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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 10 Configuring the Switch Ports
Understanding the EtherChannel
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 desirable mode can form an EtherChannel with another interface that is in desirable
or auto mode.
• An interface in auto mode can form an EtherChannel with another interface in desirable mode.
• An interface in auto mode cannot form an EtherChannel with another interface that is also in auto
mode because neither interface initiates 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 STP 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, transmits 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.
Physical Learners and Aggregate-Port Learners
Network devices are classified as PAgP physical learners or aggregate-port learners. A device is a
physical learner if it learns addresses by physical ports and directs transmissions based on that learning.
A device is an aggregate-port learner if it learns addresses by aggregate (logical) ports.
Table 10-1 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 initiate PAgP packet negotiation. This setting minimizes
the transmission of PAgP packets.
desirable Places an interface into an active negotiating state, in which the interface initiates
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.
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