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Link aggregation: A cool tip for better bandwidth & redundancy on Ethernet switches


February 04, 2015

Ronen Isaac


ETHERNET EVERYWHERE BLOG: Have you ever worried that your existing 1G link may not be cutting the mustard for some data transmission? What about link redundancy and failover with your existing Ethernet device? Wouldn’t it be nice to know that you you’ll always have failover?

Most of us think that adding bandwidth availability or link redundancy would include costly equipment investment or link upgrades, but with link aggregation, that’s just not the case.

Link aggregation (LAG) is a technique used on Ethernet switches to enable the fast and inexpensive transmission of bulk data. Link aggregation enhances or increases the network capacity while maintaining a fast transmission speed while not investing in additional hardware or communications links, thus reducing cost.

Link aggregation (LAG) is used to describe various methods for using multiple parallel network connections to increase throughput beyond the limit that one link (one connection) can achieve. For link aggregation, physical ports must reside on a single switch. Combining can either occur such that multiple interfaces share one logical address (i.e. IP) or one physical address (i.e. MAC address), or it allows each interface to have its own address.

Within the IEEE specification the Link Aggregation Control Protocol (LACP) provides a method to control the bundling of several physical ports together to form a single logical channel. LACP allows a network device to negotiate an automatic bundling of links by sending LACP packets to the peer (directly connected device that also implements LACP). LACP works by sending frames (LACPDUs) down all links that have the protocol enabled. If it finds a device on the other end of the link that also has LACP enabled, it will also independently send frames along the same links enabling the two units to detect multiple links between themselves and then combine them into a single logical link.

LACP can be configured in one of two modes: active or passive. In active mode it will always send frames along the configured links. In passive mode however, it acts as "speak when spoken to", and therefore can be used as a way of controlling accidental loops (as long as the other device is in active mode).

Some claim that the most important feature of link aggregation is link failover. With link failover, traffic from a failed link can be switched over to working links in the aggregation. For security purposes, data is transmitted over the usual link and the other link in the aggregation sits idle or can transmit data from another physical link. If the first link goes down, a signal is sent to the second link to take over data transmission. In this situation, the second link can be set to either continue taking on data from both streams at a slower transmission rate or it can be set to prioritize which data has a higher priority.

Using link aggregation is an efficient, cost effective way to acquire better bandwidth when needed and always provide a failover. Check with your hardware manufacturer on the best techniques for your hardware. More information on LAG can be found on the IEEE web site.

To read more Ethernet Everywhere blogs from Ronen Isaac, click the posts below:

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