Switching Networking Theory

LACP

Link Aggregation Control Protocol for dynamic EtherChannel negotiation and redundancy across multiple physical links

  • LACP is IEEE 802.3ad standard that dynamically negotiates and maintains EtherChannel bundles between switches
  • Provides automatic failover and load balancing across multiple physical links bundled into a single logical interface
  • Works by exchanging LACP Data Units (LACPDUs) every 30 seconds (or 1 second in fast mode) to monitor link health
  • Both sides must support LACP - if one side doesn’t respond to LACPDUs, the bundle won’t form

LACP Modes

Mode Behavior Use Case
Active Initiates LACP negotiation Best practice - actively forms channels
Passive Waits for LACP packets Used when you want the other side to initiate
On Forces bundling without LACP Legacy mode - no negotiation or monitoring
  • Active-Active or Active-Passive configurations will successfully negotiate
  • Passive-Passive will NOT work (neither side initiates)
  • On mode bypasses LACP entirely - dangerous because there’s no link monitoring
Side A Side B Result
Active Active Up
Active Passive Up
Passive Passive Down
On On Up*
On Active Down
On Passive Down

LACP Timers

Timer Interval Timeout
Slow 30s 90s
Fast 1s 3s

Notes:

  • Active: Sends LACPDUs
  • Passive: Waits for LACPDUs
  • On: Static bundling
  • Fast timer for critical links

Key LACP Parameters

  • System Priority: Lower value = higher priority (default 32768)
  • Port Priority: Lower value = higher priority (default 32768)
  • System ID: Combination of system priority + MAC address
  • Port Key: Groups ports that can bundle together (same speed/duplex)
  • Maximum 8 active links per bundle with up to 8 standby links

Configuration Example 

1
2
3

interface range gi0/1-2
 channel-group 1 mode active
 switchport mode trunk

Load Balancing Methods

  • src-mac: Source MAC address
  • dst-mac: Destination MAC address
  • src-dst-mac: XOR of source and destination MAC
  • src-ip: Source IP address (Layer 3 aware)
  • dst-ip: Destination IP address
  • src-dst-ip: XOR of source and destination IP

Vocabulary

LACPDU: Link Aggregation Control Protocol Data Unit - control frames exchanged between switches to maintain the bundle

Actor: The local switch in LACP negotiation

Partner: The remote switch in LACP negotiation

Port Channel: The logical interface created from bundled physical ports

Channel Group: Configuration command that assigns physical ports to a port channel

Notes

  • LACP provides superior monitoring compared to static bundling - detects failed links and removes them from the bundle automatically
  • Speed and duplex must match across all ports in the bundle (LACP enforces this)
  • Spanning Tree treats the entire bundle as a single link - eliminates multiple path issues
  • Use show etherchannel summary to verify bundle status and member ports
  • LACP timers can be tuned - fast mode (1 second) vs slow mode (30 seconds) for faster convergence
  • Mismatched VLAN configurations on member ports will prevent successful bundling
  • Load balancing algorithm affects traffic distribution - choose based on your traffic patterns (Layer 2 vs Layer 3)
  • LACP works across switch stacks but requires careful configuration of system priorities