Understanding LACP in Chassis Cluster Mode
In chassis cluster mode, aggregated Ethernet interfaces (ae) and redundant Ethernet interfaces (reth) coexist. Because the functionality of a redundant Ethernet interface relies on the JUNOS aggregated Ethernet framework, you can think of it as a special aggregated Ethernet interface.
A redundant Ethernet interface has active and standby links located on two nodes in a chassis cluster. All active links in a redundant Ethernet interface are located on a node, and all standby links are on the other node. The total number of active and standby links is limited to 16 (8 active and 8 standby).
LACP maintains a point-to-point LAG. Any port connected to the third point is denied. However, a redundant Ethernet interface does connect to two different systems or two remote aggregated Ethernet interfaces by design. To support LACP on both redundant Ethernet interface active and standby links, a redundant Ethernet interface can be modeled to consist of two sub-LAGs, where all active links form an active sub-LAG and all standby links form a standby sub-LAG. In this model, LACP selection logic is applied and limited to one sub-LAG at a time. In this way, two redundant Ethernet interface sub-LAGs are maintained simultaneously while all the LACP advantages are preserved for each sub-LAG.
It is necessary for the switches used to connect the nodes in the cluster to have a LAG link configured and 802.3ad enabled for each LAG on both nodes so that the aggregate links will be recognized as such and correctly pass traffic.
 | Note: The redundant Ethernet interface LAG child links from each node in the chassis cluster must be connected to a different LAG at the peer devices. If a single peer switch is used to terminate the redundant Ethernet interface LAG, two separate LAGs must be used in the switch. |
With LACP, it is essential for the redundant Ethernet interface to support hitless failover between the active and standby links in normal operation. The term hitless means that the redundant Ethernet interface state remains up during failover.
The lacpd process manages both the active and standby links of the redundant Ethernet interfaces. A redundant Ethernet interface pseudolink is in the up condition when the number of active up links is not less than the number of minimum links configured. Therefore, to support hitless failover, the LACP state on the redundant Ethernet interface standby links must be collecting and distributing before failover occurs.