Link Aggregation
Forwarding Principle
As shown in Figure 1, an Eth-Trunk link is deployed on the data link layer, that is, between the physical layer and the MAC sub-layer.
Figure 1 Eth-Trunk interface in the Ethernet protocol stack
An Eth-Trunk interface is assumed to be a physical interface on the MAC sub-layer. Therefore, frames transmitted in the MAC sub-layer only need to be delivered to the Eth-Trunk module that maintains an Eth-Trunk forwarding table.
The Eth-Trunk forwarding table is composed of the following parts:
HASH-KEY value
The key value is calculated through the hash algorithm on the MAC address or IP address in the packet.
Interface number
Eth-Trunk forwarding entries are relevant to the number of member interfaces in the Eth-Trunk. Different HASH-KEY values are mapped to different outbound interfaces.
For example, an Eth-Trunk supports a maximum of eight member interfaces. If four physical interfaces, 1, 2, 3, and 4, are bundled into an Eth-Trunk, the Eth-Trunk forwarding table contains four entries, as shown in Figure 2. In the Eth-Trunk forwarding table, the HASH-KEY values are 0, 1, 2, 3, 4, 5, 6, and 7, and the corresponding interface numbers are 1, 2, 3, 4, 1, 2, 3, and 4.
Figure 2 Example of an Eth-Trunk forwarding table
The Eth-Trunk module forwards a frame according to the Eth-Trunk forwarding table. The forwarding process is as follows:
1. The Eth-Trunk module receives a frame from the MAC sub-layer, and then extracts its source MAC address/IP address or destination MAC address/IP address.
2. The Eth-Trunk module calculates the HASH-KEY value using the hash algorithm.
3. Based on the HASH-KEY value, the Eth-Trunk module searches the Eth-Trunk forwarding table for the interface number, and then sends the frame from the corresponding interface.
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