LACP is used to make a selection of active links predictable and compatible with any vendor equipment. See IEEE STD 802.3-2002, Section 3, Clause 43.6.1, which describes how LACP allows standby and active signaling.
The 7210 SAS-T, 7210 SAS-R6, 7210 SAS-R6, 7210 SAS-Mxp, 7210 SAS-Sx/S 1/10GE (operating in standalone and standalone-VC mode), and 7210 SAS-Sx 10/100GE (operating in standalone mode) implementation of LACP supports the following:
A specific LAG member can be assigned to subgroups. The selection algorithm then assures that only members of a single subgroup are selected as active links.
The selection algorithm is effective only if LACP is enabled on a specific LAG. At the same time, it is assumed that the connected system also has LACP enabled (active or passive mode).
The algorithm selects active links based on following criteria:
Depending on the selection-criteria settings either the subgroup with the highest number of eligible links or the subgroup with the highest aggregate weight of all eligible members is selected first.
If multiple groups satisfy the selection criteria, the currently active subgroup remains active. Initially, the subgroup containing the highest priority eligible link is selected.
Only links pertaining to a single subgroup are active at any time.
An eligible member refers to a LAG member link that can potentially become active; that is, it is operationally up. If the slave-to-partner flag is set, the remote system does not disable its use (by signaling the ‟Standby” bit using LACP).
The selection algorithm works in a reverting mode. Each time the configuration or status of any link in a LAG changes, the selection algorithm is rerun. In case of a tie between two groups (one of them being currently active), the active group remains active (no reverting).