how LACP load balancing works in the backend

[ajayshenoypadavu]

I have used 2 switches, 1 user is connected for switch 1 and another connected for switch 2. I have connected 2 links connected between 2 switches and i enabled the LACP to aggregate the links. now what algorithm will be used inside the switch for the load balancing between 2 uplinks.  

[M02@rt37 · Accepted solution]

Hello @ajayshenoypadavu 

When LACP is enabled to aggregate multiple links between two switches, the switch uses EtherChannel Load Balancing to distribute traffic across the links. By default, most Cisco switches use source MAC (src-mac) or source-destination MAC (src-dst-mac) as the load-balancing algorithm, but this can vary depending on the platform.

The switch applies a hashing function based on the selected algorithm, ensuring that packets from the same source-destination pair consistently take the same link.

Cisco switches offer various load-balancing options, including source or destination MAC, source or destination IP, and source-destination TCP/UDP port hashing, which can be adjusted using the 'port-channel load-balance' command. The best algorithm depends on the network type—L2 networks typically use MAC-based load balancing, while L3 networks benefit from IP-based hashing.

[ajayshenoypadavu]

Thank you for replying, May i know bit deep on how the traffic will split between two links and what is the technology will be using whether it could be cisco or any other brand. 

[M02@rt37]

@ajayshenoypadavu 

EtherChannel and similar link aggregation technologies in other vendors (such as LAG in Juniper or MLAG in Arista) use a hash-based load-balancing mechanism to distribute traffic across multiple bundled links.

The switch does not split individual packets across multiple links due to potential issues like out-of-order packet delivery, which can degrade performance for certain applications like VoIP or video streaming. Instead, the switch calculates a hash value based on the configured parameters (such as source/destination MAC, IP address, or Layer 4 port numbers) and assigns the traffic flow to a specific link within the EtherChannel bundle.

The hash function ensures that traffic from a particular source-destination pair consistently takes the same physical link, avoiding reordering issues. However, this method may lead to uneven distribution if most traffic belongs to a small number of flows. Different vendors provide various load-balancing algorithms, and in Cisco devices, administrators can change the hashing method using the port-channel load-balance command. The effectiveness of load balancing depends on factors like the number of links in the bundle, the diversity of traffic flows, and the chosen hashing method...

[M02@rt37]

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https://www.cisco.com/c/en/us/support/docs/lan-switching/etherchannel/12023-4.html