LACP provides two key features that afford redundancy for LACP EtherChannels. These two features are LACP hot-standby ports and LACP 1:1 redundancy with fast switchover.
LACP Hot-Standby Ports
By default, when LACP is configured on ports, it tries to configure the maximum number of compatible ports in a port channel, up to the maximum allowed by the hardware, which is typically eight ports.
However, if LACP is unable to aggregate all the ports that are compatible into an EtherChannel (e.g. if the neighboring switch has hardware limitations and can only support a fewer number of ports per EtherChannel), then all the ports that cannot be actively included in the channel are put in hot-standby state and are used only if one of the active ports in the EtherChannel fails.
Cisco IOS software allows administrators to restrict the maximum number of bundled ports allowed in the port channel using the lacp max-bundle [number] command in interface configuration mode. By default, up to eight ports may be bundled into a single channel. Inversely, a port channel must have a minimum of one port configured.
However, Cisco IOS software allows this value to be changed via the port-channel min-links [number] interface configuration command on the port channel interface. This command specifies the minimum number of member ports that must be in the link-up state and bundled in the EtherChannel for the port channel interface to transition to the link-up state.
LACP 1:1 Redundancy with Fast-Switchover
The LACP 1:1 redundancy feature provides an EtherChannel configuration with one active link and the ability to perform a fast switchover to a hot-standby link. To use LACP 1:1 redundancy, configure an LACP EtherChannel with two ports: one active and one standby. In the event that the active link goes down, the EtherChannel stays up and the switch performs fast switchover to the hot-standby link. Traffic is then subsequently forwarded using that interface.
When the failed link becomes operational again (i.e. is restored to its original state), the switch performs another fast switchover to revert to the original active link. The LACP 1:1 redundancy feature must be enabled on both ends of the link.
NOTE: The configuration of LACP redundancy is beyond the scope of the SWITCH exam requirements and will not be illustrated in this chapter or the remainder of this guide.