350-501 Per-packet load balancing Practice Question
A service provider offers L3VPN services to multiple enterprise customers. One customer reports that they cannot reach some remote sites intermittently. The network uses MPLS L3VPN with MP-BGP for VPN route exchange. The PE routers are configured with route-target import and export. The customer's CE router is dual-homed to two different PEs in the same point of presence. The engineer checks the BGP table on both PEs and sees the customer routes with the correct route-target. However, pings from the CE to a remote site fail about 50% of the time, and the flapping pattern suggests load balancing issues. The engineer discovers that the remote site's network prefix is being advertised from both PEs with the same route-target but with different next-hops. The CE has equal-cost paths via both PEs. What is the most likely cause of the intermittent connectivity?
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
✓
The CE is performing per-packet load balancing across the two PEs, causing asymmetric routing.
When a CE router receives two equal-cost paths from two PEs, it may perform per-packet load balancing. This can lead to asymmetric routing where packets from the same flow take different paths, causing out-of-order delivery and connectivity failures about 50% of the time, especially if stateful devices are in the path or if the transport layer is sensitive to ordering. Option A is incorrect because the route-target import on the remote PE is properly configured (as the routes are seen with correct route-target). Option C is incorrect because BGP timer misconfiguration would cause session flapping and complete loss, not a 50% intermittent failure. Option D is incorrect because MTU mismatch would cause consistent packet drops, not intermittent failures.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The route-target import on the remote PE is missing the customer's route-target.
Why it's wrong here
If route-target import were missing, the route would not be present in the VRF at all, causing 100% failure.
- ✓
The CE is performing per-packet load balancing across the two PEs, causing asymmetric routing.
Why this is correct
Per-packet load balancing can lead to packets being sent to different PEs, potentially exiting via different remote PEs and causing return packets to arrive out of order or be dropped due to stateful inspection.
- ✗
The BGP timers are misconfigured, causing the session to flap.
Why it's wrong here
BGP timer misconfiguration would cause session flap, not 50% packet loss on a stable session.
- ✗
The MTU on the CE-PE links is mismatched.
Why it's wrong here
MTU mismatch typically causes packet drops for large packets, not intermittent connectivity for all traffic.
Quick reference
Asymmetric Encryption Algorithm Comparison
| Algorithm | Key Exchange | Signatures | Equivalent Security Key | Notes |
|---|---|---|---|---|
| RSA-3072 | Yes | Yes | 128-bit | Widely deployed; slow for bulk data |
| ECDSA P-256 | No | Yes | 128-bit | Fast signatures; standard TLS certs |
| ECDH / ECDHE | Yes | No | 128-bit | Perfect forward secrecy in TLS 1.3 |
| DH / DHE | Yes | No | 128-bit (3072-bit key) | Replaced by ECDHE in modern TLS |
| Ed25519 | No | Yes | ~128-bit | SSH keys, modern PKI |
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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