Specifications

78 Implementing IBM System Networking 10Gb Ethernet Switches
Link Aggregation Control Protocol
Link Aggregation Control Protocol (LACP) is an IEEE 802.3ad standard for grouping several
physical ports into one logical port (known as a dynamic trunk group or Link Aggregation
group) with any device that supports the standard. See the IEEE 802.3ad-2002 specification
for a full description of the standard.
The 802.3ad standard allows standard Ethernet links to form a single Layer 2 link by using
the Link Aggregation Control Protocol (LACP). Link aggregation is a method of grouping
physical link segments of the same media type and speed in full duplex, and treating them as
through they were part of a single, logical link segment. If a link in a LACP trunk group fails,
traffic is reassigned dynamically to the remaining links of the dynamic trunk group.
A port’s Link Aggregation Identifier (LAG ID) determines how the port can be aggregated. The
LAG ID is constructed mainly from the system ID and the port’s admin key, as follows:
򐂰 System ID: An integer value based on the switch’s MAC address and the system priority
assigned in the CLI.
򐂰 Admin key: A port’s Admin key is an integer value (1 - 65535) that you can configure in the
CLI. Each switch port that participates in the same LACP trunk group must have the same
admin key value. The Admin key is local significant, which means the partner switch does
not need to use the same Admin key value.
LACP automatically determines which member links can be aggregated and then aggregates
them. It provides for the controlled addition and removal of physical links for the link
aggregation. Up to 64 ports can be assigned to a single LAG, but only 16 ports can actively
participate in the LAG at a time.
Each port on the switch can have one of the following LACP modes.
򐂰 Off (default)
The user can configure this port in a regular static trunk group.
򐂰 Active
The port can form an LACP trunk. This port sends LACPDU packets to partner
system ports.
򐂰 Passive
The port can form an LACP trunk. This port responds only to the LACPDU packets sent
from an LACP active port.
Each active LACP port transmits LACP data units (LACPDUs), while each passive LACP port
listens for LACPDUs. During LACP negotiation, the admin key is exchanged. The LACP trunk
group is enabled if the information matches at both ends of the link. If the admin key value
changes for a port at either end of the link, that port’s association with the LACP trunk group
is lost.
When the system is initialized, all ports by default are in LACP off mode and are assigned
unique admin keys. To make a group of ports aggregatable, you assign them all the same
admin key. You must set the port’s LACP mode to active to activate LACP negotiation. You
can set other port’s LACP mode to passive, to reduce the amount of LACPDU traffic at the
initial trunk-forming stage.