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Chapter 7: LACP Port Trunks
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-S101
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 is a group
of ports on the switch. The ports in an aggregator are further 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 on a switch that are part of an aggregator transmit LACPDU
packets. If a switch port that is part of an aggregator does not receive
LACPDU packets from its corresponding port on the other device, it
assumes that the other port is not part of an LACP trunk. Instead, it
functions as a normal Ethernet port by forwarding 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.
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