionally disabling auto-summary.

Answer

Router(config)# router eigrp 100
Router(config-router)# no auto-summary
Router(config-router)# network 10.0.0.0

Answer

Router(config)# network 10.0.0.0
Router(config)# router eigrp 100
Router(config-router)# no auto-summary

Answer

Router(config)# router eigrp 100
Router(config-router)# no auto-summary
Router(config-router)# network 10.0.0.0
(Router(config-router)# exit)

Question 10

Match each MPLS VPN technology to its description.

Accepted Answer

MPLS Layer 3 VPN: Uses BGP to distribute customer routes between PE routers. MPLS provides multiple VPN services: Layer 3 VPNs use BGP for route exchange, Layer 2 VPNs emulate L2 circuits, TE controls traffic paths, and Inter-AS extends connectivity across providers. Common confusions involve swapping the L2 and L3 definitions.

Answer

MPLS Layer 3 VPN: Provides transparent transport of Layer 2 frames

Answer

MPLS Layer 2 VPN: Uses BGP to distribute customer routes between PE routers

Question 11

Match each MPLS protection mechanism to its description.

Accepted Answer

MPLS Fast Reroute (FRR): Protects against link or node failures by pre-computing a backup path for each LSP.. The correct matches are: A (MPLS FRR), B (MPLS TE FRR), C (MPLS PW Redundancy), D (MPLS LSP Protection). Distractors E and F swap definitions or use incorrect descriptions.

Answer

MPLS Fast Reroute (FRR): Protects against failures by using a globally reserved bandwidth pool for all backup LSPs.

Answer

MPLS Traffic Engineering Fast Reroute (TE FRR): Protects against failures by relying on IP routing convergence of the IGP.

Question 12

A service provider is deploying MPLS in its core network. The core routers are all configured with LDP. Which label operation does an ingress PE perform on the first packet of a new flow?

Accepted Answer

Push a new label onto the packet. The ingress PE (Provider Edge) router is the first router in the MPLS domain to receive an unlabeled IP packet. For the first packet of a new flow, the ingress PE must perform a label push operation, which adds an MPLS label (or a stack of labels) to the packet. This label is used by subsequent LSRs to forward the packet based on the label rather than the IP header, enabling MPLS forwarding.

Answer

Remove the label stack entirely

Answer

Pop the label (PHP)

Answer

Swap the incoming label for an outgoing label

Question 13

A network engineer is troubleshooting QoS in a service provider environment. Customer traffic is marked with DSCP AF31 (011010) at the CE. On the PE router, the policy maps trust DSCP and sets the CoS to 4. However, core routers remark the DSCP to 0. What is the most likely cause?

Accepted Answer

A policer on the core router is marking down out-of-contract traffic. The core router is likely applying a policer that marks down traffic exceeding the committed information rate (CIR). When traffic is out-of-contract, the policer can re-mark the DSCP to 0 (best effort), which overrides the trusted DSCP AF31 and the CoS 4 set at the PE. This behavior is common in service provider environments where core routers enforce traffic contracts and use policing to penalize excess traffic.

Answer

The core routers use LDP labels and ignore DSCP

Answer

The ingress PE did not set MPLS EXP bits

Answer

The MPLS EXP bits are not copied from DSCP

Question 14

A service provider is deploying Segment Routing (SR) with TI-LFA for fast convergence. Which mechanism does TI-LFA use to repair a link failure in a node segment path?

Accepted Answer

Precomputes a backup path using a segment list (label stack) that avoids the failed link. TI-LFA (Topology Independent Loop-Free Alternate) precomputes a backup path using a segment list (label stack) that explicitly avoids the failed link. This is achieved by encoding a repair path as a sequence of adjacency and node segments, ensuring that traffic is steered around the failure without loops, regardless of the network topology. The backup path is installed in the forwarding table before the failure occurs, enabling sub-50ms convergence.

Answer

Uses loop-free alternate (LFA) as defined in IPFRR

Answer

Uses RSVP-TE to signal a protected LSP

Answer

Relies on BFD to trigger a backup path in the forwarding table

Question 15

In an MPLS VPN environment, which address family is used to exchange VPNv4 routes between PE routers?

Accepted Answer

VPNv4 unicast. In an MPLS VPN environment, VPNv4 routes (which include both the IPv4 prefix and the Route Distinguisher) are exchanged between Provider Edge (PE) routers using the VPNv4 unicast address family under the BGP configuration. This address family allows PE routers to carry customer routes with unique RD values, enabling address overlap across different VPNs.

Answer

Route-target (RT)

Answer

IPv4 unicast

Answer

VPNv6 unicast