Switching Networking Theory

EtherChannel

Link aggregation technology bundling multiple physical links into logical interfaces for bandwidth and redundancy

  • Link aggregation technology that bundles multiple physical interfaces into a single logical interface to increase bandwidth and provide redundancy
  • Combines 2-8 physical links (typically same speed) into one logical channel that appears as a single interface to spanning tree and routing protocols
  • Load balances traffic across member links using various algorithms based on source/destination MAC, IP, or port combinations
  • If one physical link fails, traffic automatically redistributes across remaining active links without network disruption

Benefits and Use Cases

  • Bandwidth multiplication: 4 x 1Gbps links = 4Gbps aggregate bandwidth
  • Load distribution: Spreads traffic across multiple paths instead of blocking redundant links
  • High availability: Automatic failover if member links go down
  • Used between switches, switch-to-router connections, and server uplinks in enterprise networks
  • Critical for eliminating bottlenecks in high-traffic network segments (data centers, server farms)

EtherChannel Protocols

Protocol Type Standard Key Characteristics
PAgP Cisco Proprietary - Port Aggregation Protocol, older Cisco standard
LACP Industry Standard IEEE 802.3ad Link Aggregation Control Protocol, preferred method
Static Manual - No negotiation protocol, manual configuration only

PAgP (Port Aggregation Protocol)

  • Cisco proprietary negotiation protocol
  • Modes: auto (passive), desirable (active), on (forces channel without negotiation)
  • At least one side must be desirable for PAgP negotiation to work
  • Being phased out in favor of LACP
  • Industry standard preferred for interoperability
  • Modes: passive (responds to LACP), active (initiates LACP), on (static)
  • At least one side must be active for LACP negotiation to succeed
  • Supports priority settings and better failure detection than PAgP

Configuration Requirements

Physical Requirements

  • All member interfaces must have identical configuration:
    • Same speed and duplex settings
    • Same VLAN configuration (access or trunk)
    • Same spanning tree settings
  • Interfaces should be same type (all FastEthernet, all GigabitEthernet, etc.)
  • Maximum of 8 active links per EtherChannel group

Load Balancing Methods

  • src-mac: Source MAC address (good for multiple clients to one server)
  • dst-mac: Destination MAC address (good for one client to multiple servers)
  • src-dst-mac: Both source and destination MAC (most balanced for mixed traffic)
  • src-ip: Source IP address (Layer 3 aware, better distribution)
  • dst-ip: Destination IP address
  • src-dst-ip: Both source and destination IP (recommended for most scenarios)

Configuration Example Commands 

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interface range gi0/1-4
channel-group 1 mode active    (LACP active mode)
channel-protocol lacp          (specify LACP)

port-channel load-balance src-dst-ip    (global command)

Vocabulary

EtherChannel: Link aggregation technology bundling multiple physical links into one logical interface

PAgP: Port Aggregation Protocol - Cisco proprietary EtherChannel negotiation protocol

LACP: Link Aggregation Control Protocol - IEEE 802.3ad standard for link aggregation

Channel Group: The logical grouping number (1-6) assigned to bundle physical interfaces

Load Balancing Algorithm: Method used to distribute traffic across EtherChannel member links

Active Mode: Initiates negotiation protocol (LACP active, PAgP desirable)

Passive Mode: Responds to negotiation but doesn’t initiate (LACP passive, PAgP auto)

Notes

Configuration Gotchas

  • Mismatched interface configurations will prevent EtherChannel formation - all speeds, duplex, VLANs must match exactly
  • Using mode on bypasses negotiation protocols entirely (not recommended for production)
  • EtherChannel interfaces cannot participate in different VLANs simultaneously
  • Always verify EtherChannel status with show etherchannel summary after configuration

Practical Considerations

  • LACP is preferred over PAgP for vendor interoperability and future-proofing
  • Choose load balancing method based on traffic patterns: src-dst-ip works well for most enterprise scenarios
  • Single conversation (same src/dst pair) will always use same physical link - EtherChannel doesn’t split individual flows
  • Plan for asymmetric loading - not all applications distribute traffic evenly across channels
  • Consider that spanning tree sees EtherChannel as single link, reducing convergence complexity

Troubleshooting Tips

  • Check show etherchannel summary for member status and protocol state
  • Verify identical configuration on all member interfaces with show run interface
  • Use show etherchannel load-balance to confirm load distribution method
  • EtherChannel will err-disable if member link configurations drift apart

EtherChannel Commands

Basic Configuration

interface range [interface-range]

  • Select multiple interfaces for bundling

channel-group [number] mode [mode]

  • Assigns interfaces to EtherChannel group
  • Number: 1-6, Mode: active/passive/on

channel-protocol [lacp|pagp]

  • Specify negotiation protocol (optional)

LACP Configuration

channel-group [num] mode active

  • Initiates LACP negotiation (recommended)

channel-group [num] mode passive

  • Responds to LACP but doesn’t initiate

lacp port-priority [priority]

  • Set interface priority (1-65535)

PAgP Configuration

channel-group [num] mode desirable

  • Initiates PAgP negotiation

channel-group [num] mode auto

  • Responds to PAgP negotiation

Static Configuration

channel-group [num] mode on

  • Forces EtherChannel without negotiation
  • Not recommended for production

Load Balancing

port-channel load-balance [method]

  • Global command to set distribution method
  • Methods: src-dst-ip, src-dst-mac, etc.

Port-Channel Interface

interface port-channel [number]

  • Configure logical EtherChannel interface
  • Apply VLANs, trunking here

Verification Commands

show etherchannel summary

  • Shows all EtherChannels and member status
  • Displays protocol and port states

show etherchannel [number] detail

  • Detailed info for specific channel

show lacp neighbor

  • LACP peer information and status

show pagp neighbor

  • PAgP neighbor details

show etherchannel load-balance

  • Current load balancing method

show interfaces port-channel [number]

  • Port-channel interface statistics

Troubleshooting

show etherchannel protocol

  • Shows negotiation protocols in use

show run interface [interface]

  • Verify member interface configs match

Example Configuration 

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interface range gi0/1-4
 channel-group 1 mode active
 channel-protocol lacp

interface port-channel 1
 switchport mode trunk
 switchport trunk allowed vlan 10,20,30

port-channel load-balance src-dst-ip

Common Gotchas

  • All member interfaces must have identical:
    • Speed/duplex settings
    • VLAN configuration
    • Access/trunk mode
  • Use show etherchannel summary to verify
  • Mismatched configs cause err-disable