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

Question 1

In an MPLS network using LDP, which statement correctly describes the label distribution behavior for a given FEC?

Accepted Answer

Labels are advertised unsolicited to all neighbors, and all labels are retained even if not used (liberal label retention).. LDP uses downstream unsolicited mode by default, where each LSR advertises labels for all FECs to all neighbors without being asked, and liberal label retention stores all received labels even if not currently used.

Answer

Labels are advertised only in response to a request from an upstream neighbor (downstream on demand).

Answer

Labels are advertised unsolicited but only the best label for each FEC is retained (conservative label retention).

Answer

Labels are advertised only when the LSR has an outgoing label for the FEC (ordered label distribution).

Question 2

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

Accepted Answer

IGP (IS-IS or OSPF) with Segment Routing extensions. In SR-MPLS, the IGP (IS-IS or OSPF) is extended with Segment Routing capabilities to distribute segment identifiers (SIDs) and their associated MPLS labels. This replaces LDP, as the IGP directly advertises label bindings for prefixes and adjacencies, eliminating the need for a separate label distribution protocol. RSVP-TE, MPLS-TE, and BGP-LU serve different purposes and do not replace LDP for basic label distribution in SR-MPLS.

Answer

RSVP-TE

Answer

MPLS-TE

Answer

BGP-LU

Question 3

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). RSVP-TE uses Constrained Shortest Path First (CSPF) to compute a path that satisfies constraints such as bandwidth and explicit exclusions. CSPF is an extension of SPF that takes into account additional constraints like available bandwidth, link attributes, and administrative groups. Therefore, the path computation method is CSPF.

Answer

SPF (Shortest Path First)

Answer

Bellman-Ford

Answer

Dijkstra with ECMP

Question 4

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 5

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 protecting the link is a Fast Reroute (FRR) protection method that creates a backup tunnel specifically to bypass a particular link, protecting all LSPs that traverse that link. When the link fails, traffic is redirected through the bypass tunnel, which is pre-established and avoids the failed link. This is the standard link protection mechanism in MPLS-TE FRR.

Answer

Node protection bypass tunnel

Answer

1:1 detour tunnel per LSP

Answer

FRR using RSVP-TE path pre-emption

Question 6

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 7

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 10-byte value that uniquely identifies a multihomed Ethernet segment in EVPN — that is, a set of links from one or more CEs to two or more PEs that form a single logical segment. It is carried in the Ethernet A-D route and is the key field used for all-active or single-active multihoming procedures. Without a unique ESI, PEs cannot coordinate DF election or aliasing for the segment.

Answer

Ethernet Tag ID

Answer

Route Distinguisher (RD)

Answer

MPLS Label

Question 8

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 9

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 10

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 11

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'.. Option A is correct because 'show mpls ldp neighbor' displays the state of LDP sessions (Operational, Non-existent, etc.) and the TCP connection status to each peer, which directly reveals whether label distribution adjacencies are forming. Option E is correct because LDP only runs on interfaces where 'mpls ip' (or 'mpls ldp' on some platforms) is enabled, and 'show mpls ldp interface' confirms which interfaces are actively participating in LDP label exchange. Option B is not appropriate because MTU mismatches typically affect data forwarding or IGP adjacency formation, not the LDP session establishment itself. Option C is not appropriate because rebooting routers is a disruptive action that does not diagnose the root cause of failed LDP sessions. Option D is not appropriate because while LDP relies on underlying IP reachability (often provided by OSPF), verifying OSPF adjacencies alone does not confirm LDP-specific configuration or session state.

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 12

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 is fundamentally identified by the tuple <headend, color, endpoint>, so option C (Headend router) is correct because the policy is instantiated and originated on the headend router that encapsulates packets with the segment list. Option D (Endpoint (destination) prefix) is correct because the endpoint defines the destination of the policy, typically the BGP next-hop or a loopback prefix, and is one of the three mandatory identifying elements. Option E (Color (used to steer traffic with a specific color)) is correct because the color is an extended community that binds the policy to a specific intent and is required to associate the SR Policy with the corresponding BGP service route. Option A (Bandwidth reservation value) is not essential for basic SR Policy configuration, as bandwidth is an optional constraint used for path computation but not part of the identifying tuple. Option B (Explicit path with ERO) is also not mandatory, since an SR Policy can be dynamically computed by the PCE or headend using IGP/BGP-LS topology without a statically configured ERO.

Answer

Bandwidth reservation value

Answer

Explicit path with ERO

Question 13

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

Accepted Answer

Split-horizon filtering. Split-horizon filtering (B) is correct because in EVPN multi-homing the Designated Forwarder uses split-horizon rules so that BUM traffic received from the core on one PE is not flooded back to the same Ethernet segment via other PEs, preventing duplicate packets from reaching the multihomed CE. Designated Forwarder (DF) election (E) is correct because per Ethernet Segment and per VLAN/BUM service, only one PE is elected DF to forward BUM traffic toward the CE, which avoids multiple PEs replicating the same frames and ensures efficient, deterministic forwarding. MAC mobility (A) is not the mechanism for duplicate avoidance; it handles MAC address movement between PEs by updating the EVPN MAC/IP route and sequence number. Aliasing (C) improves load balancing by allowing all PEs in an Ethernet Segment to receive unicast traffic for a multihomed CE, but it does not itself prevent duplicate BUM packets. ARP suppression (D) reduces ARP flooding by having PEs answer ARP requests from a local cache, which is an optimization rather than a duplicate-prevention mechanism.

Answer

MAC mobility

Answer

Aliasing

Answer

ARP suppression

Question 14

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.. Option A is correct because Segment Routing encodes the path as a list of SIDs (segments) in the packet header or as an MPLS label stack, so it does not need a signaling protocol such as RSVP-TE to establish or maintain LSPs. Option C is correct because SR extends the IGP (IS-IS or OSPF) to advertise prefix and adjacency SIDs, letting routers compute paths from the distributed topology and segment information rather than relying on a centralized or per-LSP signaling mechanism. Option D is correct because in SR only the ingress node holds the segment list for a path; transit routers simply forward based on the top SID/label and do not keep per-LSP state, which improves scalability. Option B is not marked correct because SR does not inherently guarantee faster convergence than RSVP-TE FRR; both can achieve sub-50 ms protection, and convergence depends on the IGP and implementation. Option E is not marked correct because CSPF-style constrained path computation is still needed at the ingress to select a path satisfying TE constraints, even though the result is expressed as a SID list rather than signaled via RSVP.

Answer

It provides faster convergence than RSVP-TE FRR.

Answer

It eliminates the need for CSPF path computation.

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.. Option B is correct because End.DT6 is the SRv6 endpoint behavior that performs IPv6 table lookup and forwarding, which is exactly what is needed to instantiate an IPv6 L3VPN (per-VRF IPv6 routing) service over SRv6. Option D is correct because an SRv6 SID is a 128-bit IPv6 address structured as Locator + Function + optional Arguments, where the Locator identifies the node and is routable, the Function selects the local behavior, and the Arguments carry behavior-specific parameters. Option A is wrong because there is no such default behavior; a SID whose function is not explicitly defined is simply not a valid SRv6 endpoint behavior, and End.X is specifically an endpoint behavior for cross-connecting to a specific adjacency (L3 link). Option C is wrong because End.DT4 is the IPv4 table lookup behavior used for IPv4 L3VPN, not EVPN L2VPN; EVPN L2 services over SRv6 use L2-related behaviors such as End.DT2M or End.DX2. Option E is wrong because the SRH is an IPv6 extension header (Next Header 43) that is inserted in addition to the IPv6 header, not a replacement for it.

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.