Link aggregation protocols overcome spanning tree protocol’s (STP) inherent design to block loops between switches by bundling of ports to appear as one logical connection to STP. With one logical connection, STP will not block the link but forward traffic.  Further the bundling can increase overall bandwidth between the switches.

So how do link aggregation protocols load-balance traffic across multiple links?
By reviewing the data inside the frames and using fields such as

• Source MAC address
• Destination MAC address
• Source IP address
• Destination IP address
• Source port
• Destination port

or a combination of the information.

In the example below, server D is sending traffic to desktop 3. The devices are on two different switches A and B. Traffic flows between the switches because of 1G connection on ports 1. When another connection is added on port 2, the traffic still flows over ports 1, because STP will block the link. The physically added ports do not add to the bandwidth, STP logically blocks the port.
After configuring link aggregation, the physical ports are bundled into logical Port-Channel. Traffic now flies across both links. STP sees the port channel as a single link connecting the switches, no need to block. Data frames are split between the 2 links in a round robin fashion based on the load balancing algorithm.

Link Aggregration

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