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27-4
Catalyst 2950 Desktop Switch Software Configuration Guide
78-14982-01
Chapter 27 Configuring EtherChannels
Understanding EtherChannels
PAgP Modes
Table 27-1 shows the user-configurable EtherChannel modes for the channel-group interface
configuration command: on, auto, and desirable. Switch interfaces exchange PAgP packets only with
partner interfaces configured in the auto or desirable modes; interfaces configured in the on mode do
not exchange PAgP packets.
Both the auto and desirable modes allow 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 PAgP modes as long as the modes are
compatible. For example:
• An interface in the desirable mode can form an EtherChannel with another interface that is in the
desirable or auto mode.
• An interface in the auto mode can form an EtherChannel with another interface in the desirable
mode.
An interface in the auto mode cannot form an EtherChannel with another interface that is also in the
auto mode because neither interface starts PAgP negotiation.
An interface in the on mode that is added to a port channel is forced to have the same characteristics as
the already existing on mode interfaces in the channel.
Caution You should exercise care when setting the mode to on (manual configuration). All ports configured in
the on mode are bundled in the same group and are forced to have similar characteristics. If the group is
misconfigured, packet loss or spanning-tree loops might occur.
If your switch is connected to a partner that is PAgP-capable, you can configure the switch interface for
nonsilent operation by using the non-silent keyword. If you do not specify non-silent with the auto or
desirable mode, silent mode is assumed.
The silent mode is used when the switch is connected to a device that is not PAgP-capable and seldom,
if ever, sends packets. An example of a silent partner is a file server or a packet analyzer that is not
generating traffic. In this case, running PAgP on a physical port connected to a silent partner prevents
that switch port from ever becoming operational; however, the silent setting allows PAgP to operate, to
attach the interface to a channel group, and to use the interface for transmission.
Table 27-1 EtherChannel Modes
Mode Description
auto Places an interface into a passive negotiating state, in which the interface responds to PAgP
packets it receives but does not start PAgP packet negotiation. This setting minimizes the
transmission of PAgP packets.
desirable Places an interface into an active negotiating state, in which the interface starts
negotiations with other interfaces by sending PAgP packets.
on Forces the interface to channel without PAgP. 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.