350-501 Architecture Practice Test 9 — 15 Questions

Question 1

Which TWO are functions of the MPLS data plane?

Accepted Answer

Label push at ingress LSR. The MPLS data plane is responsible for forwarding packets based on labels, and label push at ingress LSR (A) is a core data-plane function: the ingress label edge router imposes (pushes) an MPLS label onto incoming IP packets before forwarding them into the MPLS domain. Label swap at transit LSR (C) is also a core data-plane function: a transit label switch router replaces the incoming label with the outgoing label according to its LFIB and forwards the packet along the LSP. By contrast, label distribution via LDP (B) is a control-plane function that builds and maintains label bindings, not packet forwarding. Metric calculation for routing (D) belongs to the routing protocol control plane (e.g., OSPF/IS-IS metric computation), and route exchange via BGP (E) is a control-plane function for exchanging reachability information, not a data-plane forwarding operation.

Answer

Label distribution via LDP

Answer

Metric calculation for routing

Answer

Route exchange via BGP

Question 2

Refer to the exhibit. The router has an outgoing label of 'Pop' for prefix ***********. What does this imply?

Accepted Answer

The router is the penultimate hop and will remove the label before forwarding. When a router has an outgoing label of 'Pop' for a prefix in MPLS, it indicates that the router is the penultimate hop (PHP) in the LSP. PHP means the router will remove (pop) the top label before forwarding the packet to the next hop, reducing the burden on the egress LSR. This is standard MPLS behavior as per RFC 3031, where the egress router signals an implicit null label (label 3) to its upstream neighbor.

Answer

The router has not learned a label for that prefix

Answer

The router will not forward traffic for that prefix

Answer

The router will push a new label for that prefix

Question 3

Refer to the exhibit. This JSON policy is used for Segment Routing Traffic Engineering. What is the purpose of the 'color' attribute?

Accepted Answer

It identifies a specific SR-TE policy to be used for traffic steering. In Segment Routing Traffic Engineering (SR-TE), the 'color' attribute is a 32-bit numerical value that, combined with the destination prefix, uniquely identifies an SR-TE policy. This allows traffic to be steered into a specific policy based on color, enabling advanced traffic engineering use cases such as multi-topology routing or service-level agreement (SLA) differentiation.

Answer

It sets the metric type for the path calculation

Answer

It defines the preference value for the candidate path

Answer

It specifies the link affinity constraint for the path

Question 4

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 (Multiprotocol BGP) is the routing protocol used to distribute VPNv4 routes between PE routers in an MPLS Layer 3 VPN. It carries the route target and route distinguisher extended communities that identify which VPN a prefix belongs to. Standard IGPs like OSPF or IS-IS cannot carry the VPNv4 address family or the required BGP extended communities.

Answer

IS-IS

Answer

OSPF

Answer

EIGRP

Question 5

A customer is experiencing intermittent packet loss in an MPLS L3VPN network. The PE routers show no errors on interfaces. Which tool can help isolate the issue by showing the complete path a packet takes through the MPLS network?

Accepted Answer

MPLS traceroute. MPLS traceroute (often implemented via LSP ping with the 'traceroute' option or the 'mpls traceroute' command) is the correct tool because it uses MPLS echo request/reply messages (RFC 8029) to traverse the label-switched path (LSP) hop by hop, revealing the exact label stack and transit LSRs. This allows identification of a failing or misbehaving node in the MPLS core that standard IP traceroute would not see, since IP traceroute relies on TTL expiry in the IP header, which may not occur in an MPLS network where labels are swapped and IP TTL is often copied from the label TTL.

Answer

Traceroute

Answer

Ping

Answer

MPLS ping

Question 6

In Segment Routing, what is the role of the 'Prefix-SID'?

Accepted Answer

It identifies a prefix in the IGP. A Prefix-SID is a Segment Routing identifier bound to an IGP prefix. It allows forwarding along the shortest path computed by the IGP to that prefix. Therefore, it identifies a prefix in the IGP, not a specific adjacency or node.

Answer

It identifies a specific adjacency

Answer

It is used for service chaining

Answer

It identifies a specific node

Question 7

Which protocol is used to exchange label binding information in a classic MPLS network without Segment Routing?

Accepted Answer

LDP. In a classic MPLS network without Segment Routing, LDP (Label Distribution Protocol) is the standard protocol used to exchange label binding information. LDP assigns and distributes labels for each FEC (Forwarding Equivalence Class) based on the underlying IGP routing table, enabling label-switched paths (LSPs) without requiring RSVP or Segment Routing extensions.

Answer

OSPF

Answer

BGP

Answer

IS-IS

Question 8

Which three are benefits of using MPLS Layer 3 VPNs over traditional VPNs? (Choose three.)

Accepted Answer

Scalability. MPLS Layer 3 VPNs are highly scalable because service providers add new customer sites by configuring VRFs and MP-BGP on the PE routers rather than building separate tunnels or circuits per site, so the provider core stays simple as the customer base grows (A). They also support multiple services over a single MPLS infrastructure, since the same provider edge can carry L3VPN, L2VPN, and other traffic separated by labels and VRFs, letting one network deliver many service types (B). Routing is simplified because the provider handles the core routing and only PE routers run MP-BGP with VPNv4/VPNv6 address families and VRFs, so customer sites exchange routes through the provider instead of managing a full mesh of tunnels or complex overlay routing (E). Lower cost (C) is not a guaranteed benefit since MPLS L3VPN deployment and provider pricing vary, and reduced security (D) is not a benefit at all—MPLS VPNs actually provide traffic separation through VRFs and can be secured with encryption.

Answer

Lower cost

Answer

Reduced security

Question 9

Which two protocols are commonly used for label exchange in an MPLS network? (Choose two.)

Accepted Answer

LDP. LDP (Label Distribution Protocol) is correct because it is the standard protocol designed specifically to distribute MPLS labels between adjacent Label Switch Routers, binding a label to each FEC so that LSRs can forward packets along label-switched paths. BGP is also correct because BGP, when extended with the labeled-unicast address family (RFC 3107), can distribute MPLS labels along with prefixes, which is essential for inter-AS MPLS and BGP-based MPLS VPNs (e.g., VPNv4/VPNv6 with route targets). OSPF and IS-IS are link-state routing protocols that distribute IP reachability information and can carry MPLS TE extensions (opaque LSAs/TLVs) for traffic engineering, but they do not themselves exchange MPLS labels. PIM is a multicast routing protocol used to build multicast distribution trees and has no role in label distribution.

Answer

OSPF

Answer

PIM

Answer

IS-IS

Question 10

Refer to the exhibit. A PE router is showing a VRF route. What does the [200/0] indicate?

Accepted Answer

Administrative distance 200 and metric 0. In the VRF route output, the [200/0] notation represents the administrative distance (200) and the metric (0) for the route. The administrative distance is a trustworthiness value used by the router to select the best path when multiple routing protocols provide routes to the same destination; a lower value is preferred. The metric is a protocol-specific cost, and here it is 0, indicating a directly connected or very low-cost path within the VRF.

Answer

Local preference 200 and MED 0

Answer

Label value 200 and metric 0

Answer

AS path length 200 and weight 0

Question 11

A service provider is deploying Segment Routing Traffic Engineering (SR-TE) with a Path Computation Element (PCE). The PCE fails to compute a path for an SR-TE policy configured with constraint 'avoid nodes in affinity red'. The core network has red affinity assigned to several nodes. Which is the most likely cause?

Accepted Answer

The PCE does not have the updated link-state database with the affinity attributes. The PCE computes paths based on the link-state database (LSDB) it receives from the IGP (e.g., OSPF or IS-IS). If the LSDB does not include the affinity (color) attributes for the nodes, the PCE cannot evaluate the 'avoid nodes in affinity red' constraint. This results in a path computation failure, even though the affinities are correctly configured on the network devices.

Answer

The headend router does not have the SR-TE policy configured with the correct path

Answer

The PCE is not reachable from the headend router

Answer

The SR-TE policy is configured with a strict explicit path that conflicts with the constraint

Answer

The red affinity is not properly configured in the network

Question 12

When deploying IS-IS in a large service provider core, what is the recommended network type on Ethernet interfaces to improve scalability?

Accepted Answer

point-to-point. In a large service provider core, configuring IS-IS interfaces as point-to-point (network type point-to-point) improves scalability by eliminating the need for Designated Router (DIS) election and the associated pseudonode LSPs. This reduces the number of LSPs in the link-state database, lowers CPU and memory overhead, and speeds up convergence, which is critical in dense core networks.

Answer

loopback

Answer

point-to-multipoint

Answer

non-broadcast

Answer

broadcast

Question 13

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?

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 14

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 15

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. Option A is correct because iBGP requires that all routers within the same AS form a full mesh of sessions, or use route reflectors (or confederations) to relax that requirement, since routes learned via iBGP are not re-advertised to other iBGP peers by default, which prevents loops but demands a full mesh or route reflectors. Option E is correct because BGP itself does not provide next-hop reachability; the next-hop IP address carried in BGP updates must be resolvable via the IGP (OSPF, IS-IS, etc.) running inside the AS, so the IGP is essential for BGP to function within the service provider core. Option B is incorrect because the MED attribute is optional and non-transitive, not mandatory for all routes. Option C is incorrect because AS path loop prevention applies to eBGP; within iBGP, the AS path is not used for loop prevention since all routers share the same AS number. Option D is incorrect because route reflectors do not modify the AS path; they use the ORIGINATOR_ID and CLUSTER_LIST attributes to prevent 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