EtherChannel - LACP fast-switchover
When creating an EtherChannel bundle using the LACP as the LAG protocol, it is possible to enable 1:1 link redundancy using the lacp fast-switchover command.
This command will essentially result in sub-second failover delay for an EtherChannel link when activated.
In a "normal" EtherChannel configuration using LACP, all links are active and share the load. If a single link fails, LACP must renegotiate which links are part of the bundle. During this renegotiation, the EtherChannel goes down temporarily. Once the negotiation is complete, the EtherChannel comes back up. This means that there will be downtime, even if it is for a split second.
However, when you enable LACP Fast Switchover, the logic behind the functionality changes: one link becomes the primary (active) link and the other becomes the standby link. When the primary link fails, the standby link takes over instantaneously ensuring virtually no disruption to the network. From a logical perspective, the EtherChannel bundle remains active because the standby link steps in.
Prior to entering the lacp fast-switchover command, you must ensure the following:
- The port channel protocol type is LACP.
- The
lacp max-bundle 1command has been entered on the port channel.
Note that the recommended setup is to configure only two ports in such a bundle, where one will act as active, and one as hot-standby for optimum performance. The most common use case for this is when you connect a server with (typically) two NICs to a switch using LACP, the fast-switchover is an ideal feature to maintain uninterrupted operation even during a link failure.
This is why the lacp max-bundle 1 command is recommended. For more info on this command, take a look at EtherChannel - max-bundle.
Links:
https://forum.networklessons.com/t/layer-3-etherchannel-on-cisco-ios-switch/1085/83?u=lagapidis
https://forum.networklessons.com/t/layer-3-etherchannel-on-cisco-ios-switch/1085/81?u=lagapidis