When creating an EtherChannel, you can't just combine any interfaces with any settings. Several conditions must first be met. Typically, you can group two to eight physical interfaces, but some advanced switches can support up to sixteen.
Note: Although you can group any number of interfaces in an EtherChannel up to the allowed limit, it is recommended to bundle exactly 2, 4, 8, or 16 to achieve optimal performance and resource distribution. This approach provides an equal amount of traffic (percentage-wise) being load-balanced across the physical interfaces within the EtherChannel.
The physical interfaces must be compatible and share the same configuration settings, or they won't bundle into a port channel. The requirements for creating an EtherChannel include the same:
- Port type
- Speed and duplex
- Interface mode
- Switchport mode
- VLAN information
This means you cannot mix port types within the port channel interface, such as FastEthernet and GigabitEthernet interfaces, while parameters such as speed and bandwidth must be the same.
Additionally, if you're using Layer 3 functionalities, you can't aggregate switched and routed ports, so you'll need to use only Layer 2 or Layer 3 interfaces simultaneously for a single EtherChannel.
All individual interfaces must be configured to operate as access ports or trunks, and if they're in access mode, they must belong to the same VLAN. However, if they're trunks, they must allow the same range of VLANs on the trunk links, and the native VLAN must be the same.
Important: While additional links provide resiliency, most of them get blocked because of STP, and the available bandwidth between the connecting devices is not increased. However, with EtherChannel, the port channel is considered a single interface by STP, so it no longer sees the bundled interfaces as separate links. As a result, all physical ports stay active and operate simultaneously.
As you can see from the image above, when three additional links are added between the switches, STP blocks all of them to avoid loops. However, after binding them into a port channel, they all continue operating under the same logical interface since STP considers them a single logical interface.