Specifications

28-6
Catalyst 2970 Switch Software Configuration Guide
78-15462-03
Chapter 28 Configuring EtherChannels
Understanding EtherChannels
LACP Modes
Table 28-2 shows the user-configurable EtherChannel LACP modes for the channel-group interface
configuration command.
Both the active and passive LACP modes enable interfaces to negotiate with partner interfaces to
determine if they can form an EtherChannel based on criteria such as interface speed and, for Layer 2
EtherChannels, trunking state and VLAN numbers.
Interfaces can form an EtherChannel when they are in different LACP modes as long as the modes are
compatible. For example:
• An interface in the active mode can form an EtherChannel with another interface that is in the active
or passive mode.
• An interface in the passive mode cannot form an EtherChannel with another interface that is also in
the passive mode because neither interface starts LACP negotiation.
LACP Interaction with Other Features
The DTP and the CDP send and receive packets over the physical interfaces 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 interfaces that are up and have LACP enabled for the
active or passive mode.
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.
Table 28-2 EtherChannel LACP Modes
Mode Description
active Places an interface into an active negotiating state in which the interface starts negotiations
with other interfaces by sending LACP packets.
passive Places an interface into a passive negotiating state in which the interface responds to LACP
packets that it receives, but does not start LACP packet negotiation. This setting minimizes
the transmission of LACP packets.
on Forces the interface to channel without PAgP or LACP. With the on mode, a usable
EtherChannel exists only when an interface group in the on mode is connected to another
interface group in the on mode.