The correct answer is to configure OSPF with the capability vrf-lite and enable the down-bit on the PE-CE link. This works because the down-bit is a flag set by the PE router when redistributing VPN routes into OSPF toward the CE; when the CE receives a route with the down-bit set, it will not re-advertise that route back to another PE, thereby preventing the routing loop that would otherwise occur in a multi-homing scenario. On the Cisco SPCOR 350-501 exam, this concept tests your understanding of loop prevention in MPLS L3VPN when OSPF is used as the IGP between PE and CE—a common trap is assuming that simply enabling OSPF on both PEs is enough, but without the down-bit, the CE could learn the same prefix from two PEs and advertise it back, creating a loop. A useful memory tip: think of the down-bit as a “do not re-announce” sign—once set, the CE treats the route as internal to the VPN, keeping the path clean for safe load-balancing across redundant PEs.
350-501 Networking Practice Question
This 350-501 practice question tests your understanding of networking. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 is deploying MPLS L3VPN over an OSPF backbone. The PE routers are configured with OSPF as the IGP. The CE router of customer A is connected to two PEs for redundancy. Which configuration is required on the PE routers to ensure that the CE router can load-balance traffic across both PEs without loops?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure OSPF with the capability vrf-lite and enable the down-bit on the PE-CE link.
Option C is correct because configuring OSPF with the capability vrf-lite and enabling the down-bit on the PE-CE link prevents routing loops in a multi-homed CE scenario. The down-bit is set by the PE when redistributing routes into OSPF, ensuring that the CE does not re-advertise those routes back to another PE, which would cause a loop. The vrf-lite capability allows the CE to understand the down-bit without requiring full MPLS/VPN functionality, enabling load-balancing across both PEs safely.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Use OSPF sham-links between the two PEs.
Why it's wrong here
Sham-links are used to connect backdoor links across an MPLS VPN, not for CE-PE loop prevention.
✗
Use the BGP cost community to adjust the path selection on the CE.
Why it's wrong here
BGP cost community influences BGP path selection, but does not prevent routing loops in OSPF.
✓
Configure OSPF with the capability vrf-lite and enable the down-bit on the PE-CE link.
Why this is correct
The down-bit prevents the CE from re-advertising routes learned from one PE to the other PE, avoiding loops.
Related concept
Read the scenario before looking for a memorised answer.
✗
Disable the DN-bit on the PE-CE OSPF interface.
Why it's wrong here
Disabling the DN-bit would allow loopback routes to be propagated, potentially causing loops.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between the down-bit (DN-bit) used in OSPF PE-CE scenarios and the sham-link concept used for OSPF area 0 extension, leading candidates to mistakenly choose sham-links for loop prevention in multi-homed CE designs.
Detailed technical explanation
How to think about this question
The down-bit (DN-bit) is a flag in the OSPFLSA Type 3, 5, or 7 that is set by the PE when redistributing VPN routes into OSPF. The CE router, when configured with capability vrf-lite, ignores LSAs with the DN-bit set, preventing those routes from being re-advertised back into the MPLS VPN. In real-world deployments, this is critical for multi-homed CEs that use OSPF as the PE-CE protocol, as it allows ECMP load-balancing across both PEs without the risk of routing loops, which would otherwise occur if the CE treated the routes as intra-area and re-advertised them.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Configure OSPF with the capability vrf-lite and enable the down-bit on the PE-CE link. — Option C is correct because configuring OSPF with the capability vrf-lite and enabling the down-bit on the PE-CE link prevents routing loops in a multi-homed CE scenario. The down-bit is set by the PE when redistributing routes into OSPF, ensuring that the CE does not re-advertise those routes back to another PE, which would cause a loop. The vrf-lite capability allows the CE to understand the down-bit without requiring full MPLS/VPN functionality, enabling load-balancing across both PEs safely.
What should I do if I get this 350-501 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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