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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
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Chapter 30 Configuring EtherChannels
Understanding EtherChannels
PAgP and LACP Interaction with Other Features
The Dynamic Trunking Protocol (DTP) and Cisco Discovery Protocol (CDP) send and receive packets
over the physical interfaces in the EtherChannel. Trunk ports send and receive PAgP and LACP protocol
data units (PDUs) on the lowest numbered VLAN.
Spanning tree sends packets over a single physical interface in the EtherChannel, regarding the
EtherChannel as one port.
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, 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.
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