350-501 MPLS and Segment Routing Practice Test 2 — 15 Questions

Question 1

A service provider is deploying MPLS L3VPN and notices that BGP next-hop resolution for VPNv4 routes fails on the PE routers. The PE routers are running OSPF as the IGP and have loopback interfaces used for LDP and BGP peering. Which configuration change should the engineer implement to ensure that the BGP next-hop is reachable?

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

Accepted Answer

Configure 'neighbor x.x.x.x update-source loopback0' under the BGP router configuration.. The BGP next-hop for VPNv4 routes is typically the loopback interface of the remote PE router. For BGP to consider the next-hop reachable, the local PE must have an IGP route to that loopback address. The 'neighbor x.x.x.x update-source loopback0' command ensures that BGP uses the loopback interface as the source IP for the TCP session, which aligns the BGP peering address with the IGP-advertised loopback, making the next-hop reachable via OSPF.

Answer

Configure 'neighbor x.x.x.x allowas-in 1' under the BGP VRF configuration.

Answer

Apply the 'neighbor x.x.x.x next-hop-self' command under the BGP VRF configuration.

Answer

Increase the 'maximum-paths' value under the BGP address-family VPNv4.

Question 2

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 3

A network engineer is troubleshooting an LDP session failure between two directly connected routers. The routers are configured with the 'mpls ldp' command under the interface. The 'show mpls ldp neighbor' command shows no neighbors. Which two additional pieces of information should the engineer verify? (Choose two.)

Accepted Answer

Verify that the interface IP addresses are in the same subnet.. LDP sessions are established only between directly connected LSRs when using the default 'mpls ldp' interface configuration. The LDP Hello messages are sent as UDP packets to the multicast address 224.0.0.2, and if the interface IP addresses are not in the same subnet, the Hello messages will not be received, preventing neighbor discovery.

Answer

Verify that TCP port 646 is open on the interface ACL.

Answer

Verify that 'mpls ip' is enabled globally.

Answer

Verify that OSPF is configured on the interface.

Question 4

Refer to the exhibit. A network engineer notices that the local label 19 for prefix 10.4.4.0/24 shows 'Untagged' in the outgoing label column. What is the most likely cause?

Accepted Answer

LDP is not enabled on the interface toward 192.168.3.3.. When LDP is not enabled on the interface toward the next-hop router (192.168.3.3), the local router cannot exchange label bindings with that neighbor. As a result, the local label for prefix 10.4.4.0/24 remains in the LIB but is not programmed into the LFIB with an outgoing label, showing 'Untagged' because the router must pop the label (or forward as IP) when sending traffic to that next hop.

Answer

The next-hop router 192.168.3.3 is unreachable.

Answer

Penultimate Hop Popping is enabled on the next-hop router.

Answer

The MTU on interface Gi0/0/2 is too small.

Question 5

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.. Segment Routing (SR) eliminates the need for the Label Distribution Protocol (LDP) and Resource Reservation Protocol with Traffic Engineering (RSVP-TE) by encoding MPLS labels directly into the Interior Gateway Protocol (IGP), such as OSPF or IS-IS. This simplifies the control plane by removing these protocols entirely, reducing operational complexity and resource overhead.

Answer

Requires only OSPF as the IGP.

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Reduces label imposition at the ingress PE.

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Faster convergence due to BGP PIC.

Question 6

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.

Question 7

A network operator is migrating from traditional MPLS LDP to Segment Routing (SR) with IS-IS. The network consists of four routers (R1-R4) in a square topology. The engineer has enabled SR on all routers and configured prefix-SIDs for loopbacks. However, when checking the MPLS forwarding table on R1, the engineer sees that some prefixes have label values that are not the prefix-SIDs. For example, the prefix for R4's loopback shows label 16004 instead of the expected 16004 (which is correct). But for another prefix, the label is 16003 instead of 16003 (correct). The engineer does not see any labels for some external routes. What is the most likely reason that some labels are missing?

Accepted Answer

External routes are not covered by prefix-SIDs; they require LDP or another label distribution mechanism.. Segment Routing (SR) with IS-IS only assigns prefix-SIDs to prefixes that are part of the IS-IS domain and explicitly configured with a SID. External routes, such as those redistributed from another protocol (e.g., BGP or OSPF), are not covered by prefix-SIDs and require a separate label distribution mechanism like LDP or a manually configured explicit-null label to be forwarded with MPLS encapsulation. The engineer's observation that some labels are missing for external routes directly points to this limitation.

Answer

The IS-IS wide metrics are not enabled on all interfaces.

Answer

Route redistribution from another protocol is not configured.

Answer

The prefix-SIDs are inconsistent across routers.

Question 8

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?

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 9

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 10

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 11

Drag and drop the steps to configure a PPPoE client on a Cisco router into the correct order.

Accepted Answer

Step 1: Create dialer interface, Step 2: Set encapsulation PPP, Step 3: Configure dialer pool, Step 4: Set authentication (PAP/CHAP), Step 5: Assign IP address negotiated. PPPoE client configuration involves creating a dialer interface, setting encapsulation, dialer pool, authentication, and IP address.

Answer

Step 1: Create dialer interface, Step 2: Set encapsulation PPP, Step 3: Set authentication, Step 4: Configure dialer pool, Step 5: Assign IP address

Answer

Step 1: Configure dialer pool, Step 2: Create dialer interface, Step 3: Set encapsulation PPP, Step 4: Set authentication, Step 5: Assign IP address

Answer

Step 1: Create dialer interface, Step 2: Set encapsulation PPP, Step 3: Assign IP address, Step 4: Configure dialer pool, Step 5: Set authentication

Question 12

Match each IS-IS term to its definition.

Accepted Answer

IS-IS Area: A set of routers sharing the same area address.. In IS-IS, an area is identified by a common area address, LSPs carry link state data, and Level-1 routers operate intra-area. Common confusions involve swapping area definitions (system ID vs. area address) and mixing Level-1/Level-2 roles.

Answer

IS-IS Area: A set of routers sharing the same system ID.

Answer

Level-1 Router: Routes between areas.

Question 13

Match each Ethernet OAM protocol to its function.

Accepted Answer

CFM: Fault management and connectivity verification at the Ethernet layer. These are Ethernet OAM and management protocols used in service provider access.

Answer

CFM: Performance monitoring and measurement

Answer

EFM: Fault management across provider networks

Question 14

A service provider is implementing MPLS in their core network. They want to ensure that labeled packets for BGP prefixes are forwarded correctly without requiring an IGP label for every BGP prefix. Which technique should be used to achieve this?

Accepted Answer

Use BGP-free core with LDP or Segment Routing. A BGP-free core with LDP or Segment Routing allows the service provider to forward labeled packets for BGP prefixes without requiring an IGP label for every BGP prefix. In this design, edge routers (PEs) impose a label stack where the outer label (IGP label) is distributed by LDP or Segment Routing for the BGP next-hop, and the inner label is the BGP label. Core routers (P routers) only need to swap the outer label based on the IGP label, not the BGP prefix, thus eliminating the need for IGP labels per BGP prefix.

Answer

Enable MPLS on all interfaces and rely on BGP label distribution

Answer

Implement LDP for BGP prefixes

Answer

Configure static labels for BGP prefixes

Question 15

An engineer is troubleshooting an MPLS VPN issue where CE1 cannot ping CE2 across the provider network. The PE routers are configured with MP-BGP and LDP. On PE1, 'show ip bgp vpnv4 vrf CUST' shows the route to CE2's loopback as valid, but 'show mpls forwarding-table' does not list a label for that prefix. What is the most likely cause?

Accepted Answer

Route target mismatch between PE1 and PE2. The route is present in the VRF BGP table on PE1 (valid), but no MPLS label is assigned for that prefix in the forwarding table. This indicates that PE1 has learned the route via MP-BGP but cannot install it into the MPLS forwarding table because the route target (RT) import policy on PE1 does not match the RT export policy on PE2. Without a matching RT, PE1 does not consider the VPNv4 route as belonging to the CUST VRF, so it cannot resolve the next hop or assign a label for forwarding.

Answer

MTU mismatch on the MPLS core interfaces

Answer

LDP is not enabled on the core link between PE1 and P

Answer

BGP session between PE1 and PE2 is not established