Understanding the Link Aggregation Control Protocol
JUNOS Software supports the Link Aggregation Control Protocol (LACP), which is a subcomponent of IEEE 802.3ad. LACP provides additional functionality for LAGs.
LACP provides a standardized means for exchanging information between partner systems on a link to allow their link aggregation control instances to reach agreement on the identity of the LAG to which the link belongs, and then to move the link to that LAG, and to enable the transmission and reception processes for the link to function in an orderly manner.
For example, when LACP is not enabled, a local LAG might attempt to transmit packets to a remote individual interface, which causes the communication to fail. (An individual interface is a non-aggregatable interface.) When LACP is enabled, a local LAG cannot transmit packets unless a LAG with LACP is also configured on the remote end of the link.
LACP does not support automatic configuration on SRX3000 and SRX5000 line devices, but partner systems are allowed to perform automatic configuration. When an SRX3000 or SRX5000 line device is connected to a fully 802.3ad-compliant partner system, static configuration of LAGs is initiated on the SRX3000 and SRX5000 line device side, and static configuration is not needed on the partner side.
 | Note: When an SRX3000 or SRX5000 line device is connected to a Juniper Networks MX Series router, static configuration of LAGs is needed at both the actor and partner systems. |
LACP is supported in standalone deployments, where aggregated Ethernet interfaces are supported, and in chassis cluster deployments, where aggregated Ethernet interfaces and redundant Ethernet interfaces are supported simultaneously.
By default, aggregated and redundant Ethernet links do not exchange link aggregation control protocol data units (PDUs), which contain information about the state of the link. You can configure Ethernet links to actively transmit link aggregation control PDUs, or you can configure the links to passively transmit them, sending out link aggregation control PDUs only when they receive them from the remote end of the same link. The local end of a child link is known as the actor and the remote end of the link is known as the partner. That is, the actor sends link aggregation control PDUs to its protocol partner that convey what the actor knows about its own state and that of the partner’s state.
| Note: Although the LACP functions on the SRX3000 and SRX5000 line devices are similar to the LACP features on Juniper Networks MX Series routers, the following LACP features on MX Series routers are not supported on SRX3000 and SRX5000 line devices: link protection, system priority, and port priority for aggregated Ethernet interfaces. Instead, SRX3000 and SRX5000 line devices provide active/standby support with redundant Ethernet interface LAGs in chassis cluster deployments. |