350-501 Architecture Practice Test 6 — 15 Questions

Question 1

An MPLS Traffic Engineering LSP fails to establish. The RSVP signaling shows 'PathErr: Admission Control Failure'. The link has sufficient bandwidth but the headend reports a lack of resources. What is the most likely cause?

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

Accepted Answer

The RSVP bandwidth is not configured on the interface or is set to 0. The 'PathErr: Admission Control Failure' with 'lack of resources' despite sufficient link bandwidth indicates that RSVP has no reservable bandwidth on the interface. This occurs when the 'ip rsvp bandwidth' command is missing or set to 0, preventing RSVP from admitting any LSP even if the physical link has capacity. The headend correctly reports the failure because the interface-level RSVP resource pool is exhausted or unconfigured.

Answer

The tunnel is requesting more bandwidth than available on any path

Answer

The MPLS forwarding table is corrupted

Answer

The headend router does not have the correct destination address

Answer

The LSP is configured with a strict explicit path that is impossible

Question 2

A service provider uses BGP communities to influence routing policies. An administrator wants to prevent a specific prefix from being advertised to any external AS. Which BGP community should be applied?

Accepted Answer

NO_EXPORT. The NO_EXPORT community (well-known community 0xFFFFFF01) tells the router not to advertise the prefix to any external BGP peer (eBGP), but allows it to be advertised within the same AS (iBGP). This matches the requirement to prevent the prefix from being advertised to any external AS, as NO_EXPORT is specifically designed to keep routes confined to the local autonomous system.

Answer

NO_ADVERTISE

Answer

INTERNET

Answer

LOCAL_AS

Answer

NONE

Question 3

In an MPLS VPN, what label operation does the egress PE perform on the VPN packet before forwarding it to the CE?

Accepted Answer

Pop the label stack. In an MPLS VPN, the egress PE (Provider Edge) router receives a packet with a label stack typically consisting of a transport label (IGP label) and a VPN label. Before forwarding the IP packet to the CE (Customer Edge), the egress PE must remove the entire MPLS label stack because the CE is an IP-only device that does not understand MPLS labels. This operation is called 'pop the label stack' (also known as penultimate hop popping, though at the egress PE it is explicit label removal).

Answer

Swap the top label

Answer

Impose a VPN label

Answer

Push a new label

Answer

Replace the transport label

Question 4

In Inter-AS MPLS Option B, which routers exchange labeled VPNv4 prefixes directly?

Accepted Answer

ASBR routers. In Inter-AS MPLS Option B, ASBR routers (Autonomous System Boundary Routers) exchange labeled VPNv4 prefixes directly across the AS boundary. This is achieved by having each ASBR perform two label operations: it pops the outer transport label from the VPNv4 prefix received from its own AS, then swaps the inner VPN label and forwards the labeled VPNv4 prefix to the neighboring ASBR in the adjacent AS. This direct exchange eliminates the need for end-to-end LSPs or MP-eBGP sessions between PE routers across different ASes.

Answer

PE routers

Answer

P routers

Answer

CE routers

Answer

Route reflectors

Question 5

A service provider is deploying Segment Routing Traffic Engineering (SR-TE) to optimize traffic flows. Which mechanism is used to steer traffic into an SR-TE policy?

Accepted Answer

Color extended community on BGP routes. In Segment Routing Traffic Engineering (SR-TE), traffic is steered into an SR-TE policy using a color extended community attached to BGP routes. The color community acts as a service-level identifier that maps BGP-learned prefixes to a specific SR-TE policy on the headend router, enabling automated traffic engineering without per-flow state or tunnel interfaces.

Answer

RSVP-TE tunnel interface

Answer

BGP prefix-independent convergence (PIC)

Answer

Manual static route pointing to the policy

Answer

LDP label distribution

Question 6

A service provider is designing a BGP-based network. Which TWO are characteristics of BGP within a service provider core?

Accepted Answer

IBGP sessions require a full mesh or route reflectors to avoid routing loops. IBGP requires either a full mesh of sessions or route reflectors to prevent routing loops. The BGP split-horizon rule states that routes learned from an iBGP peer are not advertised to another iBGP peer, so without a full mesh or route reflectors, reachability information cannot propagate within the AS. Route reflectors break this rule by allowing selected iBGP speakers to re-advertise routes, but they must be deployed with proper cluster and originator ID attributes to avoid loops.

Answer

BGP MED attribute is mandatory for all routes

Answer

BGP uses the AS path for loop prevention in iBGP

Answer

BGP route reflectors always modify the AS path

Question 7

An engineer is configuring LDP in an MPLS network. Which THREE are valid label distribution modes for LDP?

Accepted Answer

Downstream on Demand with Independent Label Distribution Control. Downstream on Demand (DoD) with Independent Label Distribution Control allows a router to request a label mapping only when needed and to advertise bindings independently, as defined in RFC 5036. Option B is correct because Downstream Unsolicited (DU) with Conservative Label Retention means a router learns all label mappings but only stores those for routes it actively uses, which is a supported mode in MPLS LDP implementations. Option C is correct because Downstream Unsolicited with Liberal Label Retention is the default and most common mode, where a router stores all label mappings received from neighbors. Options D and E are incorrect because Ordered Label Distribution Control is typically used with DoD, not DU, and Liberal Label Retention is rarely paired with DoD due to resource waste.

Answer

Downstream Unsolicited with Ordered Label Distribution Control

Answer

Downstream on Demand with Liberal Label Retention

Question 8

In IS-IS, which TWO are types of Link State PDUs (LSPs)?

Accepted Answer

Level 2 LSP. In IS-IS, Link State PDUs (LSPs) are the fundamental building blocks of the link-state database. They carry topology and prefix information and are generated by each router. There are two distinct types: Level 1 LSPs, which advertise information within the same area, and Level 2 LSPs, which advertise information between areas. Options B and E correctly identify these two types.

Answer

Partial Sequence Numbers PDU (PSNP)

Answer

Hello LSP

Answer

Complete Sequence Numbers PDU (CSNP)

Question 9

A service provider is designing its core network to carry both IPv4 and IPv6 traffic. Which BGP approach best minimizes routing table size on core routers while maintaining full reachability?

Accepted Answer

Use iBGP with route reflectors. iBGP with route reflectors minimizes the number of BGP sessions required in the core network, reducing routing table size and control-plane overhead while still maintaining full IPv4 and IPv6 reachability. Route reflectors allow iBGP speakers to advertise routes learned from other iBGP peers without requiring a full mesh, which is essential for scalability in large service provider cores.

Answer

Use eBGP between all core routers

Answer

Use static routing for all prefixes

Answer

Rely on OSPF for external routes

Question 10

An engineer notices that an MPLS LSP in the core is not establishing. Which troubleshooting step should be performed first?

Accepted Answer

Verify LDP neighbor adjacency. The first step in troubleshooting an MPLS LSP that is not establishing is to verify LDP neighbor adjacency because LDP is the protocol responsible for distributing labels across the MPLS domain. Without a working LDP session between adjacent LSRs, label bindings cannot be exchanged, and the LSP cannot be built. This is the foundational layer for MPLS forwarding, so it must be checked before higher-layer issues like routing or reachability.

Answer

Ping the far-end loopback interface

Answer

Check BGP IPv4 unicast neighbors

Answer

Check OSPF neighbor adjacency on all links

Question 11

A large ISP is designing a multicast architecture to support IPTV, requiring high availability and minimal traffic convergence. Which RP placement design is most appropriate?

Accepted Answer

Anycast-RP with multiple RPs sharing same IP. Anycast-RP with multiple RPs sharing the same IP address is the most appropriate design for a large ISP supporting IPTV because it provides high availability and load sharing without a single point of failure. In the event of an RP failure, traffic automatically converges to the nearest surviving RP using the same anycast address, minimizing disruption. This design also avoids the convergence delays inherent in dynamic RP discovery protocols like Auto-RP or BSR.

Answer

Single static RP placed on the core router

Answer

Auto-RP with one RP mapping agent

Answer

BSR with one candidate RP

Question 12

Which encapsulation technology is commonly used in service provider networks to separate subscriber traffic in an Ethernet aggregation network?

Accepted Answer

Q-in-Q (802.1ad). Q-in-Q (802.1ad) is the correct encapsulation technology because it allows service providers to stack an outer service VLAN tag (S-Tag) on top of a customer's inner VLAN tag (C-Tag), effectively separating subscriber traffic in an Ethernet aggregation network. This double-tagging mechanism preserves the customer's VLAN space while enabling the provider to aggregate multiple subscribers over a single physical port, which is essential for Metro Ethernet and DSLAM aggregation scenarios.

Answer

VXLAN

Answer

802.1Q

Answer

MPLS

Question 13

A service provider wants to implement Segment Routing in its MPLS core to simplify traffic engineering. What is the primary advantage of Segment Routing over traditional RSVP-TE?

Accepted Answer

No per-flow state on transit routers. Segment Routing (SR) eliminates the need for per-flow state on transit routers by encoding the path as a list of segment identifiers (SIDs) directly in the packet header. In contrast, RSVP-TE requires each transit router to maintain soft-state for every LSP, consuming memory and CPU. This state reduction simplifies network operation and scales better in large MPLS cores.

Answer

Improved ECMP load balancing

Answer

Better support for Fast Reroute

Answer

Requires a central controller for path calculation

Question 14

An ISP is deploying EVPN-VXLAN for its data center interconnect. Which statement about control-plane learning is correct?

Accepted Answer

MAC addresses are learned via BGP MP-BGP EVPN address family. In EVPN-VXLAN deployments for data center interconnect, MAC address learning occurs via the control plane using BGP with the MP-BGP EVPN address family (AFI 25 / SAFI 70). This eliminates the need for traditional data-plane flooding and learning, providing better scalability and control. The correct statement is that MAC addresses are learned via BGP MP-BGP EVPN address family.

Answer

MAC addresses are learned via IRB

Answer

MAC addresses are learned via OSPF

Answer

MAC addresses are learned via VXLAN data plane

Question 15

Which QoS mechanism is most effective for preventing congestion on a service provider edge link when traffic exceeds the link capacity?

Accepted Answer

Traffic shaping. Traffic shaping is the most effective QoS mechanism for preventing congestion on a service provider edge link when traffic exceeds link capacity because it buffers excess packets and smooths the transmission rate to match the configured committed information rate (CIR). Unlike policing, which drops or re-marks packets, shaping avoids tail drops and packet loss by queuing bursts, making it ideal for edge links where preserving traffic integrity is critical. Cisco IOS uses the Generic Traffic Shaping (GTS) or Class-Based Shaping (CB-Shaping) to enforce a rate limit while maintaining TCP fairness and reducing retransmissions.

Answer

Priority queuing

Answer

Traffic policing

Answer

WRED