Institute of Electrical and Electronics Engineers (IEEE) 802.3ad link aggregation enables multiple Ethernet interfaces to be grouped together and form a single link layer interface, also known as link aggregation group (LAG) or bundle. IEEE 802.3ad used the link aggregation control protocol (LACP) as its discovery protocol.  All links participating in LAG are considered members. Typically LAG would be used between an access switch and distribution switch or customer edge switch. Aggregated link can be used as normal L2 interface (access or trunk) or layer 3 interface.

Link aggregation takes place on point-to-point connections between two devices. A link aggregation group (LAG) balances traffic across the member links within an aggregated Ethernet bundle and effectively increases the uplink bandwidth. Another advantage of link aggregation is increased availability, because the LAG is composed of multiple member links. If one member link fails, the LAG continues to carry traffic over the remaining links.

Followings are the steps to configure L3 link aggregation between EX switches. In this example a single aggregated interface, ae0, is created (By default no aggregated interfaces exist)

• To create an aggregated interface, simply add an aggregated device under the [edit chassis] hierarchy as shown in the example below,

  user@EX# set chassis aggregated-devices ethernet device-count 5
  
  NOTE:"device-count" option used to define number of aggregated interfaces to be created on switch, the first interface will always start with "ae0">

• The next step is to define the parameters associated with the ae0 interface. As shown below, the ae0 interface configuration will include at least one logical unit along with the desired logical interface properties. The example below also includes LACP under the aggregated-ether-options hierarchy level. LACP configuration is not mandatory but if it is used,  at least one side will need to be configured in active mode in order to successfully establish the connection.
  

  user@EX# set interfaces ae0 aggregated-ether-options lacp active
  
  NOTE: Assign Active mode at one end of the link and Passive at the other

• To make the interface as L3 interface to route packets across the link put the aggregated interface (ae) to "intent"  family
  

  user@EX# set interface ae0 unit 0 family inet address 10.0.0.1/30

• The ae0 interface will be created after committing this portion of configuration. Verify AE interfaces by using following command,
  

  user@EX> run show interfaces terse | match ae

ae0                     up    up 
ae0.0                   up    up   inet     10.0.0.1/30 
ae1                     up    up 
ae1.0                   up    up   inet     20.0.0.1/30 
ae2                     up    down
ae2.0                   up    down eth-switch
ae3                     up    down
ae3.0                   up    down eth-switch
ae4                     up    up 
ae4.0                   up    up   eth-switch
  
  Once the ae0 interface has been created and defined within the configuration, the individual member links must be defined and associated with the ae0 bundle. This example shows a typical configuration which links member interfaces ge-0/0/10, ge-0/0/11, and ge-0/0/12 with the associated ae0 interface

• Associate member links with LAG (ae0)
  

  user@EX# set interface ge-0/0/10 ether-options 802.3ad ae0
user@EX# set interface ge-0/0/11 ether-options 802.3ad ae0
user@EX# set interface ge-0/0/12 ether-options 802.3ad ae0

• Use the show interfaces command with the desired option to verify the operational state for the aggregated interface and member links. Here we use the terse command option along with the pipe (|) option to filter the generated output. Verify that the aggregated interface and member links are operationally up
  

  user@switch> show interfaces terse | match ae0

ge-0/0/10.0             up    up   aenet    --> ae0.0
ge-0/0/11.0             up    up   aenet    --> ae0.0
ge-0/0/12.0             up    up   aenet    --> ae0.0
  
  This example shows that the ae0 interface along with all member links are operationally up

• Configure the second switch or the other side of the LAG with the same configuration.  To verify the LAG you can do a ping test to validate the link.