350-501 MPLS and Segment Routing Practice Test 9 — 15 Questions

Question 1

A service provider is deploying MPLS-TE with RSVP-TE in their core network. They notice that some LSPs are not being established due to resource contention. Which action would best address this issue without redesigning the entire traffic engineering deployment?

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

Accepted Answer

Enable preemption on RSVP-TE LSPs with appropriate priority levels.. Enabling preemption on RSVP-TE LSPs with appropriate setup and hold priorities allows higher-priority LSPs to tear down lower-priority LSPs to free up bandwidth, resolving resource contention without redesigning the entire TE deployment. This is the standard mechanism defined in RFC 3209 for managing bandwidth contention in MPLS-TE networks.

Answer

Increase the bandwidth of all core links.

Answer

Configure LSP path-option explicit paths with strict hops.

Answer

Disable RSVP-TE and use LDP for label distribution.

Question 2

A service provider is designing a greenfield MPLS core and wants to minimize control-plane complexity while still supporting traffic engineering. They plan to use Segment Routing with MPLS. Which statement about Segment Routing in this context is accurate?

Accepted Answer

Segment Routing reduces the number of protocols required in the core.. Segment Routing (SR-MPLS) eliminates the need for a separate label distribution protocol like LDP or RSVP-TE. The MPLS labels are derived directly from the IGP (IS-IS or OSPF) extensions, reducing control-plane complexity while still enabling traffic engineering via SR-TE policies and flexible path computation.

Answer

Segment Routing only works with IPv6 data plane.

Answer

Segment Routing uses RSVP-TE for label distribution.

Answer

Segment Routing requires TI-LFA to be enabled for traffic engineering.

Question 3

Drag and drop the steps for the BGP route selection process (best path selection) in order.

Accepted Answer

Weight, then Local Preference, then Locally Originated, then AS_PATH Length, then MED. BGP best path selection follows a specific order: weight, local preference, locally originated, AS_PATH length, MED.

Answer

Local Preference, then Weight, then Locally Originated, then AS_PATH Length, then MED

Answer

Weight, then Local Preference, then AS_PATH Length, then MED, then Locally Originated

Answer

Weight, then AS_PATH Length, then Local Preference, then Locally Originated, then MED

Question 4

A large enterprise uses MPLS L3VPN to connect multiple sites. They want to implement inter-AS option B (ASBR-to-ASBR MP-eBGP) for scalability. Which statement about this model is correct?

Accepted Answer

ASBRs exchange VPNv4 routes using MP-eBGP. In Inter-AS Option B, ASBRs use MP-eBGP to exchange VPNv4 routes. They do not maintain VRFs (Option A incorrect), and sessions are per address family, not per VRF (Option C incorrect). Option B is incorrect because a full mesh of MP-iBGP between PEs is not required. Option D correctly describes the VPNv4 route exchange using MP-eBGP.

Answer

Each ASB router maintains VRFs for all attached customers

Answer

Inter-AS option B requires a full mesh of MP-iBGP between all PEs

Answer

MP-eBGP sessions are established per VRF between ASBRs

Question 5

An MPLS core network uses LDP to distribute labels. An engineer wants to verify that the LDP session between two routers is up and exchanging labels. Which command should be used?

Accepted Answer

show mpls ldp neighbor. The command 'show mpls ldp neighbor' displays the status of LDP sessions, including the neighbor's IP address, session state (e.g., Operational), and label exchange activity. Since LDP is the protocol used to distribute labels in this MPLS core network, this command directly verifies that the session is up and exchanging labels between the two routers.

Answer

show mpls interfaces

Answer

show mpls l2transport binding

Answer

show mpls forwarding-table

Question 6

Which TWO statements correctly describe Segment Routing characteristics? (Select two.)

Accepted Answer

The path is encoded as a label stack at the source router. Option B is correct because Segment Routing encodes the entire forwarding path as an ordered list of segments (SIDs) in a label stack that is imposed by the ingress/source router, so the source dictates the path without per-hop signaling. Option C is correct because SR uses IGP extensions (IS-IS or OSPF) or BGP to advertise SIDs, eliminating the need for LDP or RSVP-TE to distribute labels or set up paths. Option A is wrong because routers can use different SRGB (Segment Routing Global Block) values as long as the SID-to-index mapping is consistent; the SRGB need not be identical across all routers. Option D is wrong because core routers still maintain per-prefix SID state (the SID/label mappings learned via IGP), so SR does not eliminate all per-prefix state. Option E is wrong because traffic engineering policies in SR are typically instantiated via a controller or headend using SID lists, and the SIDs themselves are advertised through IGP extensions (or BGP-LS for topology), not distributed as TE policies via BGP without IGP extensions.

Answer

All routers must be configured with the same SRGB value

Answer

SR eliminates all per-prefix state from core routers

Answer

Traffic engineering policies are distributed via BGP without any IGP extension

Question 7

Which TWO types of SIDs are defined in Segment Routing? (Select two.)

Accepted Answer

Adjacency-SID. In Segment Routing, the two fundamental SID types defined by the architecture are the Adjacency-SID (option A) and the Prefix-SID (option D). An Adjacency-SID is a local segment that is bound to a specific adjacency on a router (for example, an MPLS label or SRv6 SID representing a particular neighbor link), and it is used to force traffic over an explicit hop-by-hop path. A Prefix-SID is a globally significant segment that is bound to an IGP prefix (such as a loopback address) and is advertised by the IGP (IS-IS or OSPF) so that any node in the SR domain can steer traffic toward that destination prefix. The unmarked options do not belong because TI-LFA SID (B) is not a SID type but rather a TI-LFA repair-path mechanism that uses existing Adjacency-SIDs and Prefix-SIDs, Service-SID (C) is not a base SR SID type defined by the SR architecture (service programming is handled via other constructs), and Node-SID (E) is a specific use case of a Prefix-SID (the Prefix-SID of a node's loopback) rather than a separate SID type.

Answer

TI-LFA SID

Answer

Service-SID

Answer

Node-SID

Question 8

Which protocol is typically used to distribute MPLS labels between directly connected LSRs?

Accepted Answer

LDP. LDP (Label Distribution Protocol) is the standard protocol used to distribute MPLS labels between directly connected LSRs (Label Switch Routers) in a non-traffic-engineered MPLS network. It operates by establishing LDP sessions over TCP (port 646) and exchanging label mappings for each FEC (Forwarding Equivalence Class) based on the underlying IGP routing table, enabling hop-by-hop label switching without requiring explicit path setup.

Answer

RSVP

Answer

BGP

Answer

OSPF

Question 9

Which BGP address family must be used to exchange VPNv4 routes between PE routers in an MPLS L3VPN?

Accepted Answer

VPNv4 unicast. In an MPLS L3VPN, VPNv4 routes carry both the IPv4 prefix and the Route Distinguisher (RD) to ensure uniqueness across overlapping customer address spaces. The VPNv4 unicast address family (address-family ipv4 vpn) is the mandatory BGP address family used between PE routers to exchange these VPNv4 routes, enabling MPLS-based VPN reachability.

Answer

RT constraint

Answer

IPv4 unicast

Answer

L2VPN VPLS

Question 10

An engineer is troubleshooting MPLS traffic engineering tunnels and needs to verify path calculation and RSVP errors. Which command should be used?

Accepted Answer

show mpls traffic-eng tunnels. The 'show mpls traffic-eng tunnels' command displays detailed information about MPLS TE tunnels, including path calculation status, RSVP signaling errors, and tunnel state. This command is specifically designed to verify TE tunnel operations and troubleshoot issues such as path computation failures or RSVP resource reservation problems.

Answer

show ip route

Answer

show mpls ldp neighbor

Answer

debug mpls lfib errors

Question 11

In MPLS forwarding, what label operation does a transit LSR perform on the top label of a labeled packet?

Accepted Answer

Swap. A transit Label Switch Router (LSR) in an MPLS network receives a labeled packet and must forward it toward the egress LSR. The core operation on the top label is a swap: the incoming label is replaced with an outgoing label learned from the downstream LSR via Label Distribution Protocol (LDP) or other label distribution protocols. This maintains the label-switched path (LSP) and ensures the packet continues along the correct path.

Answer

None

Answer

Pop

Answer

Push

Question 12

Which BGP extended community is used to signal the OSPF domain ID between PE routers in an MPLS L3VPN when OSPF is the PE-CE protocol?

Accepted Answer

OSPF Domain ID. In an MPLS L3VPN where OSPF is the PE-CE protocol, the OSPF Domain ID extended community is used to signal the OSPF domain identifier between PE routers. This allows the receiving PE to determine whether the OSPF route originated from the same OSPF domain (and thus should be redistributed as an intra-area or inter-area route) or from a different domain (requiring a Type 5 LSA). The OSPF Domain ID is carried as a BGP extended community (type 0x0005 or 0x8005) and is critical for maintaining OSPF route type semantics across the MPLS backbone.

Answer

Site of Origin

Answer

OSPF Route Type

Answer

Route Target

Question 13

Which tool can verify connectivity along an MPLS Label Switched Path (LSP) and detect label loops?

Accepted Answer

MPLS LSP ping. MPLS LSP ping is the correct tool because it specifically tests connectivity along an MPLS Label Switched Path by sending MPLS echo request packets that follow the label stack, and it can detect label loops by checking the TTL field in the MPLS header. Unlike regular ICMP-based tools, MPLS LSP ping operates at the MPLS forwarding plane, verifying that the LSP is intact and that labels are correctly swapped at each hop.

Answer

SNMP

Answer

traceroute

Answer

ping

Question 14

An engineer notices that MPLS VPN traffic is taking a suboptimal path because the MPLS label binding for the BGP next-hop loopback is missing. What is the most likely cause?

Accepted Answer

LDP is not enabled on the core-facing interfaces.. MPLS VPN traffic relies on LDP to distribute labels for the BGP next-hop loopback addresses used in the VPNv4 route exchange. If LDP is not enabled on the core-facing interfaces, the local PE cannot obtain a label binding for the remote PE's loopback, causing the traffic to be forwarded via IP (without a label) or taking a suboptimal path. Enabling LDP on all core-facing interfaces resolves the missing label binding and restores optimal label-switched forwarding.

Answer

The remote PE is configured with a different VPN ID.

Answer

The local PE does not have a route to its own loopback.

Answer

LDP is not enabled on the PE-CE interface.

Question 15

In an MPLS L3VPN network with route reflectors, what is the default behavior regarding the BGP next-hop attribute for reflected VPNv4 routes?

Accepted Answer

It leaves the next-hop unchanged from the originating PE.. In an MPLS L3VPN network with route reflectors, the default behavior for VPNv4 routes is that the route reflector does not modify the BGP next-hop attribute. This is because the next-hop should remain the address of the originating PE router to ensure proper MPLS label switching and reachability. Changing the next-hop would break the MPLS forwarding path, as the P routers rely on the IGP to reach the PE's loopback, not the route reflector's.

Answer

It sets the next-hop to 0.0.0.0.

Answer

It removes the next-hop attribute.

Answer

It sets the next-hop to the route reflector's loopback.