350-501 All-active multihoming Practice Question
A large service provider is migrating its L2VPN services (VPWS and VPLS) to EVPN-based solutions to improve scalability and support multi-homing. During the rollout for a customer using VLAN-based EVPN for a data center interconnect, the operations team notices frequent MAC address flapping and broadcast storms on one of the attachment circuits connected to a PE router. The PE is configured for EVPN-MPLS with ESI multihoming using all-active mode. The customer has two PEs (PE1 and PE2) connected to the same CE via two separate Ethernet links. The MAC table on PE1 shows the same MAC address alternating between the local AC interface and the remote EVPN peers. What is the most likely cause and the correct action to resolve this issue?
⚠ Common exam trap
Candidates often think that split-horizon is causing the issue, but in all-active mode, split-horizon is enabled by default. The real issue is often DF election inconsistency, which can be addressed by per-flow load balancing.
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
✓
Implement ESI multihoming with per-flow load balancing to ensure consistent designated forwarder election.
In all-active multihoming, both PEs forward traffic for the same VLAN, but split-horizon prevents loops. However, if the designated forwarder (DF) election is inconsistent—for example, due to misconfiguration of ESI or DF algorithm—both PEs may forward broadcast/multicast traffic, causing MAC flapping and broadcast storms. Per-flow load balancing ensures consistent DF election by hashing based on flow characteristics, reducing the chance of both PEs forwarding the same MAC addresses. Option B (disabling split-horizon) would worsen loops. Option C (selective multicast) is for optimizing multicast, not for MAC flapping. Option D (MPLS vs VXLAN) does not affect MAC learning loops.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Implement ESI multihoming with per-flow load balancing to ensure consistent designated forwarder election.
Why this is correct
Proper ESI configuration and DF election prevent duplicate MACs and loops; per-flow load balancing can help but the key is correct ESI.
- ✗
Disable split-horizon on the local AC interface to allow MAC learning from the CE directly.
Why it's wrong here
Disabling split-horizon would likely cause loops and increase flapping.
- ✗
Enable selective multicast on the EVPN instance to reduce broadcast traffic.
Why it's wrong here
Selective multicast is for multicast optimization, not for MAC flapping in EVPN.
- ✗
Use MPLS encapsulation instead of VXLAN to avoid MAC address issues.
Why it's wrong here
Encapsulation type does not affect MAC learning; the issue is in the EVPN control plane.
Visual reference
Quick reference
Access Control Model Comparison
| Model | Acronym | Who Controls Access? | Best For |
|---|---|---|---|
| Discretionary Access Control | DAC | Resource owner | Small teams, file shares |
| Mandatory Access Control | MAC | System / security labels | Classified govt / military |
| Role-Based Access Control | RBAC | Administrator (via roles) | Enterprise environments |
| Attribute-Based Access Control | ABAC | Policy engine (user + resource attributes) | Fine-grained, dynamic policies |
| Rule-Based Access Control | RuBAC | System rules / ACLs | Firewall rules, network ACLs |
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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