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Catalyst 3750 Switch Software Configuration Guide
OL-8550-02
Chapter 34 Configuring EtherChannels and Link-State Tracking
Understanding EtherChannels
Figure 34-4 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 34-12.
By using PAgP, the 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
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