EtherChannel Load Distribution Explained
An EtherChannel is a group of links that work together as a single link. You can use EtherChannels for load balancing. If you configure an EtherChannel, the EtherChannel distributes traffic among all ports configured in it. EtherChannel uses pre-defined rules for load balancing. These rules override the default frame-processing function. This tutorial explains default frame processing rules and how an EtherChannel overrides them for load balancing.
Default frame processing
A switch processes frames through three stages. These stages are learning, decision-making, and forwarding. In the learning stage, it learns the MAC addresses of connected devices and adds them to the CAM table. In the decision-making stage, it selects forwarding ports for incoming frames. In the forwarding stage, it forwards frames from ports selected in stage two.
It uses the source MAC address field of incoming frames to learn the MAC addresses of connected devices, and the destination MAC address field to make forwarding decisions. It uses the CAM table to save the MAC addresses of the connected devices. A CAM table entry contains a MAC address and an associated switch port. When the switch receives an incoming frame, it looks up the destination MAC address in the CAM table. If it finds a matching entry in the CAM table, it forwards the frame from the port mentioned in the entry. The switch uses the source address of incoming frames to build CAM table entries and the destination address to make forwarding decisions.
Example
Create a practice as shown in the following example. You can use any network simulator software or real devices. This tutorial uses Packet Tracer and provides download links for all labs.
PC1 sends a frame to PC2. The frame contains the source and destination MAC addresses. The frame reaches S1 on port F0/1. The following table lists the frame processing stages on S1.
| Learning | S1 adds an entry to the CAM table. This entry associates port F0/1 with MAC1. |
| Decision-making | S1 finds MAC2 in the CAM table. Since the CAM table has no entry for MAC2, S1 decides to forward it from all ports, excluding port F0/1. |
| Forwarding | S1 forwards the frame from all selected ports. |
The frame reaches S2 on port G0/1. The following table lists the frame processing stages on S2.
| Learning | S2 adds an entry to the CAM table. This entry associates port G0/1 with MAC1. |
| Decision-making | S2 finds MAC2 in the CAM table. Since the CAM table has no entry for MAC2, S2 decides to forward it on all ports except G0/1. |
| Forwarding | S2 forwards the frame from all selected ports. |
The frame reaches PC2. PC2 replies to the frame. The frame reaches S2 on port 1. S2 takes the following steps to process the frame.
| Learning | S2 adds an entry to the CAM table. This entry associates port F0/1 with MAC2. |
| Decision-making | S2 finds MAC1 in the CAM table. Since the CAM table has an entry for MAC1, S2 decides to forward it from port G0/1. |
| Forwarding | S2 forwards the frame from port G0/1. |
The frame reaches S1 on port G0/1. S1 takes the following steps to process the frame.
| Learning | S1 adds an entry to the CAM table. This entry associates port G0/1 with MAC2. |
| Decision-making | S1 finds MAC1 in the CAM table. Since the CAM table has an entry for MAC1, S2 decides to forward it from port F0/1. |
| Forwarding | S1 forwards the frame from port F0/1. |
Currently, this network has only one link between switches. If you add another link between these switches, STP will block that link to remove the loop. Create an additional link between the switches and run the show spanning-tree command to verify it.
Download Packet Tracer LAB with a loop
As shown in the output above, STP blocked port G0/2 to remove the loop. STP allows only one link between two switches. If you want to use more than one link, you must configure an EtherChannel and add all links to it.
EtherChannel Configuration
There are two ways to configure EtherChannel: static and dynamic. No matter which option you choose to configure the EtherChannel, the channel uses the same frame-processing rules after configuration.
This tutorial is the last part of the tutorial "EhterChannel Concepts and Configurations Explained.". Other parts of this series are the following.
Chapter 01 EtherChannel Basic Concepts Explained
Chapter 02 EtherChannel Manual Configuration
Chapter 03 Port Aggregation Protocol (PAgP) Explained
Chapter 04 Link Aggregation Control Protocol (LACP) Explained
The following image shows a static EtherChannel configuration on this network.
Packet Tracer LAB With EtherChannel Configuration
The above configuration creates a new EtherChannel on both switches and adds ports G0/1 and G0/2 to it. The EtherChannel appears as a new port, po1, on the switch. After this configuration, all protocols and processes running on both switches see port po1 instead of G0/1 and G0/2. Since STP sees only one connection between switches, it does not block port po1. Use the show spanning-tree command to verify this.
After EtherChannel configuration, the switch associates MAC addresses with the EtherChannel, not with the underlying physical ports. Use the show mac-address-table command to verify this.
After EtherChannel configuration, the switch uses EtherChannel load-balancing rules to determine which physical port within the EtherChannel will forward the incoming frame. The following table lists EtherChannel load balancing rules and options.
| Layer | Keyword [option] | Description |
| 2 | src-mac | Source MAC address |
| 2 | dst-mac | Destination MAC address |
| 2 | src-dst-mac | Both source and destination MAC |
| 3 | src-ip | Source IP address |
| 3 | dst-ip | Destination IP address |
| 3 | src-dst-ip | Both source and destination IP |
| 4 | src-port | Source TCP or UDP port |
| 4 | dst-port | Destination TCP or UDP port |
| 4 | src-dst-port | Both source and destination TCP or UDP port |
All switches support MAC-based rules. IP-based rules depend on the switch's model and software version. The EtherChannel load-balancing algorithm uses the low-order bits of the message header fields. The low-order bits tend to differ more in real networks. Most of the networks use the same high-order bits. By selecting the lower-order bits, the algorithm achieves better load balancing across the links.
EtherChannel load balancing configuration and verification
Load balancing is the default on EtherChannels. The default load balancing uses the source MAC addresses of incoming frames to select the forwarding port. The following command allows you to configure a custom load-balancing option.
Switch(config)#port-channel load-balance [option]
You can select any valid option from the above table. You can use the following command to view the configured option.
Switch#show etherchannel load-balance
Regardless of the EtherChannel load balancing rule you choose, the rule identifies fields in the frame headers. Since all frames in the same application share the same header fields, the switch forwards them over the same link in the EtherChannel.
For example, you connect to a website to get some information. The website may return thousands of frames based on the requested information. Those frames flow over the same link in the EtherChannel. For example, with the load distribution rule of src-mac (meaning source MAC address), all frames with the same MAC address flow over one link.
EtherChannel load balancing advantages
EtherChannel load balancing provides the following advantages.
- EtherChannel load balancing ensures that all frames in a single application use the same link of the EtherChannel. It prevents the switch from inadvertently reordering messages in that application flow by sending one frame over a busy link while immediately sending another over an unused link.
- EtherChannel load balancing integrates the load distribution algorithm into the hardware forwarding ASIC, so it operates at the same speed as forwarding any other frame.
- EtherChannel load balancing utilizes all active links in the EtherChannel.
EtherChannel load balancing disadvantages
EtherChannel load balancing has the following disadvantages.
It adds complexity to the network. EtherChannel requires that many port-level configurations be identical across all ports. If a port in the EtherChannel has a different configuration, the EtherChannel does not work. A misconfiguration on a single port can bring down the entire EtherChannel.
Conclusion
EtherChannel load balancing provides many load distribution options so that you can examine network flows to determine which fields have the greatest variety of values: source and destination MAC addresses, IP addresses, or transport-layer port numbers. The more variety in the values in the fields, the better the result you achieve from the load balancing.
By ComputerNetworkingNotes Updated on 2025-12-03