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Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 28 Configuring EtherChannels
Understanding EtherChannels
Ports can form an EtherChannel when they are in different LACP modes as long as the modes are
compatible. For example:
•
A port in the active mode can form an EtherChannel with another port that is in the active or passive
mode.
•
A port in the passive mode cannot form an EtherChannel with another port that is also in the passive
mode because neither port starts LACP negotiation.
LACP Interaction with Other Features
The CDP sends and receives packets over the physical ports in the EtherChannel. Trunk ports send and
receive LACP PDUs on the lowest numbered VLAN.
In Layer 2 EtherChannels, the first port in the channel that comes up provides its MAC address to the
EtherChannel. If this port is removed from the bundle, one of the remaining ports in the bundle provides
its MAC address to the EtherChannel.
LACP sends and receives LACP PDUs only from ports that are up and have LACP enabled for the active
or passive mode.
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.
Ports that are configured in the 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 in the on mode.
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.
