25-6
Catalyst 2940 Switch Software Configuration Guide
78-15507-02
Chapter25 Configuring EtherChannels
Understanding EtherChannels
PAgP sends and receives PAgP PDUs only from interfaces that have PAgP enabled for the auto or
desirable mode. LACP sends and receives LACP PDUs only from interfaces that have LACP enabled for
the active or passive mode.
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, 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 di fferent hosts use different ports in the channel, but
packets from the same host use the same port in the channel (and the MA C addres s learne d by the switch
does not change).
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. You configure the load balancing and
forwarding method by using the port-channel load-balance global configuration command.
In Figure 25-3, 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.
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.