2.4 EtherChannel (Link Aggregation)

Tricking STP into allowing multiple active links. Master Port-Channels, LACP vs. PAgP negotiation protocols, and configuration best practices.

Core Explanations

The STP Workaround

If you plug four cables between two switches to increase bandwidth, Spanning Tree Protocol (STP) will immediately block three of them to prevent a loop. EtherChannel bundles those physical links into a single logical Port-Channel interface. STP sees it as one link, leaving it fully active.

LACP (802.3ad / 802.1AX)

Link Aggregation Control Protocol. The open industry standard supported by all vendors. It actively negotiates the bundle to ensure both sides are configured correctly. Uses 'Active' and 'Passive' modes. This is what you will use in the real world.

PAgP

Port Aggregation Protocol. A Cisco proprietary protocol that performs the exact same function as LACP. It uses 'Desirable' and 'Auto' modes. You must memorize these modes for the exam, but you will rarely configure PAgP in modern enterprise networks.

IOS Command Reference

Creating an LACP EtherChannel

! Step 1: Select the physical interfaces using the range command

SW1(config)# interface range g0/1 - 4

! Step 2: Bind them to a channel group (1-6) using LACP 'active'

SW1(config-if-range)# channel-group 1 mode active

! *The router will dynamically create interface Port-channel 1

Configuring the Logical Interface

! Step 3: Apply ALL configuration to the Port-Channel, NOT physical ports

SW1(config)# interface port-channel 1

SW1(config-if)# switchport trunk encapsulation dot1q

SW1(config-if)# switchport mode trunk

SW1(config-if)# switchport trunk allowed vlan 10,20,99

The Ultimate Verification Command

SW1# show etherchannel summary

! Look for the "SU" flag next to the Port-channel in the output table.
! S = Layer 2 (Switching)
! U = In Use (Working correctly)
! If you see "SD" (Down), your bundle has failed negotiation.

CCNA Exam Gotchas

The 'Mode On' Trap

Setting a channel-group to 'mode on' statically forces the bundle up without negotiation. If the switch on the other side is misconfigured, STP will fail to detect the loop and you will instantly cause a broadcast storm. Always use LACP Active/Active in production.

The 'Inconsistent Port' Suspension

Every physical port in an EtherChannel must have identical configurations (Speed, Duplex, Allowed VLANs). If you apply a command to physical interface g0/1 but forget g0/2, the switch suspends the port. Always apply changes directly to the Port-Channel interface.

Load Balancing Isn't 50/50

EtherChannel does not split a single file transfer equally across cables. It uses a hashing algorithm (usually Source-MAC to Destination-MAC) to pin specific 'conversations' to specific physical cables. One cable might carry 80% of the traffic depending on the hash.