LACP (LINK AGGREGATION CONTROL PROTOCOL) TROUBLESHOOTING - STANDARD OPERATING PROCEDURE

Knowledge Base Article

Document ID: KB-LACP-001
Version: 1.0
Last Updated: June 12, 2026
Platform: Juniper Networks (EX, QFX, MX, SRX Series)
Status: Production

1. OVERVIEW

LACP (Link Aggregation Control Protocol) enables dynamic bundling of multiple physical links into a single logical interface for increased bandwidth and redundancy. This SOP provides comprehensive troubleshooting procedures for LACP convergence, member link issues, and operational failures.

1.1 Objective

• Diagnose LACP convergence failures
• Resolve member link aggregation issues
• Fix partner/system ID mismatches
• Troubleshoot load balancing problems
• Provide escalation path for complex issues

1.2 Scope

This document covers: - Static and dynamic LACP aggregation - Link state transitions and convergence - System ID and partner ID negotiation - Load balancing algorithms and hashing - LACP timeout and heartbeat mechanisms - All switch and routing platforms - Interoperability with third-party devices

2. UNDERSTANDING LACP

2.1 LACP Architecture and States

Aggregation Mode: - Active: Port actively initiates LACP negotiation - Passive: Port waits for LACP negotiation from peer - Off: Aggregate without LACP (static aggregation)

Link States: - Standby: Link bundled but not used (backup) - Active: Link actively forwarding traffic - Disabled: Link not participating in LAG - Collecting: Receiving frames from LAG - Distributing: Transmitting frames to LAG

Key Identifiers: - System ID: MAC address of LAG device - System Priority: 0-65535 (lower = preferred) - Partner System ID: Remote LAG device MAC - Actor Port Number: Local port number in LAG - Partner Port Number: Remote port number - Operational Key: Identifier for LAG group

2.2 Common LACP Issues

3. PREREQUISITES FOR TROUBLESHOOTING

Before starting troubleshooting:

• Access to both ends of LAG (local and peer devices)
• Cable connectivity verification between aggregated ports
• Understanding of LACP negotiation between devices
• Baseline working LAG configuration (if applicable)
• Peer device configuration (if troubleshooting interop issues)
• Network topology diagram showing LAG ports
• Storage for debug traces and packet captures
• Ability to enable/disable ports safely

4. INITIAL LACP DIAGNOSTICS

Procedure 4.1: Verify Aggregated Ethernet Interface

Command:

show interfaces ae1 brief
show interfaces ae1 detail
show lacp interfaces ae1

Expected Output:

Interface: ae1
  Admin State: up
  Link State: up
  Physical Link Status: Up
  Speed: 20G
  Duplex: Full-Duplex
  MTU: 1514
  Device Flags: Present Running
  Interface Flags: SNMP-Traps RPVST Internal: 0x4000

Interpret Results:

• ✓ Interface up/up with combined speed (e.g., 20G for 2×10G)
• ✓ Physical link status “Up”
• ✗ Interface down = No aggregation
• ✗ Speed lower than expected = Some links not participating

Procedure 4.2: Check Member Link Status

Command:

show interfaces ae1 detailed
show lacp interfaces ae1

Look for Member Link Details:

Member Links:
  ge-0/0/0:
    State: up, LACP state: up
    LACP Role: Actor, Collecting/Distributing: Yes
    
  ge-0/0/1:
    State: up, LACP state: up
    LACP Role: Actor, Collecting/Distributing: Yes
    
  ge-0/0/2:
    State: down, LACP state: down
    LACP Role: Standby

Interpret Results:

• ✓ Member links in “up” and “Collecting/Distributing”
• ✓ Correct number of active links
• ✗ Links showing “standby” when should be “active”
• ✗ Links “down” when should be “up”
• ✗ Mismatch between physical and LACP state

Procedure 4.3: Check LACP Negotiation Status

Command:

show lacp interfaces ae1 extensive
show lacp neighbors

Expected Output:

Aggregated interface: ae1
LACP state: Role Actor (Preferred = Yes), Operational Key 0x0001
Partners Sys-id 00:11:22:33:44:55, Sys Priority 32768

Member interfaces:
  ge-0/0/0 LACP state: LACP State: up, Collecting/Distributing
    Actor System ID: 00:04:96:a0:f9:60, Oper Key 0x0001, Port Priority 128
    Partner System ID: 00:11:22:33:44:55, Oper Key 0x0001, Port Priority 128
    
  ge-0/0/1 LACP state: up, Collecting/Distributing
    Actor System ID: 00:04:96:a0:f9:60, Oper Key 0x0001, Port Priority 128
    Partner System ID: 00:11:22:33:44:55, Oper Key 0x0001, Port Priority 128

Verify:

• ✓ All links show same Partner System ID (peer device MAC)
• ✓ All links show same Operational Key
• ✓ Collecting/Distributing state for active links
• ✗ Different Partner System IDs = Multiple peers
• ✗ Different Operational Keys = Key mismatch
• ✗ Not Collecting/Distributing = Link not active

Procedure 4.4: Verify LACP Configuration

Command:

show configuration interfaces ae1
show configuration interfaces ge-0/0/0 | grep "802.3ad"

Expected Output:

set interfaces ae1 aggregated-ether-options lacp active
set interfaces ae1 aggregated-ether-options lacp fast
set interfaces ae1 unit 0 family inet address 10.0.0.1/24

set interfaces ge-0/0/0 ether-options 802.3ad ae1
set interfaces ge-0/0/1 ether-options 802.3ad ae1

Check Configuration:

• ✓ LACP mode is “active” or “passive” (not “off”)
• ✓ All member interfaces configured with “802.3ad ae1”
• ✓ Same aggregation ID (ae1) on all members
• ✗ Mixed modes (some active, some off) = Configuration mismatch
• ✗ Different ae IDs = Wrong aggregation group

Procedure 4.5: Check LACP Statistics

Command:

show lacp statistics ae1
show lacp statistics interfaces ge-0/0/0

Expected Output:

Interface: ge-0/0/0
  LACP PDU Transmit: 1234
  LACP PDU Receive: 1235
  LACP PDU Discard: 0
  Timeout: Fast
  LACP Error: No Error

Interpret Results:

• ✓ PDU Transmit and Receive both increasing
• ✓ PDU Discard = 0 (no lost LACP packets)
• ✓ No LACP errors reported
• ✗ PDU Receive = 0 = No LACP packets from peer
• ✗ High Discard count = Dropped LACP packets
• ✗ LACP errors = Protocol issues

5. STEP-BY-STEP TROUBLESHOOTING WORKFLOWS

SCENARIO 1: LACP Not Converging / All Links Down

Step 1.1: Verify Physical Links are Up

show interfaces ge-0/0/0 terse
show interfaces ge-0/0/1 terse

# Should show:
ge-0/0/0          up    up
ge-0/0/1          up    up

# If showing "down", troubleshoot physical link first
# See Interface Flap Troubleshooting SOP

Step 1.2: Verify LACP Mode Configuration

show configuration interfaces ae1 | grep "lacp"

# Should show either:
set interfaces ae1 aggregated-ether-options lacp active
# OR
set interfaces ae1 aggregated-ether-options lacp passive

# Check both sides match (at least one should be active)

If Mode Mismatch Found:

# Both passive = No negotiation will occur
# Fix: Set at least one to active
set interfaces ae1 aggregated-ether-options lacp active
commit

# Verify LACP starts converging
show lacp interfaces ae1

Step 1.3: Check Member Interface Configuration

show configuration interfaces | grep -A 2 "ge-0/0/"

# Each member should show:
set interfaces ge-0/0/0 ether-options 802.3ad ae1
set interfaces ge-0/0/1 ether-options 802.3ad ae1

# If any missing:
set interfaces ge-0/0/0 ether-options 802.3ad ae1
set interfaces ge-0/0/1 ether-options 802.3ad ae1
commit

Step 1.4: Verify No Disable Statements

show configuration interfaces ge-0/0/0 | grep -i disable
show configuration interfaces ae1 | grep -i disable

# If disabled:
delete interfaces ge-0/0/0 disable
delete interfaces ae1 disable
commit

# Verify interfaces come up:
show interfaces ae1 terse

Step 1.5: Check LACP PDU Exchange

# Monitor LACP packet exchange
show lacp statistics ae1

# PDU Receive should be > 0
# If = 0, peer not sending LACP

# Verify peer configuration:
# On peer device:
show interfaces ae1 detail
show lacp interfaces ae1

# If peer not configured as LACP:
# Configure peer with LACP active

Step 1.6: Test Link Convergence Timing

# LACP convergence should happen quickly
# Monitor interface state changes:
show log messages | grep -i "ge-0/0/\|lacp\|aggregate" | tail -30

# Should see transitions:
# - Link detected (ge-0/0/0 up)
# - LACP negotiation starts (LACP PDU received)
# - Link becomes active (typically <3 seconds)

If Convergence Slow (>10 seconds):

# Check LACP timeout setting
show configuration interfaces ae1 | grep -i "timeout\|fast"

# Enable fast timeout for quicker convergence
set interfaces ae1 aggregated-ether-options lacp fast
commit

# Wait 5-10 seconds for convergence
show lacp interfaces ae1

SCENARIO 2: Only Some Links Aggregating (Partial LAG)

Step 2.1: Identify Down Member Links

show lacp interfaces ae1 extensive | grep -A 3 "ge-0/0"

# Look for links showing:
# State: down
# LACP state: standby
# Collecting/Distributing: No

Step 2.2: Check Physical Link Status

# For each down link:
show interfaces ge-0/0/0 detail

# Check for:
# Admin State: up/down
# Link State: up/down
# Link Transitions: (how many times link went down/up)

# If physical link down:
show interfaces ge-0/0/0 diagnostics optics  (if fiber)
show interfaces ge-0/0/0 statistics | grep -i error

Physical Troubleshooting:

# If CRC errors present:
# Link quality issue - test with different cable

# If link flapping:
# See Interface Flap Troubleshooting SOP

# If all links on same port failed:
# May be port hardware issue

Step 2.3: Verify Link Configuration

# Check if link properly configured as member
show configuration interfaces ge-0/0/0 | grep "802.3ad"

# If missing:
set interfaces ge-0/0/0 ether-options 802.3ad ae1
commit

# Verify link comes up:
show interfaces ge-0/0/0 terse
wait 5 seconds
show lacp interfaces ae1

Step 2.4: Check System ID Consistency

show lacp neighbors | grep "Sys-id\|Oper Key"

# All member links should show same:
# - Partner System ID (peer device MAC)
# - Operational Key

# If differs:
# Link may be connected to different port group
# Verify physical connections are correct

Step 2.5: Verify Port Priority

show lacp interfaces ae1 extensive | grep "Port Priority"

# If different ports have different priorities:
# Higher priority number = lower preference
# All member links should have same priority

# If different:
show configuration interfaces ge-0/0/0 | grep -i priority

# Set all to same:
set interfaces ge-0/0/0 ether-options 802.3ad ae1 primary
set interfaces ge-0/0/1 ether-options 802.3ad ae1 primary
commit

SCENARIO 3: No Load Balancing (Single Link Carrying All Traffic)

Step 3.1: Verify Multiple Links are Distributing

show interfaces ae1 statistics | grep "input\|output"

# Should show traffic distributed
# If all traffic on one link:
show interfaces ge-0/0/0 statistics | grep "input\|output"
show interfaces ge-0/0/1 statistics | grep "input\|output"

# Compare byte counts - should be similar if multiple flows

Step 3.2: Check Hash Policy Configuration

show configuration | grep -i "hash-policy"

# Expected output:
set chassis aggregated-ether-options hash-policy layer-3-4

# Different policies:
# - layer-2: Uses source/dest MAC
# - layer-3: Uses source/dest IP
# - layer-3-4: Uses IP + TCP/UDP port (best for most)
# - layer-4: Uses TCP/UDP port only

If No Hash Policy Set:

# Set appropriate hash policy
set chassis aggregated-ether-options hash-policy layer-3-4
commit

# Verify distribution across multiple flows
show interfaces ae1 statistics

Step 3.3: Analyze Traffic Pattern

# Single flow will only use one link (this is correct behavior)
# Multiple different flows will use different links

# Test with multiple flows:
# - From different source IP to different dest IP
# - Different source ports
# 
# Monitor distribution:
show interfaces ae1 statistics | grep -E "ge-0/0/[0-1]"

Step 3.4: Check for Asymmetric Traffic

# If using per-packet load balancing:
set chassis aggregated-ether-options hash-policy layer-3-4 per-packet
commit

# Per-packet distribution ensures all packets distributed
# But may cause out-of-order delivery
# Use only if required for true load balancing

SCENARIO 4: LACP Flapping (Links Oscillating)

Step 4.1: Check Link Stability

show log messages | grep -i "ge-0/0/\|lacp" | tail -30

# Look for repeated up/down transitions
# Each transition = potential issue

# Count transitions:
show log messages | grep "ge-0/0/0" | grep -E "(up|down)" | wc -l

# High count = unstable link

Step 4.2: Check Physical Link Quality

# LACP flapping often caused by physical issues
show interfaces ge-0/0/0 statistics | grep -i "error\|crc"

# Collect all physical diagnostics
show interfaces ge-0/0/0 diagnostics optics  (if fiber)
show interfaces ge-0/0/0 detail | grep -i "admin\|link"

Physical Troubleshooting:

# If errors found:
# - Test with different cable
# - Clean connectors
# - Test with loopback adapter
# - Replace SFP if fiber
# (See Interface Flap Troubleshooting SOP)

Step 4.3: Check LACP Timeout Configuration

show configuration interfaces ae1 | grep -i timeout

# LACP timeout should be appropriate:
# - Fast (3 seconds): For stable networks
# - Slow (30 seconds): For unstable networks

# If timeout too short for link quality:
set interfaces ae1 aggregated-ether-options lacp slow
commit

# Wait 5 minutes to verify stability
show log messages | grep "ge-0/0/0"

Step 4.4: Check Peer Device Stability

# If only this device's links flapping:
# Issue likely local

# If peer device also showing flaps:
# Coordinate with peer team for troubleshooting
# May be network issue between devices

6. LOG COLLECTION PROCEDURES

6.1 Basic LACP Status Logs

Execute on BOTH local and peer devices:

# LACP interface status
show interfaces ae1 brief > lacp_ae_status.txt
show interfaces ae1 detail >> lacp_ae_status.txt
show interfaces ae1 extensive >> lacp_ae_status.txt

# Member link status
show interfaces ge-0/0/0 terse >> lacp_ae_status.txt
show interfaces ge-0/0/1 terse >> lacp_ae_status.txt

# LACP negotiation details
show lacp interfaces ae1 > lacp_negotiation.txt
show lacp interfaces ae1 extensive >> lacp_negotiation.txt
show lacp neighbors >> lacp_negotiation.txt

# LACP statistics
show lacp statistics ae1 > lacp_statistics.txt
show lacp statistics interfaces ge-0/0/0 >> lacp_statistics.txt
show lacp statistics interfaces ge-0/0/1 >> lacp_statistics.txt

# Configuration
show configuration interfaces ae1 > lacp_config.txt
show configuration interfaces ge-0/0/0 >> lacp_config.txt
show configuration interfaces ge-0/0/1 >> lacp_config.txt

6.2 System Logs

Command:

show log messages | grep -i "lacp\|aggregate\|ge-0/0/" | last 300 > lacp_messages.log

show log messages | grep -i "member\|802.3ad" | last 100 >> lacp_messages.log

show log messages | last 500 > lacp_full_log.txt

6.3 Advanced Diagnostics

For Complex Issues:

# Interface statistics and errors
show interfaces ge-0/0/0 statistics > lacp_link_stats.txt
show interfaces ge-0/0/0 extensive >> lacp_link_stats.txt
show interfaces ae1 statistics >> lacp_link_stats.txt

# Protocol errors
show interfaces ge-0/0/0 detail | grep -i "error\|crc\|drop" > lacp_errors.txt
show interfaces ae1 detail | grep -i "error\|crc\|drop" >> lacp_errors.txt

# Traffic distribution
show interfaces ge-0/0/0 | match "input rate\|output rate" > lacp_distribution.txt
show interfaces ge-0/0/1 | match "input rate\|output rate" >> lacp_distribution.txt

7. DIAGNOSTIC COMMANDS REFERENCE

View LAG Status

show interfaces ae1 brief
show interfaces ae1 detail
show lacp interfaces ae1
show lacp neighbors

View Member Links

show interfaces ge-0/0/0 terse
show interfaces ge-0/0/0 detail
show lacp interfaces ae1 extensive

View Statistics

show lacp statistics ae1
show lacp statistics interfaces ge-0/0/0
show interfaces ae1 statistics
show interfaces ge-0/0/0 statistics

View Configuration

show configuration interfaces ae1
show configuration interfaces ge-0/0/0
show configuration | grep "hash-policy"

Monitor Real-time

monitor interfaces ae1
monitor lacp interfaces ae1
monitor interfaces ge-0/0/0

Check Errors

show interfaces ae1 extensive | grep error
show interfaces ge-0/0/0 statistics | grep error
show lacp statistics ae1 | grep -i discard

8. COMMON ROOT CAUSES & SOLUTIONS

9. INTEROPERABILITY CONSIDERATIONS

Juniper-to-Juniper LACP

# Juniper switches typically support all LACP modes
# Compatibility usually excellent if both running current Junos

Juniper-to-Third-Party LACP

Common Issues:

# 1. System ID difference
show lacp neighbors
# Verify remote MAC matches peer device

# 2. Key mismatch
# All member ports must have same operational key
show lacp interfaces ae1 extensive | grep "Oper Key"

# 3. Port number confusion
# Actor Port Number should match physical port
# Partner Port Number may differ between vendors

# 4. Timeout incompatibility
# Ensure timeout compatible with peer device
# Use slow timeout for conservative interop
set interfaces ae1 aggregated-ether-options lacp slow

10. ESCALATION CRITERIA

Escalate to JTAC if:

1. LACP fails to converge despite proper configuration
2. LACP flapping despite physical link stable
3. All member links down for unknown reason
4. Interoperability issue with third-party device
5. Partner ID constantly changing
6. System cannot aggregate >2 links in LAG
7. Hardware error in aggregation
8. LACP consuming excessive CPU

Provide JTAC with: - All logs from Section 6 - Complete LAG configuration - Peer device configuration (if available) - Diagnostic output from both sides - Timeline of issues - Hardware/software versions

11. OPERATIONAL BEST PRACTICES

Configuration Template

# Configure aggregated interface
set interfaces ae1 aggregated-ether-options lacp active
set interfaces ae1 aggregated-ether-options lacp fast
set interfaces ae1 unit 0 family inet address 10.0.0.1/24
set interfaces ae1 unit 0 family inet mtu 1500

# Configure member interfaces
set interfaces ge-0/0/0 ether-options 802.3ad ae1
set interfaces ge-0/0/1 ether-options 802.3ad ae1
set interfaces ge-0/0/2 ether-options 802.3ad ae1
set interfaces ge-0/0/3 ether-options 802.3ad ae1

# Configure hash policy globally
set chassis aggregated-ether-options hash-policy layer-3-4

commit

Monitoring and Maintenance

# Regular health check
show lacp interfaces ae1
show lacp statistics ae1
show interfaces ae1 statistics

# Monitor for flaps
show log messages | grep "lacp\|aggregate" | wc -l

# Verify load distribution
show interfaces ge-0/0/0 statistics
show interfaces ge-0/0/1 statistics

# Plan redundancy testing
# - Test link failure scenarios
# - Verify failover happens smoothly
# - Document behavior

12. DOCUMENT HISTORY

For Support: Contact Network Operations Center or JTAC
Document Owner: Network Engineering Team
Last Reviewed: June 12, 2026