EtherChannel Link Aggregation Control Protocol (LACP)

Link Aggregation Control Protocol or LACP is an IEEE standard and a part of the IEEE 802.3ad specification that allows you to combine several physical Ethernet links in our network devices to form a single logical link and enable load balancing in our interfaces.

We can configure LACP EtherChannel with a maximum of 16 Ethernet interfaces of the same type. In a LAG or Link Aggregation Group, up to eight member links can be in active mode, and the other eight links can be in standby mode.

 

The 2 Link Aggregation Control Protocol Configuration Modes

We have two Link Aggregation Control Protocol (LACP) modes, and these are the following:

Active – The interface actively sends LACP packets in its attempt to form an LACP connection.

Passive – The interface can respond to LACP negotiation but will never initiate on its own.

 

Here’s an overview of the different modes and combinations and if link aggregation will work or not.

ActivePassive
ActiveYesYes
PassiveYesNo

 

LACP Initial Configuration Check

We have minimum requirements for Link Aggregation Control Protocol (LACP) to work. These are the following:

1. One side should at least be in Active Mode.

2. All port speed and duplex should be the same.

3. Switchport mode is the same (access or trunk).

4. Virtual Local Area Network (VLAN) passing on both sides should match.
 

EtherChannel LACP Configuration

The first requirement for link aggregation is at least one side should be in Active mode. For our example, we will configure Switch1 to be in Active Mode and the other network switch, Switch2, to be in Passive Mode.

Now, using our sample network topology below, let’s configure LACP on our network switches:

lacp

Switch 1 – Active Mode

Switch1#conf t
Switch1(config)#interface range fa0/1 - 2
Switch1(config-if-range)#speed 100
Switch1(config-if-range)#duplex full
Switch1(config-if-range)#switchport mode trunk
Switch1(config-if-range)#channel-group 1 mode active
Switch1(config-if-range)#end

 

Switch 2 – Passive Mode

Switch2#conf t
Switch2(config)#interface range fa0/1 - 2
Switch2(config-if-range)#speed 100
Switch2(config-if-range)#duplex full
Switch2(config-if-range)#switchport mode trunk
Switch2(config-if-range)#channel-group 1 mode passive
Switch2(config-if-range)#end

 

The logs on our switch shows that Port-Channel1 came up and link aggregation is working:

*Sep 5 15:30:06.378: %LINK-3-UPDOWN: Interface Port-channel1, changed state to up

*Sep 5 15:30:07.378: %LINEPROTO-5-UPDOWN: Line protocol on Interface Port-channel1, changed state to up

 

How to Verify LACP?

We can use the ‘show etherchannel <channel-group number> port-channel’ command to verify link aggregation and our port channel status:

Switch1#show etherchannel 1 port-channel
                Port-channels in the group:
                ---------------------------
Port-channel: Po1    (Primary Aggregator)
------------
Age of the Port-channel   = 0d:00h:01m:05s
Logical slot/port   = 16/0          Number of ports = 2
HotStandBy port = null
Port state          = Port-channel Ag-Inuse
Protocol            =   LACP
Port security       = Disabled
Ports in the Port-channel:
Index   Load   Port     EC state        No of bits
------+------+------+------------------+-----------
  0     00     Fa0/1    Active             0
  0     00     Fa0/2    Active             0
Time since last port bundled: 0d:00h:00m:50s Fa0/2

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