Cisco IOS Port-Channel / LACP

Cisco IOS / IOS-XEIntermediate

LACP EtherChannel (802.3ad) port-channel configuration for Cisco IOS/IOS-XE switches and routers. Supports L2 (trunk/access) and L3 (routed) port-channels.

↓ example.txt

What is it?

A Port-Channel (also called EtherChannel or Link Aggregation) bundles multiple physical interfaces into a single logical interface, multiplying bandwidth and providing redundancy. IEEE 802.3ad LACP (Link Aggregation Control Protocol) is the standard — both sides negotiate which links to bundle and detect failures. A 4-port GigabitEthernet port-channel gives you 4 Gbps of aggregate bandwidth (with load balancing) and if one link fails, the remaining links continue carrying traffic without any convergence delay.

✓ When to use

·Uplinks between access and distribution switches that need more than 1 Gbps
·Server connections where 1 Gbps is a bottleneck (use 2-4 port NIC teaming + switch port-channel)
·Connections between core switches or between a switch and a router
·Anywhere you need link redundancy without spanning tree blocking ports
·WAN router interfaces where multiple T1/E1 or xDSL lines need to be aggregated

✗ When NOT to use

·Connecting a single workstation — LACP requires a switch on both ends (or server NIC teaming)
·Mixing different speed interfaces in one port-channel — all member ports must be the same speed
·When spanning tree is already providing redundancy and bandwidth is sufficient

How it compares

vs Spanning Tree (STP)STP blocks redundant links to prevent loops — you pay for the cable but get no bandwidth from it. Port-channel uses ALL links simultaneously. Port-channel is always preferred over STP-blocked links for critical uplinks.

vs Cisco NX-OS vPCvPC (Virtual Port-Channel) allows a port-channel to span two physical switches — the server or downstream switch sees one port-channel but connects to two upstream switches. More resilient than standard port-channel.

vs Active/Passive bonding (Linux)Linux bonding mode 4 (LACP) is equivalent to Cisco LACP active mode. Linux bonding mode 1 (active-backup) provides failover but no bandwidth aggregation.

Prerequisites

→Both ends of the port-channel must be configured — the switch on the other side needs the same port-channel configuration
→All member interfaces must have the same speed, duplex, VLAN configuration, and trunk settings
→On IOS: 'switchport trunk encapsulation dot1q' must be configured BEFORE 'switchport mode trunk' on some platforms
→LACP 'active' mode on at least one side — never use 'on' mode with LACP (incompatible)

Config Generator

Hostname *

Port-Channels

Member Interfaces

Verification commands

show etherchannel summary

Show all port-channels, member interfaces, and LACP status

Expected: Port-channel shows 'SU' (Layer 2, in use). Members show 'P' (bundled in port-channel). If 'I' (standalone), LACP not forming.

show lacp neighbor

Show LACP PDU exchange with the neighbor switch

Expected: Shows the neighbor's system ID, port priority, and state. If empty, LACP PDUs aren't being received.

show interfaces port-channel <id>

Show port-channel interface statistics and state

Expected: Line protocol up/up. Check input/output errors — these indicate physical layer issues on member links.

Debug commands

show lacp internal

Show LACP state machine for each member port

Common mistakes & fixes

⚡ Port-channel shows 'SD' or members show 'I' (individual) instead of 'P' (bundled)

Cause: LACP mode mismatch (both sides passive), or interface configuration mismatch between members

Fix: Set at least one side to 'active': 'channel-group <id> mode active'. Verify all members have identical speed, duplex, and VLAN config.

⚡ Port-channel is up but traffic imbalanced across members

Cause: Load balance algorithm not optimal for the traffic type

Fix: Change load balance method: 'port-channel load-balance src-dst-ip' for routed traffic, 'src-dst-mac' for L2 traffic. Check with 'show etherchannel load-balance'.

⚡ Cannot add interface to port-channel — 'incompatible interface removed'

Cause: Interface has different speed, duplex, VLAN, or STP configuration than existing port-channel members

Fix: Default the interface first: 'default interface <if>', then reconfigure it to match other members exactly before adding to channel-group.