Some Layer 2-capable network equipment devices support LAG protected links in an active/standby mode but without LACP. This is commonly referred to as static LAG. In order to interwork with these products, the 7705 SAR supports configuring LAG without LACP.
LACP provides a standard means of communicating health and status information between LAG peers. If LACP is not used, the peers must be initially configured in a way that ensures that the ports on each end are connected and communicating. Otherwise, LAG will not be active. Which LAG peer is made active is a local decision. If the port priority settings are the same for all ports, it is possible that the two ends will select ports on different physical links and LAG will not be active. Decide the primary link by setting the port priority for the LAG on each peer to ensure that the active ports on each end coincide with the same physical link.
The key parameters for configuring static LAG are selection-criteria (set to best-port) and standby-signaling (set to power-off). The selection criteria is used to determine which selection algorithm decides the primary port (the active port in a no-fault condition). It is always the subgroup with the best-port (the highest-priority port - lowest configured value) that is chosen as the active subgroup. The selection criteria must be set to best-port before standby signaling can be placed in power-off mode. Once the selection criteria is set to best-port, setting the standby-signaling parameter to power-off causes the transmitters on the standby ports to be powered down.
After a switchover caused by a failure on the active link, the transmitters on the standby link are powered on. The switch time for static LAG is typically longer than it is with LACP, due to the time it takes for the transmitters to come up and transmission to be established. When the fault is restored, static LAG causes a revertive switch to take place. The revertive switch is of shorter duration than the initial switchover since the system is able to prepare the other side for the switch and initiate the switchover once it is ready.
Since the transmitters on the standby link are off, it is not possible for the LAG to respond to a physical disconnect (fault) on the standby link. This means that it is possible to have a failure on the active link result in a switch to a failed standby link.