How a LAG Forms

LAGs form automatically with LACP enabled on the device. There are four criteria for forming a LAG. Both actor and partner ports must:

1. Operate in full duplex mode.
2. Have matching local LAG and physical port admin keys for the device controlling LAG formation.
3. Operate in parallel in that a LAG can have only two devices associated with it.
4. Consist of two or more physical actor to partner port pairings unless the single port LAG feature is enabled.

LAG Formation displays a LAG formation example containing three devices with five 100Mbps ports and three 1Gb ports configured. For this example, all ports are operating in full-duplex mode, and the admin key for all LAG ports has been set to 100. Device A is the actor and therefore determines which ports will join a LAG. Devices B and C are the partners.

In our example two LAGs have formed because the actor ports are shared between two partner devices. Attempting to form a single LAG using all the actor ports would have broken the rule that actor and partner ports must operate in parallel.

LAG Formation

Actor ports 1 - 3 on device A directly connect to partner ports 1 - 3 on device B:

• We have already stated that all ports are operating in full-duplex mode, so rule 1 is satisfied for all three ports.
• Investigating the port admin keys, we see that ports 1 and 2 on device A are set to 100 (the same setting as all LAG ports on the device), while port 3 on device A is set to 200. Because the port admin keys are the same for both the LAG port and these physical ports, ports 1 and 2 satisfy rule 2. Because the admin key for physical port 3 is different from any possible LAG for this device, port 3 can not be part of any LAG.
• Because ports 1 and 2 for both the actor and partner operate in parallel with each other, rule 3 is satisfied for these ports.
• Rule 4 is satisfied, regardless of whether single port LAGs are enabled, because there are two aggregatable port pairings between devices A and B.

For these reasons, LAG 1 (lag.0.1) is formed using actor and partner ports 1 and 2.

Actor ports 4 - 8 on device A directly connect to partner ports 4 - 8 on device C:

• Because all ports are operating in full-duplex mode, rule one is satisfied for all five ports.
• Investigating port admin keys, we see that ports 4 - 6 on device A are set to 100 (the same setting as all LAG ports on the device), while ports 7 and 8 on device A are set to 300 and 400, respectively. Because port admin keys for all LAGs and the physical ports 4 - 6 are the same, physical ports 4 - 6 satisfy rule 2. Because the admin key settings for physical ports 7 and 8 do not agree with any LAG admin key setting on the device, ports 7 and 8 can not be part of any LAG.
• Because ports 4 - 6 for both the actor and partner operate in parallel with each other, rule 3 is satisfied for these ports.
• Rule 4 is satisfied, regardless of whether single port LAG is enabled, because there are three aggregatable port pairings between devices A and C.

For these reasons, LAG 2 is formed using actor and partner ports 4 - 6.

Note

Port speed is not a consideration in the forming phase for LAGs. LAG 2 contains 100Mbps and 1Gb port members.