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Chapter 7: LACP Port Trunks
104
Overview
LACP (Link Aggregation Control Protocol) port trunks perform the same
function as static trunks. They increase the bandwidth between network
devices by distributing the traffic load over multiple physical links. The
advantage of an LACP trunk over a static port trunk is its flexibility. While
implementations of static trunking tend to be vendor specific, the AT-S112
Management software implementation of LACP is compliant with the IEEE
802.3ad standard, making it interoperable with equipment from other
vendors that also comply with the standard. Therefore, you can create an
LACP trunk between an Allied Telesis device and network devices from
other manufacturers.
Another advantage is that ports in an LACP trunk can function in a
standby mode. This adds redundancy and resiliency to the trunk. If a link
in a static trunk goes down, the overall bandwidth of the trunk is reduced
until the link is re-established or another port is added to the trunk. In
contrast, an LACP trunk can automatically activate ports in a standby
mode when an active link fails so that the maximum possible bandwidth of
the trunk is maintained.
For example, assume you create an LACP trunk of ports 1 to 6 on a switch
and the switch is using ports 1 to 4 as the active ports and ports 5 and 6 as
reserve. If an active port loses its link, the switch automatically activates
one of the reserve ports to maintain maximum bandwidth of the trunk.
The main component of an LACP trunk is an aggregator which manages a
group of ports on the switch. On the AT-GS950/16PS switch, the ports
assigned to a trunk group are automatically assigned to an aggregator.
Only one aggregator can be assigned to each trunk group. With LACP
activated, each active trunk group is referred to as an aggregate trunk.
An aggregate trunk can consist of any number of ports on a switch, but
only a maximum of eight ports can be active at a time. If an aggregate
trunk contains more ports than can be active at once, the extra ports are
placed in a standby mode. Ports in the standby mode do not pass network
traffic, but they do transmit and accept LACP Data Unit (LACPDU)
packets, which the switch uses to search for LACP-compliant devices.
Only ports that are part of an aggregator transmit LACPDU packets. A port
that is part of an aggregator assumes that the other port is not part of an
LACP trunk if it does not receive LACPDU packets from its corresponding
port on the other device. Instead, it functions as port in standby mode and
does not forward network traffic. However, it does continue to send
LACPDU packets. If it begins to receive LACPDU packets, it automatically
transitions to an active or standby mode as part of an aggregate trunk.