Allied Telesis AT-S111, AT-GS950/48PS manual 106

Chapter 7: LACP Port Trunks

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-S111 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-GS950/48PS 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.

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