350-501 Architecture Practice Test 5 — 15 Questions

Question 1

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 2

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 3

An SP is migrating from an MPLS LDP-based network to Segment Routing. They want to ensure that existing LDP LSPs continue to work alongside SR LSPs during the migration. Which mechanism should be configured?

Accepted Answer

LDP-SR interworking. LDP-SR interworking (RFC 8661) allows LDP and Segment Routing LSPs to coexist and interoperate during migration. It enables LDP-signaled LSPs to stitch with SR LSPs by mapping LDP labels to SR prefix-SIDs, ensuring end-to-end MPLS forwarding without service disruption. This mechanism is specifically designed for seamless coexistence without requiring a full network cutover.

Answer

SRGB

Answer

BGP-LU

Answer

MPLS TE

Question 4

In a carrier-supporting carrier (CSC) architecture, the customer carrier runs MPLS in its network. Which technology is used to exchange VPN routes between the provider carrier and the customer carrier?

Accepted Answer

MP-BGP with VPNv4 address family. In a carrier-supporting-carrier (CSC) architecture, the customer carrier runs MPLS in its network and needs to exchange VPN routes (e.g., customer VPN prefixes) with the provider carrier. MP-BGP with the VPNv4 address family is the correct technology because it carries both the IPv4 prefix and the Route Distinguisher (RD), enabling the provider carrier to distinguish overlapping VPN routes from different customer VPNs. This is defined in RFC 4364 and is the standard inter-AS VPN option for CSC scenarios.

Answer

OSPF

Answer

EIGRP

Answer

IPv4 BGP

Question 5

A service provider is deploying multicast over MPLS L3VPN. Which mechanism is used to provide multicast in the core between PE routers?

Accepted Answer

mLDP. In an MPLS L3VPN multicast deployment, the core network between PE routers uses a label-based multicast mechanism rather than a traditional PIM protocol. mLDP (Multipoint LDP) is the correct choice because it establishes point-to-multipoint (P2MP) LSPs in the MPLS core, enabling efficient multicast transport without requiring PIM in the core. This approach is defined in RFC 6388 and is commonly used for MVPN (Multicast VPN) implementations.

Answer

IGMP

Answer

PIM-SM

Answer

PIM-DM

Question 6

Which two are requirements for deploying Segment Routing in a service provider network? (Choose two.)

Accepted Answer

IGP supporting segment routing (OSPF or IS-IS). Segment Routing (SR) relies on an IGP (OSPF or IS-IS) that has been extended to advertise prefix-SIDs and adjacency-SIDs. The IGP extensions (RFC 8665 for IS-IS, RFC 8666 for OSPF) enable routers to compute source-routed paths without a separate signaling protocol. This is a fundamental requirement because SR-MPLS encodes the path as a stack of labels derived directly from the IGP SIDs.

Answer

TE tunnels must be configured

Answer

LDP must be enabled

Answer

All routers must run BGP

Question 7

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

Accepted Answer

Scalability. MPLS Layer 3 VPNs use a full-mesh of MP-BGP sessions between Provider Edge (PE) routers to exchange VPNv4 routes, which allows the network to scale to thousands of VPNs and customers without requiring a full mesh of tunnels. This is far more scalable than traditional VPNs that rely on point-to-point tunnels (e.g., IPsec or GRE) which create an O(n²) problem for connectivity.

Answer

Lower cost

Answer

Reduced security

Question 8

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

Accepted Answer

LDP. LDP (Label Distribution Protocol) is a standardized protocol (RFC 5036) specifically designed for distributing MPLS labels between routers to establish Label Switched Paths (LSPs) without requiring an IGP to carry label information. BGP (Border Gateway Protocol) can also exchange MPLS labels, particularly in MPLS VPN and Inter-AS scenarios, where it carries VPNv4 routes with MPLS labels using the BGP label-mapping capability (RFC 3107).

Answer

OSPF

Answer

PIM

Answer

IS-IS

Question 9

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 10

Refer to the exhibit. What is the significance of the entry with 'Pop Label'?

Accepted Answer

It indicates the penultimate hop popping (PHP). In MPLS, the 'Pop Label' entry in a forwarding table indicates the Penultimate Hop Popping (PHP) operation. PHP is a standard behavior (RFC 3031) where the penultimate router removes the MPLS label before forwarding the packet to the egress LSR, reducing the egress router's processing load. This is explicitly signaled via the 'implicit-null' label (label value 3) in LDP or RSVP-TE.

Answer

It indicates a merge operation

Answer

It indicates the label is removed at this router

Answer

It indicates an error

Question 11

Refer to the exhibit. Which statement is true about this configuration?

Accepted Answer

LDP is automatically enabled on the interface. The `mpls ip` command under interface configuration enables LDP on that interface, allowing it to automatically discover LDP neighbors and exchange label bindings. This command implicitly enables LDP without requiring manual neighbor configuration, which is the default behavior for label distribution in many MPLS deployments.

Answer

LDP is manually configured

Answer

MPLS VPN is enabled

Answer

MPLS forwarding is disabled

Question 12

A service provider is designing a Layer 3 MPLS VPN for a customer with two sites. The customer requires fast convergence in case of a PE-CE link failure. Which routing protocol should be used between PE and CE to achieve the fastest convergence?

Accepted Answer

EBGP with BFD. EBGP with BFD provides the fastest convergence for PE-CE link failures because BFD can detect link failures in sub-second intervals (as low as 50 ms) and trigger BGP to withdraw routes immediately, without waiting for routing protocol timers. This is critical for Layer 3 MPLS VPNs where fast failover is required, and EBGP is commonly used as the PE-CE routing protocol in service provider environments.

Answer

Static routing with object tracking

Answer

IS-IS with SPF tuning

Answer

EIGRP

Answer

OSPF with fast hello timers

Question 13

A service provider is troubleshooting BGP route selection for prefixes received from two different peers. The first peer prepends its AS path twice, making it longer than the second peer's path. However, the router still prefers the route with the longer AS path. Which additional attribute could cause this behavior?

Accepted Answer

The route has a higher local preference. BGP selects the best path based on a sequence of comparison steps. The first step after considering the next-hop reachability is to prefer the route with the highest weight (Cisco proprietary), then the highest local preference. Since the router prefers the longer AS path, a higher local preference (option D) on the first peer's route overrides the shorter AS path of the second peer, as local preference is evaluated before AS path length in the BGP best-path selection algorithm.

Answer

The route has a lower MED

Answer

The route has a higher weight

Answer

The route has a lower origin type

Answer

The route has a lower neighbor router ID

Question 14

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 15

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