Allied Telesis AT-S106 - Manual (Page 86)

Chapter 7: LACP Port Trunks
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LACP 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-S106
Management software 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 reestablished 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 11 to 20 on a
switch and the switch is using ports 11 to 18 as the active ports and ports
19 and 20 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. Before the ports of a trunk are physically
connected and linked up between two switches with LACP activated, each
port on the switch is assigned an individual aggregator. After the network
cables are connected between the trunk ports and the ports are linked up,
the first aggregator in the trunk group assumes the aggregator role for all
active ports in the trunk group. Depending on your network topology, the
switch ports can be grouped into one or more trunks, referred to as
aggregate trunks.
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 a normal Ethernet port by
forwarding network traffic. However, it does continue to send LACPDU