350-501 Networking Practice Question
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?
⚠ Common exam trap
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.
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 OSPF with the capability vrf-lite and enable the down-bit on the PE-CE link.
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.
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.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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