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16-3
Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter 16 Understanding and Configuring EtherChannel
Overview of EtherChannel
Understanding Manual EtherChannel Configuration
Manually configured EtherChannel ports do not exchange EtherChannel protocol packets. A manually
configured EtherChannel forms only when you enter configure all ports in the EtherChannel compatibly.
Understanding PAgP EtherChannel Configuration
PAgP supports the automatic creation of EtherChannels by exchanging PAgP packets between LAN
ports. PAgP packets are exchanged only between ports in auto and desirable modes.
The protocol learns the capabilities of LAN port groups dynamically and informs the other LAN ports.
Once PAgP identifies correctly matched Ethernet links, it facilitates grouping the links into an
EtherChannel. The EtherChannel is then added to the spanning tree as a single bridge port.
Both the auto and desirable modes allow PAgP to negotiate between LAN ports to determine if they can
form an EtherChannel, based on criteria such as port speed and trunking state. Layer 2 EtherChannels
also use VLAN numbers.
LAN ports can form an EtherChannel when they are in different PAgP modes if the modes are
compatible. For example:
• A LAN port in desirable mode can form an EtherChannel successfully with another LAN port that
is in desirable mode.
• A LAN port in desirable mode can form an EtherChannel with another LAN port in auto mode.
• A LAN port in auto mode cannot form an EtherChannel with another LAN port that is also in auto
mode, because neither port will initiate negotiation.
Understanding IEEE 802.3ad LACP EtherChannel Configuration
Cisco IOS Release 12.2(25)EWA and later releases support IEEE 802.3ad LACP EtherChannels. LACP
supports the automatic creation of EtherChannels by exchanging LACP packets between LAN ports.
LACP packets are exchanged only between ports in passive and active modes.
Table 16-1 EtherChannel Modes
Mode Description
on Mode that forces the LAN port to channel unconditionally. In the on mode, a usable
EtherChannel exists only when a LAN port group in the on mode is connected to another
LAN port group in the on mode. Because ports configured in the on mode do not negotiate,
there is no negotiation traffic between the ports.
auto PAgP mode that places a LAN port into a passive negotiating state, in which the port
responds to PAgP packets it receives but does not initiate PAgP negotiation.
desirable PAgP mode that places a LAN port into an active negotiating state, in which the port
initiates negotiations with other LAN ports by sending PAgP packets.
passive LACP mode that places a port into a passive negotiating state, in which the port responds
to LACP packets it receives but does not initiate LACP negotiation.
active LACP mode that places a port into an active negotiating state, in which the port initiates
negotiations with other ports by sending LACP packets.