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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-25303-03
Chapter 1 Configuring EtherChannels and Link-State Tracking
Understanding EtherChannels
Figure 1-4 Relationship of Physical Ports, Logical Port Channels, and Channel Groups
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.
Port Aggregation Protocol
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. You can use PAgP only
in single-switch EtherChannel configurations; PAgP cannot be enabled on cross-stack EtherChannels.
For more information, see the “EtherChannel Configuration Guidelines” section on page 1-12.
By using PAgP, the switch or switch stack learns the identity of partners capable of supporting PAgP and
the capabilities of each port. It then dynamically groups similarly configured ports (on a single switch
in the stack) 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.
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Channel-group
binding
Physical ports
Logical
port-channel
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