Content

| 3.9 |  | EtherChannel |
| 3.9.2 |  | Frame distribution |

EtherChannel balances the traffic load across the links in a channel by applying an algorithm to a portion of the binary pattern extracted from addresses in the Ethernet frame or encapsulated data packets. The algorithm output is a numerical value that is then associated with one of the links in the channel. On IOS-based switches, Catalyst 4000 series switches and Catalyst 5000/5500 family switches, EtherChannel load balancing can be based on either source MAC or destination MAC addresses. On Catalyst 6000 family switches, the load-balancing policy (frame distribution) can be based on a MAC address, IP address, or port number. With source MAC address forwarding, when packets are forwarded to an EtherChannel, they are distributed across the ports in the channel based on the source MAC address of the incoming packet. Therefore, to provide load balancing, packets from different hosts use different ports in the channel, but packets from the same host use the same port in the channel.

With destination MAC address forwarding, when packets are forwarded to an Ether-Channel, they are distributed across the ports in the channel based on the MAC address of the destination host in the incoming packet. Therefore, packets to the same destination are forwarded over the same port, and packets to a different destination might be sent on a different port in the channel.

The most recent EtherChannel frame distribution process is based on a Cisco-proprietary hashing algorithm. The algorithm is deterministic. Given the same addresses and session information, it always hashes to the same port in the channel, preventing out-of-order packet delivery.

Although EtherChannel frame distribution based on MAC address, IP address, or Layer 4 port number is possible, the configurable options allow for distribution based on source only (MAC, IP, or port), destination only (MAC, IP, or port), or both source and destination (MAC, IP, or port). The mode selected applies to all EtherChannels configured on the switch. Use the option that provides the greatest variety in the configuration. For example, if the traffic on a channel is going to a single MAC address only, using source addresses, IP addresses, or Layer 4 port numbers as the basis for frame distribution might provide better frame distribution than selecting MAC addresses as the basis.

In Figure , an EtherChannel connects a Catalyst 2950 to a router. The switch is also connected to four workstations. Because the router is referenced by the switch with a single MAC address, source-based forwarding on the switch EtherChannel ensures that the switch uses all available bandwidth to the router. The router is configured for destination-based forwarding because the large number of workstations ensures that the traffic is evenly distributed from the router EtherChannel.

Cisco 7000, 7200, 7500, and 8500 routers support Fast EtherChannels via logical port channel interfaces binding Fast Ethernet interfaces. Cisco 8500 routers also support Gigabit EtherChannel.