Owner's Manual

128 | Chapter 5. Configuring Slots and Ports on a Switch
NETGEAR 8800 User Manual
After you enable load-sharing, the LACP protocol is enabled by default. You configure
dynamic link aggregation by first assigning a primary, or logical, port to the group, or LAG and
then specifying the other ports you want in the LAG.
LACP, using an automatically generated key, determines which links can aggregate. Each
link can belong to only one LAG. LACP determines which links are available. The
communicating systems negotiate priority for controlling the actions of the entire trunk (LAG),
using LACP, based on the lowest system MAC number. You can override this automatic
prioritization by configuring the system priority for each LAG.
After you enable and configure LACP, the system sends PDUs (LACPDUs) on the LAG ports.
The LACPDUs inform the remote system of the identity of the sending system, the
automatically generated key of the link, and the desired aggregation capabilities of the link. If
a key from a particular system on a given link matches a key from that system on another
link, those links are aggregatable. After the remote system exchanges LACPDUs with the
LAG, the system determines the status of the ports and whether to send traffic on which
ports.
Among those ports deemed aggregatable by LACP, the system uses those ports with the
lowest port number as active ports; the remaining ports aggregatable to that LAG are put into
standby status. Should an active link fail, the standby ports become active, also according to
the lowest port number. (See
Configuring LACP on page 133 for the number of active and
standby LACP links supported per platform.)
All ports configured in a LAG begin in an unselected state. Based on the LACPDUs
exchanged with the remote link, those ports that have a matching key are moved into a
selected state. If there is no matching key, the ports in the LAG remain in the unselected
state.
However if more ports in the LAG are selected than the aggregator can handle because of
the system hardware, those ports that fall out of the hardware’s capability are moved into
standby state. The lowest numbered ports are the first to be automatically added to the
aggregator; the rest go to standby. As the name implies, these ports are available to join the
aggregator if one of the selected ports should go offline.
You can configure the port priority to ensure the order that ports join the aggregator. However,
that port must first be added to the LAG before you can configure the LACP settings. Again, if
more than one port is configured with the same priority, the lowest-numbered port joins the
aggregator first.
After the ports in the LAG move into the selected state, LACP uses the mux portion of the
protocol to determine which ports join the aggregator and can collect and distribute traffic. A
few seconds after a port is selected, it moves into the mux state of waiting, and then into the
mux state of attached. The attached ports then send their own LACP sync messages
announcing that they are ready to receive traffic.
The protocol keeps sending and receiving LACPDUs until both sides of the link have echoed
back each other’s information; the ends of the link are then considered synchronized. After
the sync messages match up on each end, that port is moved into the aggregator (into the
mux state of collecting-distributing) and is able to collect and distribute traffic.