LACP Port Modes

LACP supports the automatic creation of port channels by exchanging LACP packets between ports. LACP does this by learning the capabilities of port groups dynamically and informing the other ports. Once LACP identifies correctly matched Ethernet links, it facilitates grouping the links into a port channel. Once an LACP mode has been configured, it can only be changed if a single interface has been assigned to the specified channel group. LACP supports two modes: active and passive. These two modes of operation are described in the following sections.

LACP Active Mode

LACP active mode places a switch port into an active negotiating state in which the switch port initiates negotiations with remote ports by sending LACP packets. Active mode is the LACP equivalent of PAgP desirable mode. In other words, in this mode, the switch port actively attempts to establish an EtherChannel with another switch that is also running LACP.

LACP Passive Mode

When a switch port is configured in passive mode, it will negotiate with an LACP channel only if it receives another LACP packet. In passive mode, the port responds to LACP packets that the interface receives but does not start LACP packet negotiation. This setting minimises the transmission of LACP packets. In this mode, the port channel group attaches the interface to the EtherChannel bundle. This mode is similar to the auto mode that is used with PAgP.

It is important to remember that the active and passive modes are valid on non-PAgP interfaces only. However, if you have a PAgP EtherChannel and want to convert it to LACP, then Cisco IOS software allows you to change the protocol at any time. The only caveat is that this change causes all existing EtherChannels to reset to the default channel mode for the new protocol. Table 33.2 below shows the different LACP combinations and the result of their use in establishing an EtherChannel between two switches:

Table 33.2 – EtherChannel Formation Using Different LACP Modes