350-501 MPLS and Segment Routing Practice Test 8 — 15 Questions

Question 1

A network architect is designing an MPLS L3VPN service over a Segment Routing core. The PE routers are Cisco IOS XR devices running IS-IS with SR-MPLS. The architect must ensure that VPNv4 routes are advertised between PE routers. Which two statements about the BGP configuration on the PE routers are correct? (Choose two.)

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

Accepted Answer

The PE routers must activate the VPNv4 address family under the IBGP session and configure route targets for the VRFs.. For MPLS L3VPN over SR-MPLS, PE routers must establish IBGP sessions (typically between loopbacks) and activate the VPNv4 address family. They must also configure route targets on VRFs to control route import/export. The SR-MPLS core provides reachability to the BGP next-hops via Prefix-SIDs, so no additional MPLS forwarding configuration is needed for the core. Route reflectors are optional for scalability, and send-label is not used for VPNv4.

Answer

The PE routers must configure a route reflector because IBGP full mesh is not possible in an SR-MPLS core.

Answer

The PE routers must use the 'neighbor x.x.x.x send-label' command to exchange MPLS labels for VPNv4 routes.

Answer

The PE routers must enable 'mpls bgp forwarding' on the core-facing interfaces to allow MPLS label forwarding for VPN traffic.

Question 2

A service provider is using Segment Routing over MPLS with OSPFv2 as the IGP. The engineer wants to enable SR-MPLS on all routers. Which configuration is required to advertise Prefix-SIDs in OSPFv2 on a Cisco IOS XR router?

Accepted Answer

Under router ospf, configure segment-routing mpls, then under the area, configure interface Loopback0 and prefix-sid index 1.. In Cisco IOS XR, enabling SR-MPLS in OSPFv2 requires segment-routing mpls under router ospf, and then under the OSPF area, you configure the interface (typically a loopback) with a prefix-sid index. This causes the router to advertise the Prefix-SID in OSPFv2 Extended Prefix Opaque LSAs. The SRGB must also be configured, but it is not sufficient by itself.

Answer

Under router ospf, configure segment-routing mpls, then under the interface, configure sr-prefer.

Answer

Under router ospf, configure segment-routing mpls, then under the global configuration, configure segment-routing mpls srgb 16000 23999.

Answer

Under router ospf, configure segment-routing mpls and then under the interface, configure prefix-sid index.

Question 3

A service provider is implementing Segment Routing over an MPLS network using IS-IS as the IGP. The network engineer must ensure that the Prefix-SIDs are advertised correctly and that the MPLS forwarding plane is programmed. Which TWO statements about Segment Routing with IS-IS are correct? (Choose two.)

Accepted Answer

The Prefix-SID is advertised in the IS-IS Prefix-SID sub-TLV, which is carried within the IS-IS Extended IP Reachability TLV (TLV 135).. In IS-IS Segment Routing, the Prefix-SID is carried in the Prefix-SID sub-TLV within the Extended IP Reachability TLV (TLV 135). The MPLS forwarding entry uses the label advertised by the next-hop, which is calculated from the SRGB and the SID index. These two statements correctly describe the advertisement and forwarding behavior.

Answer

The Prefix-SID is advertised in the IS-IS Router Capability TLV (TLV 242).

Answer

The Prefix-SID is only advertised if the router is configured with an SRGB that matches the SRGB of all other routers in the domain.

Answer

The MPLS forwarding entry for a Prefix-SID is an incoming label mapped to an outgoing label and next-hop, where the outgoing label is the Prefix-SID of the destination plus the SRGB offset.

Question 4

An engineer is configuring an MPLS L3VPN on a Cisco IOS XR PE router. The customer requires that the PE router use a unique Route Distinguisher (RD) for each VRF and that the RD be automatically generated based on the router's BGP router ID and a unique VRF identifier. Which configuration achieves this requirement?

Accepted Answer

Enable the auto-rd command under the VRF configuration, which automatically generates an RD using the BGP router ID and the VRF ID.. The auto-rd command under the VRF address-family in Cisco IOS XR automatically generates an RD using the BGP router ID and a unique VRF identifier. This meets the requirement for automatic, unique RDs per VRF. Manual configuration, global BGP commands, or assuming a default do not provide the desired automatic generation.

Answer

Configure the RD using the rd auto command under the global BGP configuration.

Answer

Use the default RD assignment, which automatically uses the BGP router ID and VRF name.

Answer

Configure the RD manually using the rd command with the router's loopback IP and a unique number for each VRF.

Question 5

A service provider is deploying MPLS Traffic Engineering (TE) with RSVP-TE on Cisco IOS XR routers. An engineer configures an explicit path with strict hops for a TE tunnel. After committing the configuration, the tunnel remains down. Which command should the engineer use to verify that the explicit path is correctly configured and that RSVP-TE has reserved resources along the path?

Accepted Answer

show mpls traffic-eng tunnels. The command show mpls traffic-eng tunnels is the most comprehensive for verifying MPLS TE tunnel status and configuration. It displays the tunnel's operational state, the explicit path, and RSVP reservation details. When a tunnel is down, this command helps identify whether the issue is with the explicit path, RSVP reservation, or other factors. Other commands focus on different protocols or lack the necessary detail.

Answer

show route ipv4

Answer

show mpls ldp discovery

Answer

show rsvp session

Question 6

A service provider is deploying Segment Routing over an IPv6 (SRv6) core. An engineer must configure a Cisco IOS XR router to support SRv6 locator advertisement in IS-IS. Which command is required to enable SRv6 under the IS-IS address family?

Accepted Answer

address-family ipv6 unicast; segment-routing srv6. To enable SRv6 advertisement in IS-IS on Cisco IOS XR, the engineer must enter the IS-IS IPv6 unicast address family and issue the 'segment-routing srv6' command. This instructs IS-IS to advertise the configured SRv6 locators as part of its link-state information. The global SRv6 configuration, including locator definition, is a prerequisite, but the advertisement is activated under the routing protocol.

Answer

locator LOC1

Answer

sid-format srv6

Answer

segment-routing srv6

Question 7

A network engineer is implementing Segment Routing over MPLS (SR-MPLS) in an IS-IS network. The engineer must ensure that the SRGB is consistent across all routers to avoid forwarding issues. Which TWO statements about the SRGB in Cisco IOS XR are correct? (Choose two.)

Accepted Answer

The SRGB must be identical on all routers in the SR domain for Prefix-SIDs to work correctly.. The SRGB in Cisco IOS XR defaults to 16000-23999 and must be consistent across all routers in the SR domain. The Prefix-SID index is added to the SRGB base to form the label, so differing SRGBs would cause label mismatches. The SRGB is configured globally, not per interface, and cannot be different per router without breaking forwarding.

Answer

The SRGB range can be modified per interface to optimize label allocation.

Answer

The SRGB can be different on each router as long as the Prefix-SID indices are unique.

Answer

The SRGB is configured under the global MPLS configuration and applies to all interfaces.

Question 8

An engineer is troubleshooting an MPLS L3VPN over an SR-MPLS core. Customer traffic from CE1 to CE2 is not being forwarded. The PE routers have established an IBGP session and exchanged VPNv4 routes. The engineer verifies that the MPLS forwarding table on the ingress PE contains the VPN label and the transport label (Prefix-SID) for the egress PE. However, the core routers do not have an entry for the VPN label. What is the most likely cause?

Accepted Answer

The core routers do not have an MPLS label binding for the VPN label because it is only advertised between PE routers.. In MPLS L3VPN, the VPN label is only advertised between PE routers via IBGP. Core (P) routers do not receive VPN label bindings and do not need them; they forward traffic based on the transport label (the Prefix-SID in SR-MPLS). The egress PE uses the VPN label to identify the correct VRF. Therefore, the absence of VPN label entries on core routers is expected and not a fault. The actual problem likely lies elsewhere, such as a misconfiguration on the PE routers.

Answer

The core routers are missing a route to the egress PE loopback address.

Answer

The core routers have not been enabled for Segment Routing, so they cannot forward the transport label.

Answer

The core routers are not configured with the customer VRF.

Question 9

A service provider is designing a new MPLS core network using Segment Routing with MPLS data plane. They require traffic engineering capabilities to optimize bandwidth utilization. Which technology should be used to compute optimal paths based on IGP link attributes and bandwidth constraints?

Accepted Answer

SR-TE (Segment Routing Traffic Engineering). SR-TE (Segment Routing Traffic Engineering) is the correct choice because it uses a centralized or distributed controller to compute optimal paths based on IGP link attributes (such as metric, TE metric, affinity) and bandwidth constraints, encoding the path as a segment list in the packet header. Unlike RSVP-TE, SR-TE does not require per-flow state in the core routers, making it more scalable for bandwidth optimization in an MPLS Segment Routing network.

Answer

RSVP-TE with FRR

Answer

LDP over SR

Answer

OSPF with MPLS-TE extensions

Question 10

A service provider is deploying a new MPLS core with Segment Routing and requires fast convergence upon link failure. They plan to use TI-LFA (Topology Independent Loop-Free Alternate). What is a prerequisite for TI-LFA to provide protection against any single link failure?

Accepted Answer

IGP must be a link-state protocol with complete topology information (OSPF or IS-IS).. TI-LFA relies on the IGP having a complete view of the network topology to compute a post-convergence path that avoids the failed link. OSPF and IS-IS are link-state protocols that flood link-state advertisements (LSAs) or link-state packets (LSPs) to provide this full topology database, which is essential for TI-LFA to calculate a loop-free backup path for any single link failure.

Answer

BGP-LU must be enabled for label distribution.

Answer

LDP must be enabled on all interfaces.

Answer

RSVP-TE must be configured with FRR.

Question 11

Which TWO statements about MPLS label switching are correct? (Choose two.)

Accepted Answer

The transit LSR performs label swapping.. Option A is correct because a transit (intermediate) LSR receives a labeled packet, looks up the incoming label in its LFIB, swaps it for the outgoing label, and forwards the packet toward the next hop. Option C is correct because the ingress LSR (ingress edge router) is responsible for imposing (pushing) an MPLS label onto the IP packet as it enters the MPLS domain, creating the label stack entry. Option B is incorrect because the CE (Customer Edge) router is outside the MPLS domain and receives plain IP packets, not labeled frames. Option D is incorrect because PHP is performed by the penultimate (second-to-last) LSR, which pops the label before forwarding to the egress LSR, not by the egress router itself. Option E is incorrect because the egress LSR typically removes the label (or receives an already-popped packet via PHP) and forwards based on the IP header, rather than performing label swapping.

Answer

The CE receives a frame with an MPLS label.

Answer

PHP (Penultimate Hop Popping) causes the egress router to pop the label.

Answer

The egress LSR performs label swapping before forwarding.

Question 12

Which THREE of the following are benefits of using Segment Routing over LDP in an MPLS network? (Choose three.)

Accepted Answer

Enables traffic engineering without RSVP-TE.. Option A is correct because Segment Routing can steer traffic along explicit paths using segment lists (SR policies) without deploying RSVP-TE, since the path is encoded in the packet header rather than signaled hop-by-hop. Option B is correct because SR leverages existing IGP (IS-IS or OSPF) extensions to distribute segment identifiers, removing the need to run a separate label-distribution protocol like LDP alongside the IGP. Option C is correct because with SR the IGP itself distributes SID/label information and traffic engineering is handled via SR policies, so LDP and RSVP-TE can be eliminated from the core. Option D is not a benefit of SR over LDP, as neither protocol provides inherent protection against label spoofing; that requires separate mechanisms. Option E is incorrect because SR does not require BGP-LU for label distribution—labels come from IGP-advertised SIDs, and BGP-LU is an independent mechanism used for inter-domain scenarios.

Answer

Provides inherent security against label spoofing.

Answer

Requires BGP-LU for label distribution.

Question 13

An engineer is designing an MPLS network and needs to ensure that VPN traffic between two PE routers is label-switched. The PE routers are connected via a P router and have a full mesh of iBGP sessions. Which label distribution method is required for the VPN labels?

Accepted Answer

MP-BGP. In MPLS VPN architectures, VPN labels (also known as service labels) are distributed using Multiprotocol BGP (MP-BGP). MP-BGP carries VPN-IPv4 routes that include both the route distinguisher (RD) and the VPN label in the Network Layer Reachability Information (NLRI). This allows PE routers to exchange per-VRF label bindings, enabling label-switched VPN traffic across the MPLS core. Without MP-BGP, the PE routers cannot signal the VPN-specific labels required for end-to-end label switching.

Answer

Segment Routing

Answer

RSVP-TE

Answer

LDP

Question 14

Which TWO are benefits of using Segment Routing (SR) over traditional LDP-based MPLS?

Accepted Answer

Eliminates the need for LDP and RSVP-TE protocols.. Option A is correct because Segment Routing removes the need to run LDP for label distribution and RSVP-TE for traffic engineering, since SR uses the IGP (with extensions) to distribute segment identifiers and can steer traffic via segment lists instead of RSVP-TE signaled LSPs. Option E is correct because SR supports traffic engineering without RSVP-TE by encoding explicit paths as ordered segment lists (e.g., using SR-TE policies), eliminating RSVP-TE's per-flow state and signaling overhead. Option B is not correct because SR does not require only OSPF; it works with OSPF, IS-IS, and even BGP-based label distribution, so restricting it to OSPF is inaccurate. Option C is not correct because SR does not inherently reduce label imposition at the ingress PE; the ingress PE still imposes a label stack (often a segment list), which can be larger than a single LDP label. Option D is not correct because BGP PIC is a BGP convergence feature independent of SR, and SR's benefits are not defined by BGP PIC.

Answer

Requires only OSPF as the IGP.

Answer

Reduces label imposition at the ingress PE.

Answer

Faster convergence due to BGP PIC.

Question 15

A service provider has a network with PE1, P1, P2, and PE2 connected in a chain: PE1-P1-P2-PE2. The IGP is IS-IS with wide metrics, and MPLS LDP is enabled on all interfaces. The goal is to provide L3VPN services between PE1 and PE2. The engineer has configured MP-iBGP between PE1 and PE2, and the VPNv4 routes are exchanged. However, when a CE router behind PE1 tries to reach the CE behind PE2, the traffic fails. The engineer checks the MPLS forwarding table on PE1 and sees that the label for the BGP next-hop (PE2's loopback) is 'Untagged'. The BGP next-hop is reachable via ICMP. What is the most likely cause of this issue?

Accepted Answer

LDP is not enabled on the interface between PE1 and P1.. The MPLS forwarding table on PE1 shows the label for PE2's loopback as 'Untagged', which indicates that LDP has not assigned a label for that prefix on the incoming interface. Since LDP is responsible for distributing labels for IGP routes (like loopbacks) in an MPLS LDP-enabled network, the missing label means LDP is not operational on the link between PE1 and P1. Without a label, PE1 cannot push an MPLS label for the BGP next-hop, causing the VPNv4 traffic to be dropped or forwarded as IP, which fails because the core routers (P1, P2) do not have the VPN route.

Answer

The IS-IS metric between PE1 and P1 is too high, causing suboptimal routing.

Answer

The MPLS TTL propagation is disabled, causing packets to be dropped.

Answer

The MTU on the link between PE1 and P1 is set to 1500, causing fragmentation.