If an XOR mathematical operation is performed on the last two bits of the source MAC address and the destination MAC address, the operation yields one of four results:
Each of these values points to a link in the Fast EtherChannel bundle. In the case of a two-port Fast EtherChannel, only a single bit is used in the XOR operation.
In order to distribute frames across the links in a channel, the EtherChannel reduces part of the binary pattern that the addresses in the frame form to a numerical value that selects one of the links in the channel. EtherChannel frame distribution uses a Cisco-proprietary hash algorithm. The algorithm is deterministic. If the same addresses and session information are used, the same port in the channel is always hashed. This method prevents out-of-order packet delivery.
The Cisco-proprietary hash algorithm computes a value in the 0-7 range. With this value as a basis, a particular port in the EtherChannel is chosen. The port setup includes a mask which indicates which values the port accepts for transmission. With the maximum number of eight ports in a single EtherChannel, each port accepts only one value. If the EtherChannel has four ports, each port accepts two values, and so on.
Note: The hash algorithm cannot be configured or changed to load balance the traffic among the ports in an EtherChannel.
Three Link EtherChannel Scenario
If the hash is perfect, when a port channel has three links, the flows are divided as 3/3/2. What the hash is based on determines the flow. For example, the hash can be based on one of these:
If the default is used, each distinct src/dst IP pair constitutes a flow. Then, it is necessary to factor in how fat each flow is since this can change the picture dramatically.
If the hash is perfect and all flows are equal, the three-link channel traffic is unevenly balanced at 3/3/2. In this scenario, in order to achieve perfect load-balancing, create a channel with two, four, or eight links.
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