- A
Check the IGP (OSPF/IS-IS) configuration on the routers. BGP-LS relies on IGP to obtain link-state information, and if IGP does not have full visibility, BGP-LS will not either.
Why wrong: BGP-LS advertises link-state information from the router's IGP database. While IGP must have full topology, the fact that the RR receives BGP-LS from all routers suggests IGP is working.
- B
Apply a prefix-list on the PCE to filter out unwanted BGP-LS prefixes, as the PCE may be overwhelmed.
Why wrong: The problem is missing data, not too much data. Filtering would worsen the issue.
- C
Configure the RR to send BGP-LS routes to the PCE. Verify that the RR has a BGP session with the PCE in the address-family link-state.
The PCE needs to receive BGP-LS updates from the RR. If the RR is not configured to advertise BGP-LS to the PCE, the PCE's topology will be incomplete.
- D
Verify that the PCE itself has a BGP-LS adjacency to each router, bypassing the RR.
Why wrong: While possible, this is not a standard design; PCEs typically receive topology via RR to reduce peering complexity. The issue is with the existing RR-PCE session.
Quick Answer
The correct answer is to configure the route reflector to send BGP-LS routes to the PCE, specifically by establishing a BGP session with the PCE under the address-family link-state. This resolves the incomplete topology database because the route reflector, while receiving BGP-LS NLRI from all routers, is not automatically advertising those link-state routes to the PCE; BGP-LS requires explicit neighbor configuration and address-family activation for redistribution. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of BGP-LS propagation in an SR-MPLS network, where the PCE relies on the RR as a central collector—a common trap is assuming that BGP-LS sessions from routers to the RR automatically flow through to the PCE. Remember the memory tip: “RR collects, but does not reflect BGP-LS unless told to—activate the link-state AF to the PCE.”
350-501 Automation and Assurance Practice Question
This 350-501 practice question tests your understanding of automation and assurance. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A service provider operates a large MPLS network with Segment Routing (SR) and BGP-LS enabled on all routers. They have deployed a centralized Path Computation Element (PCE) to compute SR-TE policies for optimal traffic engineering. The PCE is configured to receive the network topology via BGP-LS from a route reflector (RR). Recently, the PCE has been unable to compute paths for certain destinations, and logs show that the topology database is missing some links and nodes. The engineer verifies that all routers have BGP-LS configured and are peering with the RR. The RR's BGP table shows the BGP-LS NLRI received from all routers. However, the PCE sees only a subset of the topology. Which action should the engineer take to resolve the issue?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Configure the RR to send BGP-LS routes to the PCE. Verify that the RR has a BGP session with the PCE in the address-family link-state.
The PCE is not receiving the full topology because the RR is not sending BGP-LS routes to the PCE. The most likely cause is that the RR is not configured to advertise BGP-LS to the PCE. Option B is correct: the engineer should add the PCE as a BGP neighbor on the RR and ensure that the address-family link-state is activated. Option A is wrong because the BGP-LS sessions from routers to the RR are already working. Option C is wrong because the PCE itself likely has BGP-LS configured; the issue is the path before the PCE. Option D is wrong because the problem is not about policy filtering on the PCE; missing nodes/links indicate incomplete topology, not excessive data.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Check the IGP (OSPF/IS-IS) configuration on the routers. BGP-LS relies on IGP to obtain link-state information, and if IGP does not have full visibility, BGP-LS will not either.
Why it's wrong here
BGP-LS advertises link-state information from the router's IGP database. While IGP must have full topology, the fact that the RR receives BGP-LS from all routers suggests IGP is working.
- ✗
Apply a prefix-list on the PCE to filter out unwanted BGP-LS prefixes, as the PCE may be overwhelmed.
Why it's wrong here
The problem is missing data, not too much data. Filtering would worsen the issue.
- ✓
Configure the RR to send BGP-LS routes to the PCE. Verify that the RR has a BGP session with the PCE in the address-family link-state.
- ✗
Verify that the PCE itself has a BGP-LS adjacency to each router, bypassing the RR.
Why it's wrong here
While possible, this is not a standard design; PCEs typically receive topology via RR to reduce peering complexity. The issue is with the existing RR-PCE session.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-501 OSPF questions on adjacency and route selection.
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Automation and Assurance — study guide chapter
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FAQ
Questions learners often ask
What does this 350-501 question test?
Automation and Assurance — This question tests Automation and Assurance — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: Configure the RR to send BGP-LS routes to the PCE. Verify that the RR has a BGP session with the PCE in the address-family link-state. — The PCE is not receiving the full topology because the RR is not sending BGP-LS routes to the PCE. The most likely cause is that the RR is not configured to advertise BGP-LS to the PCE. Option B is correct: the engineer should add the PCE as a BGP neighbor on the RR and ensure that the address-family link-state is activated. Option A is wrong because the BGP-LS sessions from routers to the RR are already working. Option C is wrong because the PCE itself likely has BGP-LS configured; the issue is the path before the PCE. Option D is wrong because the problem is not about policy filtering on the PCE; missing nodes/links indicate incomplete topology, not excessive data.
What should I do if I get this 350-501 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-501 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 24, 2026
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