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Cisco IE 3000 Switch Software Configuration Guide
OL-13018-03
Chapter 38 Configuring EtherChannels and Link-State Tracking
Understanding EtherChannels
Caution
You should use care when using the
on
mode. This is a manual configuration, and ports on both ends of
the EtherChannel must have the same configuration. If the group is misconfigured, packet loss or
spanning-tree loops can occur.
Load Balancing and Forwarding Methods
EtherChannel balances the traffic load across the links in a channel by reducing part of the binary pattern
formed from the addresses in the frame to a numerical value that selects one of the links in the channel.
EtherChannel load balancing can use MAC addresses or IP addresses, source or destination addresses,
or both source and destination addresses. The selected mode applies to all EtherChannels configured on
the switch. You configure the load balancing and forwarding method by using the
port-channel
load-balance
global configuration command.
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 EtherChannel, they are
distributed across the ports in the channel based on the destination host’s MAC address of the incoming
packet. Therefore, packets to the same destination are forwarded over the same port, and packets to a
different destination are sent on a different port in the channel.
With source-and-destination MAC address forwarding, when packets are forwarded to an EtherChannel,
they are distributed across the ports in the channel based on both the source and destination MAC
addresses. This forwarding method, a combination source-MAC and destination-MAC address
forwarding methods of load distribution, can be used if it is not clear whether source-MAC or
destination-MAC address forwarding is better suited on a particular switch. With source-and-destination
MAC-address forwarding, packets sent from host A to host B, host A to host C, and host C to host B
could all use different ports in the channel.
With source-IP address-based forwarding, when packets are forwarded to an EtherChannel, they are
distributed across the ports in the EtherChannel based on the source-IP address of the incoming packet.
Therefore, to provide load-balancing, packets from different IP addresses use different ports in the
channel, but packets from the same IP address use the same port in the channel.
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, packets are sent to an EtherChannel and
distributed across the EtherChannel ports, 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
Figure 38-3
, an EtherChannel from a switch that is
aggregating data from four workstations communicates with a router. Because the router is a
