In EVPN multihoming (ESI-LAG) deployments, enabling Spanning Tree Protocol (RSTP) on access switch uplinks toward the EVPN fabric can cause intermittent connectivity, MAC instability, and traffic blackholing.

This article explains the symptoms, root cause, verification steps, and resolution.

Environment

• QFX Series (EVPN/VXLAN fabric, ESI-LAG multihoming)
• EX Series (access layer switch)
• EVPN multihoming (dual-homed access switch to QFX cores)
• LACP aggregated uplinks (ae interface)

Symptoms

• Core switches appear intermittently unreachable or “down” from monitoring systems
• Intermittent loss of connectivity from access layer devices
• MAC address flapping or instability
• Uneven or asymmetric traffic forwarding
• Unexpected port blocking on access switch uplink

Problem Description

An EX access switch was dual-homed to QFX EVPN core switches using ESI-LAG. Despite correct EVPN and LACP configuration, the network experienced intermittent connectivity issues.

Verification & Evidence

1. STP active on uplink (unexpected)

user@access-switch> show spanning-tree interface

Interface       State  Role

ae0             FWD    DESG

2. BPDU transmission toward EVPN core

user@access-switch> show spanning-tree statistics interface

Interface BPDUs Sent BPDUs Received

ae0 18000+ 0

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Interpretation:

• The access switch is sending BPDUs
• No BPDUs are received from the QFX cores
• This indicates a control-plane mismatch, since EVPN fabrics do not run STP

. EVPN multihoming was correctly configured

Observed during troubleshooting:

• ESI configuration consistent across both QFX devices
• LACP on AE interface operational
• VLAN configuration aligned
• Physical interfaces healthy

This ruled out:

• Physical issues
• LACP problems
• Misconfigured EVPN/ESI

Root Cause

A control-plane conflict between:

• EVPN (QFX cores) → Handles loop prevention via:
  • ESI multihoming
  • Designated Forwarder (DF) election
• RSTP (EX access switch) → Independently performing loop prevention

Since:

• QFX EVPN fabric does not participate in STP
• The EX switch receives no BPDUs

The EX switch:

• Assumes itself as root bridge
• Continues sending BPDUs
• May incorrectly block uplinks to prevent perceived loops

This results in:

• Traffic blackholing
• MAC instability
• Inconsistent forwarding behavior

Impact

• Intermittent user traffic drops
• MAC learning inconsistencies
• Suboptimal use of uplinks (no proper load balancing)
• False link state alarms in monitoring systems

Solution

Disable STP on uplinks toward EVPN fabric

set protocols rstp interface ae0 disable

commit

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(Optional – recommended if physical members are configured individually):

set protocols rstp interface xe-0/0/0 disable

set protocols rstp interface xe-1/0/0 disable

commit

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Post-Fix Expected Behavior

• No STP participation on uplinks
• No BPDU transmission toward EVPN core
• Both uplinks forwarding traffic (active-active via LACP)
• Stable MAC learning
• No unexpected port blocking
• Consistent connectivity

Design Guidance

Best Practice for EVPN Multihoming

• EVPN provides loop prevention natively (no STP required)
• STP should:
  • Be disabled on uplinks
  • Only run on edge/access-facing ports (if needed)

Do NOT enable STP on:

• ESI-LAG uplinks
• EVPN-connected aggregation links

Conclusion

This issue was caused by RSTP running on an uplink toward an EVPN multihoming fabric, creating a loop-prevention conflict between STP and EVPN.

Disabling STP on the uplink resolved:

• Connectivity instability
• Traffic loss
• Incorrect port blocking

https://www.juniper.net/documentation/us/en/software/nce/evpn-lag-multihoming-guide/topics/concept/evpn-lag-guide-introduction.html
https://www.juniper.net/documentation/us/en/software/junos/evpn/topics/topic-map/easy-evpn-lag-config-script.html
https://www.juniper.net/documentation/us/en/software/nce/evpn-lag-multihoming-guide/topics/task/evpn-lag-guide-configuration-best-practices.html
https://www.juniper.net/documentation/us/en/software/nce/evpn-lag-multihoming-guide/topics/task/evpn-lag-guide-esi-types-lacp.html