Arista Interface Flapping: Troubleshooting Guide

Interface flapping on Arista switches can cause network instability, trigger STP reconvergence, and disrupt traffic. This guide covers common causes and diagnostic steps — manually and with Inishi.

Quick Diagnosis with Inishi

If you're using Inishi, just ask:

"Why is interface Ethernet1 flapping on switch-01?"

Inishi will:

1. Check interface status and error counters
2. Review recent logs for flap events
3. Correlate with transceiver diagnostics
4. Identify the root cause

Skip the manual troubleshooting below and try Inishi free → (opens in a new tab)

Manual Troubleshooting

Step 1: Check Interface Status

switch-01# show interfaces Ethernet1
Ethernet1 is up, line protocol is up (connected)
  Hardware is Ethernet, address is 001c.73a1.b234
  MTU 9214 bytes, BW 10000000 Kbit
  Full-duplex, 10Gb/s, auto negotiation: off
  Last link flapped 0:05:32 ago
  5 input errors, 0 CRC, 0 frame, 5 overrun, 0 ignored
  0 output errors, 0 collisions

Key indicators:

• Last link flapped — How long since the last flap
• input errors, CRC, frame — Physical layer issues
• overrun — Buffer issues (usually transient)

Step 2: Check Flap History

switch-01# show interfaces Ethernet1 status history
Time                   Status
2024-01-15 14:32:05    down
2024-01-15 14:32:07    up
2024-01-15 14:28:12    down
2024-01-15 14:28:14    up
2024-01-15 14:25:03    down
2024-01-15 14:25:05    up

Pattern analysis:

• Regular intervals → Auto-negotiation issues or external device
• Random intervals → Physical layer (cable, SFP, port)
• Correlated with other events → Check for spanning tree or LACP issues

Step 3: Check Transceiver Diagnostics

switch-01# show interfaces Ethernet1 transceiver detail
Ethernet1
  Administrative down: False
  Transceiver Type: SFP-10G-SR
  Tx Power: -2.3 dBm
  Rx Power: -8.5 dBm
  Temperature: 42.5 C
  Voltage: 3.29 V

Thresholds to watch:

Step 4: Check Logs

switch-01# show logging last 100 | grep Ethernet1
Jan 15 14:32:05 switch-01 Ebra: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet1, changed state to down
Jan 15 14:32:07 switch-01 Ebra: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet1, changed state to up
Jan 15 14:32:05 switch-01 Phy: %PHY-5-LINK_DOWN: Ethernet1 link status changed to down

Step 5: Check MLAG Status (if applicable)

switch-01# show mlag
MLAG Configuration:
domain-id             : production
local-interface       : Vlan4094
peer-address          : 10.255.255.2
peer-link             : Port-Channel1

MLAG Status:
state                 : active
negotiation status    : connected
peer-link status      : up

If the flapping interface is part of an MLAG, check that MLAG is stable.

Common Causes and Fixes

1. Bad Cable or Fiber

Symptoms:

• CRC errors incrementing
• Flapping at irregular intervals
• Low Rx power on transceiver

Diagnosis:

show interfaces Ethernet1 counters errors
show interfaces Ethernet1 transceiver

Fix:

1. Clean fiber connectors with IPA wipes
2. Reseat the cable/patch cord
3. Replace cable if errors persist
4. Try a different port to isolate

2. Failing SFP/Transceiver

Symptoms:

• Rx/Tx power out of spec
• High temperature
• Flapping increases over time

Diagnosis:

show interfaces Ethernet1 transceiver detail
show interfaces Ethernet1 transceiver thresholds

Fix:

1. Reseat the SFP
2. Clean SFP and fiber connectors
3. Replace SFP with known-good spare
4. Check for compatibility (genuine vs third-party)

3. Auto-Negotiation Mismatch

Symptoms:

• Flapping every 30-60 seconds (negotiation timeout)
• Duplex/speed mismatch in logs

Diagnosis:

show interfaces Ethernet1 status
show running-config interfaces Ethernet1

Fix:

switch-01(config)# interface Ethernet1
switch-01(config-if-Et1)# speed forced 10000full
switch-01(config-if-Et1)# no speed auto

4. STP Topology Changes

Symptoms:

• Multiple interfaces flapping together
• STP TCN (Topology Change Notification) in logs

Diagnosis:

show spanning-tree interface Ethernet1 detail
show logging | grep SPANTREE

Fix:

1. Enable BPDU Guard on edge ports
2. Configure PortFast on access ports
3. Investigate upstream STP issues

5. LACP Issues

Symptoms:

• Port-Channel members flapping
• LACP timeout messages in logs

Diagnosis:

show lacp interface Ethernet1 detailed
show port-channel 1 detailed

Fix:

1. Verify LACP mode matches on both ends (active or passive)
2. Check that system-id and key match
3. Verify LACP rate (fast vs normal)

6. Errdisabled State

Symptoms:

• Interface shows errdisabled
• No traffic passing

Diagnosis:

show interfaces Ethernet1 status
show errdisable recovery

Fix:

switch-01(config)# interface Ethernet1
switch-01(config-if-Et1)# shutdown
switch-01(config-if-Et1)# no shutdown

Or enable auto-recovery:

switch-01(config)# errdisable recovery cause all
switch-01(config)# errdisable recovery interval 300

Prevention: Proactive Monitoring

With Inishi

Set up health polling to catch flapping before it impacts users:

1. Enable interface monitoring on your Arista switches
2. Inishi tracks flap counts and error rates
3. Get alerts when thresholds are exceeded

Set up monitoring → (opens in a new tab)

With Event-Handler (Native)

Configure Arista's built-in event handler to alert on interface changes:

switch-01(config)# event-handler interface-flap
switch-01(config-handler-interface-flap)# trigger on-intf Ethernet1 operstatus
switch-01(config-handler-interface-flap)# action bash logger "Interface flap detected"

Diagnostic Commands Summary

Let Inishi Do the Work

Manual troubleshooting works, but it requires expertise and time. With Inishi:

• Ask "Why is this interface flapping?" in plain English
• Get correlated analysis across status, errors, and logs
• See proposed fixes before they run
• Keep audit trails for compliance

Start your free trial → (opens in a new tab) — 14 days, 5 devices, no credit card required.