Cisco Systems 3560 사용자 설명서

After you configure an EtherChannel, configuration changes applied to the port-channel interface apply 
to all the physical ports assigned to the port-channel interface. Configuration changes applied to the 
physical port affect only the port where you apply the configuration. To change the parameters of all 
ports in an EtherChannel, apply configuration commands to the port-channel interface, for example, 
spanning-tree commands or commands to configure a Layer 2 EtherChannel as a trunk.

The Port Aggregation Protocol (PAgP) is a Cisco-proprietary protocol that can be run only on Cisco 
switches and on those switches licensed by vendors to support PAgP. PAgP facilitates the automatic 
creation of EtherChannels by exchanging PAgP packets between Ethernet ports. 

By using PAgP, the switch learns the identity of partners capable of supporting PAgP and the capabilities 
of each port. It then dynamically groups similarly configured ports into a single logical link (channel or 
aggregate port). Similarly configured ports are grouped based on hardware, administrative, and port 
parameter constraints. For example, PAgP groups the ports 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.

 shows the user-configurable EtherChannel PAgP modes for the channel-group interface 

configuration command. 

Switch ports exchange PAgP packets only with partner ports configured in the auto or desirable modes. 
Ports configured in the on mode do not exchange PAgP packets.

Both the auto and desirable modes enable ports to negotiate with partner ports to form an EtherChannel 
based on criteria such as port speed and, for Layer 2 EtherChannels, trunking state and VLAN numbers. 

Ports can form an EtherChannel when they are in different PAgP modes as long as the modes are 
compatible. For example:

A port in the desirable mode can form an EtherChannel with another port that is in the desirable or 
auto mode.

A port in the auto mode can form an EtherChannel with another port in the desirable mode. 

A port in the auto mode cannot form an EtherChannel with another port that is also in the auto mode 
because neither port starts PAgP negotiation.

If your switch is connected to a partner that is PAgP-capable, you can configure the switch port 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. 

auto 

Places a port into a passive negotiating state, in which the port responds to PAgP packets 
it receives but does not start PAgP packet negotiation. This setting minimizes the 
transmission of PAgP packets.

desirable  Places a port into an active negotiating state, in which the port starts negotiations with 

other ports by sending PAgP packets.