Specifications

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Catalyst 2970 Switch Software Configuration Guide
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Chapter 28 Configuring EtherChannels
Understanding EtherChannels
administrative, and port parameter constraints. For example, PAgP groups the interfaces with the same
speed, duplex mode, native VLAN, VLAN range, and trunking status and type. After grouping the links
into an EtherChannel, PAgP adds the group to the spanning tree as a single switch port.
PAgP Modes
Table 28-1 shows the user-configurable EtherChannel PAgP modes for the channel-group interface
configuration command.
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.
Table 28-1 EtherChannel PAgP 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 (or the Link Aggregation Control Protocol
[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.