350-501 Automation and Assurance Practice Question
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 Route Reflector (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 C is correct: the engineer should verify that the RR has a BGP session with the PCE in the address-family link-state and ensure that BGP-LS routes are being sent. Option A is wrong because BGP-LS relies on IGP for link-state information, but the issue is not with IGP visibility; the RR already receives BGP-LS from all routers. Option B is wrong because filtering out prefixes would reduce topology information, not solve missing data. Option D is wrong because the PCE does not need direct BGP-LS adjacency to each router; it can receive topology via the RR.
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.
Why this is correct
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.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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