LACP: Best Practices and Configuration

LACP (Link Aggregation Control Protocol, 802.3ad / 802.1AX) aggregates multiple physical links into a single logical link to increase bandwidth, provide redundancy, and load-balance traffic. It is a multi-vendor standard. This guide covers LACP best practices for avoiding common pitfalls. 2026.

How LACP Works

• Aggregation of 2-8 links into one logical port channel
• LACPDUs exchanged every 1s (fast) or 30s (slow)
• Each system identifies the other system by priority + MAC
• Ports are identified by priority + port number
• Negotiation: active+active and active+passive = OK. Passive+passive = no bundle

LACP Modes

• Active: sends LACPDUs regularly
• Passive: responds only when it receives LACPDUs
• On / Static: aggregation without LACP (unsafe, with no fault detection)
• Recommended: active+active on both sides

Load-Balancing Algorithms

The selected algorithm determines how flows are distributed across the links:

• src-mac: hashed on the source MAC (poorly balanced for 1 server → N clients)
• dst-mac: hashed on the destination MAC
• src-dst-mac: hashed on the combination (standard)
• src-dst-ip: IP-based (suitable for L3 routing)
• src-dst-ip-port (L4 hash): best for multiple flows with the same src/dst (e.g., multi-connection HTTP/S)
• Recommendation for Cisco Catalyst: src-dst-ip-port

Best Practices

• Always use LACP active+active (do not use 'on' mode)
• At least 2 links per port channel (redundancy)
• Maximum of 8 active links per port channel (hardware limit)
• Use the same speed/duplex/VLAN settings on all members
• Use an L4 hash algorithm for effective load balancing
• Disable PortFast on the port channel (not applicable)
• A LAG across different switches requires MC-LAG/vPC/MLAG (not standard LACP)

Cisco IOS Configuration

• interface range Gi1/0/1-2
• channel-group 1 mode active
• description Uplink-to-Core
• interface Port-channel1
• switchport mode trunk
• switchport trunk allowed vlan 10,20,30
• port-channel load-balance src-dst-ip-port (global command)

Juniper Junos Configuration

• set chassis aggregated-devices ethernet device-count 10
• set interfaces ae0 aggregated-ether-options lacp active
• set interfaces ge-0/0/0 ether-options 802.3ad ae0
• set interfaces ge-0/0/1 ether-options 802.3ad ae0
• set interfaces ae0 unit 0 family ethernet-switching interface-mode trunk vlan members all

MC-LAG / vPC: Multi-Chassis LACP

Enables 1 server/switch to form a LAG to 2 different switches for high availability:

• Cisco Nexus: vPC (Virtual PortChannel)
• Arista: MLAG (Multi-Chassis LAG)
• Juniper: MC-LAG
• Configuration: 2 switches connected by a peer link present 1 shared MC-LAG ID to the server

EVPN ESI-LAG (Modern Approach)

• Replaces proprietary MC-LAG with the EVPN standard
• Active-active multihoming via ESI (Ethernet Segment Identifier)
• Supported by Juniper, Cisco Nexus, Arista, Nokia, and SONiC
• Recommended for modern EVPN-VXLAN fabrics

Troubleshooting

• show etherchannel summary: port-channel status
• show lacp neighbors: LACP peers
• show interfaces port-channel 1: aggregate counters
• show spanning-tree interface Po1: STP on the bundle
• Common issues: speed/duplex mismatch, LACP mode mismatch, or a port in err-disable state

Pitfalls

• Adding a port with an existing VLAN configuration: the configuration is lost and must be reapplied to the port channel
• 'on' mode without LACP: unsafe, with no negotiation or failure detection
• Too many links: links 9+ remain on standby and are not used for load balancing
• src-mac load balancing with 1 server: all traffic leaves through 1 link

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