350-501 MPLS and Segment Routing — 100 Questions

Question 1

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?

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

Accepted Answer

The neighbor is missing 'send-community extended' under address-family ipv4 unicast.. For BGP-SR, the label information is carried in the BGP Prefix-SID attribute (type 40), which is a separate path attribute, not an extended community. However, the 'send-community extended' command under the neighbor address-family is required to enable the advertisement of this attribute to the neighbor. Without it, the PE will not send the label information to the CE, even if the route-policy is correctly applied.

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 2

Which three features are unique to Segment Routing when compared to traditional MPLS with LDP? (Choose three)

Accepted Answer

Source-based path selection. A is correct because Segment Routing (SR) encodes the path as an ordered list of segment identifiers (SIDs) in the packet header, allowing the source node to explicitly specify the forwarding path. This is fundamentally different from traditional MPLS with LDP, where each router independently assigns labels and forwards packets based on the destination IP, with no source control over the path taken.

Answer

Stateful TE tunnels

Answer

Centralized controller (PCE) requirement

Question 3

A large service provider runs a dual-stack MPLS core with Segment Routing (SR-MPLS) and IS-IS as the IGP. The network has been operating stably for months. Recently, a new PE router (PE-5) was added and configured with IS-IS and SR. After the addition, some remote prefixes in the VRF on other PEs become unreachable. Troubleshooting reveals that the BGP next hop for those prefixes is the loopback of another PE (PE-3), but the MPLS forwarding table on PE-3 shows no label for the prefix. The IS-IS database on PE-3 shows the prefix-SID for PE-5's loopback, but the forwarding table does not have a label for that prefix. Commands like 'show mpls forwarding-table' show no entry for PE-5's loopback. What is the most likely cause and correct action?

Accepted Answer

Check and adjust the SRGB configuration on PE-5 to match the range used by other routers. The issue is that PE-3 does not have a label for PE-5's loopback because the prefix-SID for PE-5 is not programmed in the LFIB. This commonly happens when the SRGB (Segment Routing Global Block) on PE-5 does not overlap with PE-3's SRGB, or when the label index is not configured correctly. Since the IS-IS database shows the prefix-SID, the IGP is advertising it, but the label may be out of range. The correct action is to check and align the SRGB configuration on all routers. Option A (clear IS-IS adjacency) is a generic reset that might not fix the underlying mismatch. Option B (enable MPLS on the interface) is already done if LDP was used, but SR uses IGP, not LDP. Option D (redistribute into BGP) is not relevant.

Answer

Clear the IS-IS adjacency between PE-3 and PE-5

Answer

Enable MPLS on the interface connecting to PE-5

Answer

Configure BGP to redistribute IS-IS routes into BGP

Question 4

A service provider has deployed Segment Routing (SR-MPLS) with OSPF as the IGP. They enabled TI-LFA for link protection. During a maintenance window, they shut down a core link between two P routers. Expected behavior is that TI-LFA should provide sub-50ms failover. However, after the shutdown, traffic loss exceeds 10 seconds. Analysis shows that the backup path uses a segment list that includes a node SID from a router that is currently unreachable due to the same failure. The TI-LFA backup path calculation appears to have included a node that is dependent on the failed link. What design issue is most likely causing this?

Accepted Answer

Configure the backup path to use adjacency SIDs instead of node SIDs. The traffic loss occurs because TI-LFA's backup path computation includes a node SID from a router that is only reachable via the failed link. TI-LFA uses the post-convergence topology, but if it includes node SIDs from routers that are dependent on the failed link, the backup path fails. Using adjacency SIDs instead of node SIDs ensures that the backup path uses link-local labels, bypassing the unreliable node. This provides a valid backup path independent of the failed node's reachability, achieving sub-50ms failover.

Answer

Increase the prefix-SID index on all routers to avoid conflicts

Answer

Reduce the IGP metric on the backup links to ensure they are perceived as shorter paths

Answer

Disable TI-LFA and rely on LFA

Question 5

In Segment Routing for MPLS (SR-MPLS), which TLV is used in IS-IS to advertise the Node SID?

Accepted Answer

TLV 135. In IS-IS for SR-MPLS, the Node SID is advertised using TLV 135 (Extended IP Reachability) with the SR-MPLS SID/Label sub-TLV (sub-TLV 3). TLV 135 carries the prefix reachability information, and the sub-TLV attaches the SID to that prefix. TLV 149 (SR-MPLS SID/Label Binding TLV) is used for external prefixes or mapping server advertisements, not for directly attached Node SIDs.

Answer

TLV 232

Answer

TLV 22

Answer

TLV 149

Question 6

In Segment Routing (SR-MPLS), a label allocated from the Segment Routing Global Block (SRGB) for a specific router's loopback is known as what?

Accepted Answer

Node SID. In Segment Routing MPLS, a Node SID is a specific type of Prefix SID that is allocated from the SRGB and assigned to the loopback address of a specific router. While the Prefix SID is a broader term that can refer to any prefix, the Node SID uniquely identifies a particular node. Therefore, for a specific router's loopback, it is correctly known as a Node SID (option B). Option C (Prefix SID) is too general and not the specific term for this scenario.

Answer

Anycast SID

Answer

Prefix SID

Answer

Adjacency SID

Question 7

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?

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 8

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 9

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 10

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 11

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 12

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

Accepted Answer

The transit LSR performs label swapping.. A transit Label Switch Router (LSR) in an MPLS network performs label swapping: it receives a labeled packet, replaces the incoming label with an outgoing label from its LFIB (Label Forwarding Information Base), and forwards the packet toward the egress LSR. This is the fundamental operation of an LSR in the core of an MPLS domain, as defined in RFC 3031.

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 13

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.. Segment Routing (SR) can perform traffic engineering using SR-TE policies (via a controller or PCEP) or SR Flexible Algorithm, without requiring RSVP-TE. This eliminates the complexity of RSVP-TE's soft-state signaling and per-LSP state maintenance, relying instead on source-routed segment lists encoded in the packet header.

Answer

Provides inherent security against label spoofing.

Answer

Requires BGP-LU for label distribution.

Question 14

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 15

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 16

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 17

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 18

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 19

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 20

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.

Answer

Reduces label imposition at the ingress PE.

Answer

Faster convergence due to BGP PIC.