Practical example: Two Gigabit ports are aggregated into a 2Gbps port

The cases shared in this issue are related to wired network issues.

Background of the problem
We often receive feedback from users that when using our switches that support aggregation (whether static aggregation or LACP) to connect to NAS or switches, the single-terminal file transfer or throughput cannot achieve the bandwidth stacking effect and can only run at the full bandwidth of one port.

In the above two topologies, when using switches to aggregate or switches to aggregate directly with NAS servers, and using computers to read/send files from NAS point-to-point, it will appear that the rate of a single physical port (1G) can only be fully utilized and 2G transmission cannot be achieved. Users will think that it is a problem with the switch equipment and that bandwidth cannot be superimposed.

Troubleshooting ideas
Confirm that there is no problem with the aggregation settings;
Confirm that there is no problem with the quality of the network cable;
Confirm that the bottleneck of computer-to-computer throughput is not on the computer;


There are several switch algorithms:

Source MAC address: This algorithm hashes the source MAC address of the packet entering the switch, and then assigns a physical port to transmit the packet based on the result of the HAS calculation;



Basic Analysis
(1) Confirm that the above points are completely set correctly. There is no problem with the settings or physical lines that affects the judgment of this problem;

(2) Understand the switch aggregation mechanism and the corresponding switch processing behavior of several aggregation algorithms:

The switch port aggregation algorithm mechanism: Since the switch does not support application layer analysis, it does not support connection balance after aggregation. Therefore, the switch algorithm is based on each data packet entering the switch and is calculated and then assigned to a physical port for transmission.


Destination MAC address: This algorithm hashes the destination MAC address of the packet entering the switch, and then assigns a physical port to transmit the packet according to the result of the HAS calculation;
Source destination MAC address: This algorithm hashes the source MAC address and MAC address of the packet entering the switch, and then assigns a physical port to transmit the packet according to the result of the HAS calculation;
The other three types of source IP, destination IP, source destination IP mechanism is the same as the MAC mechanism, but IP is used for HAS calculation, and then the port is matched. When there are more layer 2 data packets in some traffic scenarios, a MAC-based algorithm is needed to divert the traffic.


Taking the source and destination MAC addresses as an example, the hashing calculation is performed based on the source and destination MAC addresses as the input conditions to obtain the forwarding port number. Two different sets of source and destination MAC addresses may have the same port number after being processed by the algorithm. For example, if two ports are converged, many HAS values ​​will eventually be evenly distributed to these two ports.

Problem Summary


After the switch performs port aggregation, since it is a single terminal, the HAS value obtained by the switch algorithm corresponds to one physical port, and data can only be forwarded on one port, so it can only run at the full rate of one port. Only in the scenario of multi-terminal use can the aggregation of the switch reflect the function of bandwidth stacking.




For this type of problem, users think that all ports should be fully utilized if the ports are aggregated. However, this is not actually possible because the switch does not match based on the number of connections, but matches ports based on the HAS value algorithm of each packet. Once you understand this mechanism, this problem is easy to understand.