20+ practice questions focused on MPLS and Segment Routing — one of the most tested topics on the Cisco SPCOR / CCNP Service Provider Core 350-501 exam. Each question includes a detailed explanation so you learn why the right answer is correct.
Start MPLS and Segment Routing PracticeRefer 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?
Explanation: 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.
Which three features are unique to Segment Routing when compared to traditional MPLS with LDP? (Choose three)
Explanation: 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.
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?
Explanation: 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.
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?
Explanation: 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.
In Segment Routing for MPLS (SR-MPLS), which TLV is used in IS-IS to advertise the Node SID?
Explanation: 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.
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Practice all MPLS and Segment Routing questions1. Baseline your knowledge
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2. Review every explanation
For each question — right or wrong — read the full explanation. Understanding why an answer is correct is more valuable than knowing the answer itself.
3. Focus on exam traps
MPLS and Segment Routing questions on the 350-501 frequently use trap wording. Look for subtle differences in answers that test your precision, not just general knowledge.
4. Reach 80% consistently
Do repeated sessions until you score 80%+ three times in a row. Then move to mixed-mode practice to test cross-topic recall under realistic conditions.
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