Link Aggregation Control Protocol (LACP) is a standards-based protocol defined in IEEE 802.3ad that enables the bundling of multiple physical network links into a single logical channel called an EtherChannel or port channel. This aggregation provides increased bandwidth, redundancy, and load balan…Link Aggregation Control Protocol (LACP) is a standards-based protocol defined in IEEE 802.3ad that enables the bundling of multiple physical network links into a single logical channel called an EtherChannel or port channel. This aggregation provides increased bandwidth, redundancy, and load balancing between connected network devices.
LACP operates by exchanging Link Aggregation Control Protocol Data Units (LACPDUs) between switches or between a switch and another device. These packets contain information about the system priority, MAC address, port priority, and port number, which helps devices negotiate and form the aggregated link.
There are two LACP modes that ports can be configured with. Active mode means the port actively sends LACP packets to initiate negotiation with the partner device. Passive mode means the port responds to LACP packets but does not initiate the negotiation process. For an EtherChannel to form using LACP, at least one side must be in active mode.
LACP provides several advantages over static EtherChannel configuration. It offers dynamic link management, meaning if a physical link fails, LACP automatically removes it from the bundle and redistributes traffic across remaining links. When the failed link recovers, LACP adds it back to the channel. This provides fault tolerance and ensures continuous network availability.
The protocol supports up to 16 physical ports in a bundle, with 8 ports active and 8 in hot standby mode. LACP uses system and port priorities to determine which ports become active members of the channel.
When configuring LACP on Cisco switches, you use the channel-group command with the mode active or mode passive options. All ports in the EtherChannel must have matching configurations including speed, duplex, VLAN assignments, and trunk settings for successful bundle formation. LACP is preferred over the Cisco proprietary PAgP protocol due to its vendor-neutral nature and interoperability benefits.
Link Aggregation Control Protocol (LACP) - Complete CCNA Guide
Why LACP is Important
Link Aggregation Control Protocol (LACP) is a critical technology in modern networking that allows you to combine multiple physical links into a single logical link. This provides several key benefits:
• Increased Bandwidth: By bundling multiple links together, you can multiply your available throughput • Redundancy: If one link fails, traffic continues flowing through the remaining links • Load Balancing: Traffic is distributed across all active links in the bundle • Cost Efficiency: Use multiple lower-speed links instead of upgrading to expensive higher-speed connections
What is LACP?
LACP is an IEEE standard protocol (802.3ad, now part of 802.1AX) that enables the automatic negotiation and formation of link aggregation groups, also called EtherChannels in Cisco terminology or port channels. LACP ensures that both ends of a connection agree on which ports are part of the aggregated link and monitors the health of individual links.
Key LACP Terminology: • EtherChannel/Port Channel: The logical interface created by bundling physical interfaces • LACP Modes: Active and Passive • PAgP: Cisco's proprietary alternative to LACP (Port Aggregation Protocol) • System Priority: Determines which switch controls the aggregation decisions • Port Priority: Determines which ports are preferred when more than the maximum are available
How LACP Works
LACP Modes: • Active: The switch actively sends LACP packets to negotiate an EtherChannel • Passive: The switch only responds to LACP packets but does not initiate negotiation
Mode Combinations for Successful EtherChannel Formation: • Active + Active = Yes (forms EtherChannel) • Active + Passive = Yes (forms EtherChannel) • Passive + Passive = No (neither side initiates)
LACP Configuration Steps: 1. Ensure physical interfaces have matching configurations (speed, duplex, trunk mode, allowed VLANs) 2. Assign interfaces to a channel group with the LACP mode 3. Configure the port-channel interface with desired settings
• System Priority: Default is 32768. Lower value = higher priority. The switch with the lower system priority makes decisions about which ports to include. • Port Priority: Default is 32768. Lower value = higher priority. Used when there are more ports than allowed in the bundle (maximum 16 ports, only 8 active).
Verification Commands: • show etherchannel summary - Overview of all EtherChannels • show etherchannel port-channel - Detailed port-channel information • show lacp neighbor - View LACP neighbor details • show lacp internal - View local LACP information
Exam Tips: Answering Questions on LACP
1. Remember Mode Combinations: For LACP to form, at least one side must be in active mode. Passive + Passive will never form an EtherChannel. This is a frequently tested concept.
2. Distinguish LACP from PAgP: • LACP uses active and passive modes (IEEE standard) • PAgP uses desirable and auto modes (Cisco proprietary) • You cannot mix LACP and PAgP on the same EtherChannel
3. Configuration Consistency: All interfaces in an EtherChannel must have identical configurations including speed, duplex, switchport mode, and VLAN assignments. Mismatches cause the EtherChannel to fail.
4. Know the Static Option: Using mode on forces EtherChannel formation with no negotiation protocol. Both sides must use 'on' mode, which provides no link monitoring and is generally not recommended.
5. Port-Channel Interface Behavior: Configuration applied to the port-channel interface is inherited by member interfaces. Apply configurations to the port-channel, not individual physical interfaces.
6. Maximum Ports: LACP supports up to 16 ports in a bundle, with 8 active and 8 in standby (hot-standby). PAgP supports only 8 ports total.
7. Common Exam Scenarios: • Troubleshooting EtherChannel formation failures • Identifying correct mode combinations • Selecting appropriate commands for verification • Choosing between LACP and PAgP based on requirements
8. Layer 2 vs Layer 3 EtherChannels: EtherChannels can be configured as Layer 2 (switchport) or Layer 3 (routed) interfaces. Know the difference and when each is used.
Quick Reference Table for Exam:
| Mode Combination | Result | | Active + Active | Forms | | Active + Passive | Forms | | Passive + Passive | Does NOT Form | | On + On | Forms (no negotiation) | | On + Active/Passive | Does NOT Form |