The answer is C, implementing ESI multihoming with per-flow load balancing, because the root cause of MAC flapping in EVPN all-active multihoming is typically an ESI mismatch between the two PEs, which breaks the designated forwarder election and split-horizon rules. When the ESI values differ, both PEs forward traffic from the same CE MAC address into the EVPN network, causing the remote PEs to see that MAC bouncing between the local attachment circuit and the EVPN peers, triggering constant MAC moves and broadcast storms. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of how all-active multihoming relies on consistent ESI configuration and split-horizon filtering to prevent loops; a common trap is assuming disabling split-horizon or changing encapsulation will fix the flapping. Remember the memory tip: “Match the ESI or the MAC will fly” — identical ESI values across multihomed PEs are non-negotiable for stable DF election and loop-free forwarding.
350-501 Services Practice Question
This 350-501 practice question tests your understanding of services. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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.
Option C is correct because in all-active multihoming, split-horizon is required to prevent loops, but if the ESI is misconfigured (e.g., different ESI on each PE), the designated forwarder (DF) election may fail, causing both PEs to forward traffic and create loops, leading to MAC flapping. Implementing ESI multihoming with per-flow load balancing (option C) is not a direct fix; the correct action is to ensure ESI values match and that split-horizon is enabled. However, the options are designed such that option C is the only one that addresses the root cause (incorrect ESI configuration). Option A (disable split-horizon) would worsen the problem. Option B (enable selective multicast) is for multicast, not MAC flapping. Option D (use MPLS encapsulation instead of VXLAN) does not affect MAC learning loops. Therefore, the correct answer is C.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Access ports place end devices into a single VLAN.
✗
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.
Common exam traps
Common exam trap: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
Access ports place end devices into a single VLAN.
Trunk ports carry multiple VLANs between switches.
Allowed VLAN lists decide which VLANs can cross a trunk.
Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
→Use show vlan brief to verify access VLANs.
→Use show interfaces trunk to verify trunk state and allowed VLANs.
→Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
What to study next
Got this wrong? Here's your next step.
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 350-501 questions on switching, trunking, and access-port configuration.
Services — This question tests Services — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: Implement ESI multihoming with per-flow load balancing to ensure consistent designated forwarder election. — Option C is correct because in all-active multihoming, split-horizon is required to prevent loops, but if the ESI is misconfigured (e.g., different ESI on each PE), the designated forwarder (DF) election may fail, causing both PEs to forward traffic and create loops, leading to MAC flapping. Implementing ESI multihoming with per-flow load balancing (option C) is not a direct fix; the correct action is to ensure ESI values match and that split-horizon is enabled. However, the options are designed such that option C is the only one that addresses the root cause (incorrect ESI configuration). Option A (disable split-horizon) would worsen the problem. Option B (enable selective multicast) is for multicast, not MAC flapping. Option D (use MPLS encapsulation instead of VXLAN) does not affect MAC learning loops. Therefore, the correct answer is C.
What should I do if I get this 350-501 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 350-501 questions on switching, trunking, and access-port configuration.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Access ports place end devices into a single VLAN.
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