350-501 MPLS and Segment Routing Practice Test 13 — 15 Questions

Question 1

In SR-MPLS, which mechanism replaces LDP for label distribution?

Accepted Answer

IGP (IS-IS or OSPF) with Segment Routing extensions. Segment Routing replaces LDP by using IGP extensions (IS-IS or OSPF) to advertise Prefix-SIDs (Node-SIDs) and Adjacency-SIDs. In IS-IS, TLV 242 (SR Capabilities) advertises the SRGB, while Node-SIDs are advertised via Prefix-SID sub-TLVs (e.g., TLV 135 for IPv4), eliminating the need for a separate label distribution protocol.

Answer

RSVP-TE

Answer

MPLS-TE

Answer

BGP-LU

Question 2

An engineer configures an RSVP-TE tunnel with a bandwidth of 100 Mbps and an explicit path that excludes a specific interface. The tunnel fails to come up. Which term describes the path computation method used by RSVP-TE to find a feasible path given these constraints?

Accepted Answer

CSPF (Constrained Shortest Path First). CSPF (Constrained Shortest Path First) is the algorithm used by RSVP-TE to compute a path that satisfies constraints such as bandwidth, explicit path, and link affinities.

Answer

SPF (Shortest Path First)

Answer

Bellman-Ford

Answer

Dijkstra with ECMP

Question 3

In SR-MPLS, when a router receives a packet with an MPLS label corresponding to an Adjacency SID, what operation does it perform?

Accepted Answer

It pops the label and forwards the packet out of the specific interface.. An Adjacency SID represents a specific link; when a router receives a packet with that label, it pops the label and forwards the packet out of the associated interface, because the Adjacency SID is a 'penultimate hop popping' type of SID.

Answer

It performs a recursive lookup in the FIB to determine the next hop.

Answer

It pushes an additional label and forwards.

Answer

It swaps the label with the next-hop label and forwards.

Question 4

An engineer is implementing MPLS-TE Fast Reroute for link protection. Which protection method provides a backup tunnel that bypasses a specific link, protecting all LSPs traversing that link?

Accepted Answer

Bypass tunnel protecting the link. A bypass tunnel is a single tunnel protecting a link, carrying traffic for all LSPs that traverse that link. Detour tunnels are per-LSP.

Answer

Node protection bypass tunnel

Answer

1:1 detour tunnel per LSP

Answer

FRR using RSVP-TE path pre-emption

Question 5

In Segment Routing over IPv6 (SRv6), what is the function of the 'Locator' part of an SRv6 SID?

Accepted Answer

It identifies the node and is used for routing the packet to that node.. The Locator is the portion of the SRv6 SID that identifies the node (or the segment endpoint) and is used for routing to that node.

Answer

It indicates the length of the SID.

Answer

It encodes an argument for the SRv6 function.

Answer

It specifies the function to be performed on the packet.

Question 6

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 7

In SR-MPLS TE, a headend router is configured with an SR Policy that includes an explicit candidate path with segment list [16001, 16003]. What do these numbers represent?

Accepted Answer

MPLS label values representing Node SIDs or Adjacency SIDs. In SR-MPLS, segment lists are composed of MPLS labels. Node SIDs are typically from the SRGB (e.g., 16001 could be a Node SID for a router). Adjacency SIDs would be link-specific labels. The numbers are labels, not interface indices or metric values.

Answer

Interface indices

Answer

Metric values for path calculation

Answer

IPv4 addresses encoded as numbers

Question 8

Which LDP label retention mode stores all received label bindings even if they are not currently used for forwarding?

Accepted Answer

Liberal label retention. Liberal label retention stores all label bindings, allowing quick convergence when a path changes, while conservative retention stores only bindings from the next-hop.

Answer

Conservative label retention

Answer

Ordered label distribution

Answer

Downstream unsolicited

Question 9

In SRv6, which endpoint behavior is used for decapsulating the outer IPv6 header and performing an IPv4 lookup (e.g., for L3VPN)?

Accepted Answer

End.DT4. End.DT4 is the SRv6 endpoint behavior that decapsulates the outer IPv6 header and then performs a lookup in the IPv4 VPN table. End.DT6 is for IPv6, and End.DT2U is for L2VPN.

Answer

End.DT6

Answer

End.DT2U

Answer

End.DX4

Question 10

An engineer is troubleshooting an MPLS network where some routers are not exchanging labels via LDP. Which two steps are most appropriate for diagnosing the issue? (Choose two.)

Accepted Answer

Verify LDP neighbor adjacencies using 'show mpls ldp neighbor'.. Verifying LDP sessions with 'show mpls ldp neighbor' and checking interfaces with 'show mpls ldp interface' are standard first steps. Checking MTU or OSPF neighbors are less direct.

Answer

Check the MTU size on all interfaces.

Answer

Reboot all routers to reset LDP sessions.

Answer

Verify that OSPF neighbor relationships are established.

Question 11

In SR-MPLS traffic engineering, which three components are essential for configuring an SR Policy on a headend router? (Choose three.)

Accepted Answer

Headend router. In SR-MPLS traffic engineering, an SR Policy on a headend router is defined by three essential components: the headend router itself (the router that originates the policy), the endpoint (destination prefix), and a color value that distinguishes multiple policies to the same endpoint. Bandwidth reservation (A) is optional and not part of the basic policy identifier. Explicit path with ERO (B) is a way to specify the path but is not a required component of the policy definition; segment lists are used instead. Therefore, the correct choices are C (headend), D (endpoint), and E (color).

Answer

Bandwidth reservation value

Answer

Explicit path with ERO

Question 12

In EVPN multi-homing, which two mechanisms are used to avoid duplicate packets and ensure efficient forwarding? (Choose two.)

Accepted Answer

Split-horizon filtering. Designated Forwarder (DF) election ensures only one PE forwards traffic to the CE for broadcast/multicast/unknown unicast, and split-horizon prevents a PE from forwarding packets back to the ES from which they were received.

Answer

MAC mobility

Answer

Aliasing

Answer

ARP suppression

Question 13

Which three statements accurately describe the benefits of Segment Routing over RSVP-TE for traffic engineering? (Choose three.)

Accepted Answer

It does not require a signaling protocol like RSVP.. SR-TE reduces state by not requiring per-LSP state on intermediate nodes, supports distributed intelligence via IGP, and does not require a separate protocol like RSVP. However, SR-TE still requires CSPF for path computation, and RSVP-TE also supports fast reroute.

Answer

It provides faster convergence than RSVP-TE FRR.

Answer

It eliminates the need for CSPF path computation.

Question 14

Which two actions are performed by an MPLS label edge router (LER) when it receives an unlabeled IP packet destined for a prefix with an LSP? (Choose two.)

Accepted Answer

It forwards the packet based on the MPLS label.. The LER (or ingress LSR) imposes a label stack (push operation) on the packet, then forwards it based on the label. It does not swap labels (that's done by LSRs), nor does it pop the label (egress LER does).

Answer

It performs a recursive lookup in the LFIB.

Answer

It performs a label swap operation.

Answer

It pops the top label and forwards based on the inner header.

Question 15

A network engineer is designing a Segment Routing over IPv6 (SRv6) deployment. Which TWO statements are true about the SRv6 SID format and behaviors? (Choose two.)

Accepted Answer

End.DT6 behavior is used for IPv6 L3VPN services.. SRv6 SID is 128 bits and consists of locator, function, and optional argument. End.DT6 is used for IPv6 L3VPN services. End.DT4 is for IPv4 L3VPN, not for L2VPN. End.X is not the default behavior; the default is End. The SRH is an extension header that does not replace the IPv6 header. Therefore, the correct answers are B and D.

Answer

End.X behavior is a default behavior for any SID that is not explicitly defined.

Answer

End.DT4 behavior is used for EVPN L2VPN services over SRv6.

Answer

The SRH (Segment Routing Header) is an extension header that replaces the IPv6 header when SR is used.