EtherChannel

EtherChannel is a port link aggregation technology. It allows you to group multiple physical Ethernet links between two switches and treat them as a single logical link. That way Spanning Tree only sees this single Port-Channel, so it doesn’t block anything.

1. Protocols

When configuring EtherChannel, you have to choose a negotiation protocol:

LACP (Link Aggregation Control Protocol): The IEEE standard (802.3ad).

• Active: The port actively sends LACP negotiation packet to the other switch to try and form an EtherChannel.
• Passive: The port is silent but listening. It will not initiate the negotiation, but it will respond and form and EtherChannel if it receives LACP packets.

PAgP (Port Aggregation Protocol): Cisco’s proprietary version.

*Desirable: The port actively sends PAgP negotiation packets to the other switch (equivalent to LACP’s Active). *Auto: (equivalent to LACP’s Passive but with PAgP packets).

Static (ON): Forces the channel without negotiation. Dangerous because if one side is misconfigured, it can create an STP loop.

*On: The port is forcefully bundled.

2. Configuration of LACP

2.1 Configure Switch A

SwitchA(config)# interface range GigabitEthernet 0/1 - 2
SwitchA(config-if-range)# channel-group 1 mode active
SwitchA(config-if-range)# exit

2.2 Configure the Logical Port-Channel on Switch A

Once the channel group is created, a virtual interface called Port-channel 1 appears. You configure your VLANs or trunking on this logical interface, NOT the physical ones anymore.

SwitchA(config)# interface Port-channel 1
SwitchA(config-if)# switchport mode trunk
SwitchA(config-if)# exit

2.3 Configure Switch B

SwitchB(config-if-range)# channel-protocol lacp
SwitchB(config-if-range)# channel-group 1 mode active
SwitchB(config-if-range)# exit

SwitchB(config)# interface Port-channel 1
SwitchB(config-if)# switchport mode trunk
SwitchB(config-if)# end

3. Verification commands

SwitchA# show etherchannel summary
SwitchA# show interfaces trunk

EtherChannel Protocols & Modes

While setting the mode (e.g., channel-group 1 mode active) automatically selects the protocol in the background, you can explicitly lock it at the interface level using the channel-protocol [lacp | pagp] command. The Mode Mapping Cheat Sheet:

• LACP (IEEE Standard): active (initiates) / passive (waits)
• PAgP (Cisco Proprietary): desirable (initiates) / auto (waits)
• Static (No Negotiation): on / on

Rule of thumb: Never mix protocols on the same link. An active port will never form a channel with a desirable port.