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30-6
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
OL-10101-02
Chapter 30 Configuring EtherChannels
Understanding EtherChannels
EtherChannel On Mode
EtherChannel on mode can be used to manually configure an EtherChannel. The on mode forces a port
to join an EtherChannel without negotiations. It can be useful if the remote device does not support PAgP
or LACP. With the on mode, a usable EtherChannel exists only when both ends of the link are configured
in the on mode.
This mode is not available on Long-Reach Ethernet (LRE) switches, because LRE interfaces do not
support an EtherChannel-only mode.
Ports that are configured with on mode in the same channel group must have compatible port
characteristics, such as speed and duplex. Ports that are not compatible are suspended, even though they
are configured with on mode.
Caution You should exercise care when using the on mode. This is a manual configuration, and ports on both
ends of the EtherChannel must have a similar configuration. If the group is misconfigured, packet loss
or spanning-tree loops can occur.
Understanding Load Balancing and Forwarding Methods
EtherChannel balances the traffic load across the links in a channel by randomly associating a newly
learned MAC address with one of the links in the channel.
With source-MAC address forwarding, packets forwarded to an EtherChannel 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. The MAC address learned by the switch does not change).
With destination-MAC address forwarding, packets forwarded to an EtherChannel are distributed across
the ports in the channel based on the destination host MAC address of 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. You configure the load balancing and forwarding method
by using the port-channel load-balance global configuration command.
In Figure 30-3, multiple workstations are connected to a switch, and an EtherChannel connects the
switch to the router. Source-based load balancing is used on the switch end of the EtherChannel to ensure
that the switch efficiently uses the bandwidth of the router by distributing traffic from the workstation
across the physical links. Since the router is a single MAC address device, it uses destination-based load
balancing to efficiently spread the traffic to the workstations across the physical links in the
EtherChannel.