350-501 MPLS and Segment Routing — Timed 100 Questions

Question 1

An engineer is troubleshooting an MPLS L3VPN where customers behind CE1 cannot reach a specific prefix behind CE2. The PE routers are using OSPF as the IGP and LDP for label distribution. On PE2, the prefix is present in the VRF routing table, but not in the VRF forwarding table. What is the most likely cause?

100 minute time limit — choose an answer to begin.

Accepted Answer

The route is missing a label in the LFIB on PE2.. The prefix is present in the VRF routing table (RIB) but missing from the VRF forwarding table (FIB) on PE2. This indicates that the route has been learned via BGP and installed in the RIB, but the MPLS VPN label (the inner label) required to forward the packet across the MPLS core is absent. Without a valid label in the LFIB, the CEF (FIB) cannot install the route, causing the reachability failure. Option D correctly identifies this missing label in the LFIB as the root cause.

Answer

MTU mismatch is causing the VPN label to be dropped.

Answer

OSPF is not redistributing the BGP routes into the IGP on PE2.

Answer

The VRF is not properly configured on PE2's interface toward CE2.

Question 2

An engineer is deploying MPLS in the core and wants to ensure that all core routers use the same label for a specific prefix, regardless of which router originated it. Which MPLS label allocation mode should be used?

Accepted Answer

Per-prefix label mode. Per-prefix label mode (option C) is correct because it assigns a single label for a specific prefix across all core routers, regardless of which router originated the route. This ensures label consistency, which is critical for proper MPLS forwarding and troubleshooting. In contrast, per-next-hop or per-interface modes would create different labels for the same prefix based on the next hop or interface, breaking the requirement for uniform label allocation.

Answer

Per-interface label mode

Answer

Per-next-hop label mode

Answer

Per-VRF label mode

Question 3

During an MPLS network migration from LDP to Segment Routing, an engineer notices that some routers are not advertising Prefix-SIDs for certain loopbacks. The IGP is OSPF. What configuration is required on these routers to advertise Prefix-SIDs?

Accepted Answer

Configure 'segment-routing mpls' globally and assign a SID index under the OSPF router process for the loopback.. In OSPF, Prefix-SIDs for loopbacks are advertised by configuring 'segment-routing mpls' globally and then assigning a SID index under the OSPF router process using the 'prefix-sid index' command for the specific loopback network. This ties the SID to the OSPF prefix advertisement, enabling SR-MPLS forwarding without LDP.

Answer

Enable 'mpls ldp autoconfig' on the loopback interface.

Answer

Enable 'segment-routing mpls' globally and configure 'prefix-sid index' under the loopback interface.

Answer

Configure 'segment-routing mpls set-adjacency-sid' on the loopback.

Question 4

Refer to the exhibit. A PE router in an MPLS L3VPN network shows the above output. The VRF CUSTOMER contains two routes. Which statement about forwarding for these routes is true?

Accepted Answer

Traffic to 10.1.1.0/24 will be label-switched with an implicit null label.. The output shows that for the VRF CUSTOMER, the route to 10.1.1.0/24 has a next hop of 192.168.1.2 and is associated with label 3 (implicit null). In MPLS L3VPN, label 3 instructs the upstream router to pop the MPLS label and forward the packet using IP routing to the directly connected next hop. Therefore, traffic to 10.1.1.0/24 will be label-switched with an implicit null label, meaning the label is removed before forwarding to the CE router.

Answer

Both routes are reachable via MPLS.

Answer

Traffic to 10.2.2.0/24 will be forwarded using the BGP next hop label.

Answer

Traffic to 10.2.2.0/24 will be forwarded using IP routing.

Question 5

Refer to the exhibit. An engineer configured Segment Routing on a router. However, the router does not advertise a Prefix-SID for the loopback0 interface. What is the most likely reason?

Accepted Answer

The 'prefix-sid index' command is not associated with the loopback prefix; it should be configured under the OSPF process for that specific prefix.. In Cisco IOS XR, the 'prefix-sid index' command must be configured under the OSPF process for the specific prefix (e.g., 'prefix-sid index 100' under 'router ospf 1' for the loopback0 prefix). Configuring it under the interface does not associate the SID with the prefix in OSPF's link-state database, so the router will not advertise the Prefix-SID. The exhibit shows the command under the loopback interface, which is incorrect for OSPF Segment Routing.

Answer

The loopback is not included in any OSPF network statement.

Answer

The 'segment-routing mpls' command is missing under the loopback interface.

Answer

The 'mpls ip' command is missing on the loopback interface.

Question 6

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?

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 7

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 session establishment requires the two directly connected interfaces to have IP addresses in the same subnet, because LDP first discovers neighbors via UDP port 646 multicast (224.0.0.2) on the local link, and that discovery only works if the interfaces are on a common IP subnet — so option A is correct. LDP then establishes a TCP session on port 646 to the neighbor's LDP router ID, which must be reachable via the unicast routing table; if the router ID (often a loopback) is not advertised or routable, the TCP session cannot come up, making option D correct. Option B is not required because LDP's TCP port 646 is used for the session itself, but the question is about neighbor discovery failing (no neighbors shown), and an interface ACL blocking 646 would typically be a separate concern from the initial discovery; more importantly, LDP discovery uses UDP 646, not TCP, so checking TCP 646 alone is not the right verification. Option C is incorrect because 'mpls ip' is not a global command — MPLS forwarding is enabled per interface with 'mpls ip' (or 'mpls ldp' in IOS-XE), and the scenario already states 'mpls ldp' is configured under the interface. Option E is incorrect because LDP does not require OSPF specifically; any IGP (or even static routes) that provides reachability to the LDP router IDs is sufficient, so OSPF is not a mandatory check.

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 8

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 9

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 10

Refer to the exhibit. An engineer is configuring Segment Routing for BGP (BGP-SR) on a PE router to assign labels to prefixes learned from a CE. The route-policy SET-LABEL is applied to the neighbor under the address-family ipv4 unicast. However, the CE prefix 10.1.1.0/24 is not receiving the label. What is the most likely reason?

Accepted Answer

The neighbor is missing 'send-community extended' under address-family ipv4 unicast.. The correct option is C: the neighbor is missing 'send-community extended' under address-family ipv4 unicast. BGP Segment Routing (BGP-SR) uses the BGP Prefix-SID attribute, which is a BGP attribute carried in an optional transitive path attribute and requires extended community encoding; without 'send-community extended' enabled on the neighbor, the label-index/Prefix-SID information is not advertised, so the CE prefix 10.1.1.0/24 never receives the label. Option A is wrong because route-policy syntax uses valid match/set clauses and 'destination' is not a required replacement for 'ip prefix-list'. Option B is wrong because update-source affects the TCP session source address, not label allocation or advertisement. Option D is wrong because BGP-SR uses 'set label-index' (mapping to the Prefix-SID), not 'set label'.

Answer

The route-policy syntax is incorrect; 'destination' should be 'ip prefix-list'.

Answer

The update-source should be the interface facing the CE, not Loopback0.

Answer

The 'set label-index' command should be 'set label' for BGP-SR.

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

Question 16

A network engineer is designing an MPLS core using Segment Routing. They want to minimize label stack depth while still providing fast convergence using TI-LFA. Which prefix-SID assignment strategy should be used?

Accepted Answer

Advertise prefix-SIDs for all IGP prefixes. Advertising prefix-SIDs for all IGP prefixes allows TI-LFA to compute backup paths using any prefix in the network, not just loopbacks. This minimizes label stack depth by enabling TI-LFA to use a single prefix-SID for the repair path, avoiding the need for additional labels or explicit path constructs. Fast convergence is achieved because TI-LFA precomputes a backup next-hop that can be activated immediately upon failure, and having all prefixes as SIDs ensures the backup path can be expressed with minimal label imposition.

Answer

Assign a node-SID to each loopback interface

Answer

Use per-interface label mode from SRGB

Answer

Allocate labels per-VRF on PE routers

Question 17

Refer to the exhibit. An engineer notices that R1 has an LDP neighbor but 'show mpls forwarding-table' on R1 shows no label bindings for prefixes learned from R2. What is the most likely cause?

Accepted Answer

R1 is configured for 'label distribution on-demand'. R1 shows an LDP neighbor (session is up) but no label bindings for prefixes from R2. This occurs when R1 is configured for 'label distribution on-demand' (RFC 5036), meaning it only requests label bindings for prefixes in its routing table, not all prefixes from the peer. Since R1 has not yet needed those specific prefixes, it has not requested labels, so the forwarding table remains empty.

Answer

The LDP discovery source is incorrect

Answer

The peer LDP identifier is not reachable

Answer

LDP session is not established

Question 18

A service provider is migrating from LDP to Segment Routing in an IS-IS network. After enabling IS-IS with segment-routing on all routers, they observe that some prefixes still receive labels from LDP instead of from SR. Which configuration is most likely missing on these routers?

Accepted Answer

Missing 'segment-routing mpls' command under IS-IS. When migrating from LDP to Segment Routing in an IS-IS network, the 'segment-routing mpls' command must be explicitly enabled under the IS-IS routing process. Without this command, IS-IS will not allocate MPLS labels for prefixes using the Segment Routing (SR) extension, causing the router to fall back to LDP for label distribution. This is the most common missing configuration when SR labels are not being assigned.

Answer

OSPF segment-routing is still configured

Answer

TI-LFA is not enabled

Answer

SRGB range conflicts with MPLS label range

Question 19

An engineer is configuring an MPLS Traffic Engineering tunnel using RSVP-TE. The tunnel is intended to carry voice traffic with strict delay requirements. Which RSVP-TE object should be used to enforce a maximum delay path?

Accepted Answer

Use the 'path-option' with 'metric delay' under the tunnel. The 'path-option with metric delay' command instructs RSVP-TE to compute a path that minimizes cumulative delay rather than IGP cost or hop count. This directly enforces a maximum delay constraint for voice traffic by selecting the path with the lowest measured or configured delay metric across the MPLS network.

Answer

Enable 'record-route' on the tunnel

Answer

Configure 'fast-reroute' protection

Answer

Specify an explicit-path (ERO) manually

Question 20

A network operator is deploying Segment Routing with TI-LFA across an OSPF network. After configuring OSPF with 'segment-routing mpls' and 'fast-reroute per-prefix', they notice that backup paths are not being installed for some prefixes. 'Show ip ospf segment-routing protected' shows 'No FRR backup' for those prefixes. What is a possible reason?

Accepted Answer

Adjacency-SIDs are not configured on neighboring routers. TI-LFA (Topology Independent Loop-Free Alternate) for per-prefix fast-reroute in OSPF requires that adjacency-SIDs be configured on neighboring routers. Without adjacency-SIDs, OSPF cannot compute the necessary repair paths to provide backup protection, resulting in 'No FRR backup' for those prefixes.

Answer

LDP label distribution is still active

Answer

The prefix-SID index is not globally unique

Answer

OSPF does not support TI-LFA for per-prefix prefixes