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Understanding EtherChannels
With destination-IP-address-based forwarding, when packets are forwarded to an EtherChannel, they are
distributed across the ports in the EtherChannel based on the destination-IP address of the incoming
packet. Therefore, to provide load-balancing, packets from the same IP source address sent to different
IP destination addresses could be sent on different ports in the channel. But packets sent from different
source IP addresses to the same destination IP address are always sent on the same port in the channel.
With source-and-destination IP address-based forwarding, when packets are forwarded to an
EtherChannel, they are distributed across the ports in the EtherChannel based on both the source and
destination IP addresses of the incoming packet. This forwarding method, a combination of source-IP
and destination-IP address-based forwarding, can be used if it is not clear whether source-IP or
destination-IP address-based forwarding is better suited on a particular switch. In this method, packets
sent from the IP address A to IP address B, from IP address A to IP address C, and from IP address C to
IP address B could all use different ports in the channel.
Different load-balancing methods have different advantages, and the choice of a particular
load-balancing method should be based on the position of the switch in the network and the kind of
traffic that needs to be load-distributed. In
communicates with a router. Because the router is a single-MAC-address device, 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.
Use the option that provides the greatest variety in your configuration. For example, if the traffic on a
channel is going only to a single MAC address, using the destination-MAC address always chooses the
same link in the channel. Using source addresses or IP addresses might result in better load balancing.
Figure 31-3
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
31-8
Figure
Load Distribution and Forwarding Methods
Switch with
source-based
forwarding enabled
EtherChannel
Cisco router
with destination-based
forwarding enabled
Chapter 31
31-3, an EtherChannel of four workstations
Configuring EtherChannels
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