33-5
Catalyst 3750 Switch Software Configuration Guide
78-16180-02
Chapter 33 Configuring EtherChannels
Understanding EtherChannels
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 33-11.
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.
PAgP Modes
Table 33-1 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.
A port in the on mode that is added to a port channel is forced to have the same characteristics as the
already existing on mode ports in the channel.
Table 33-1 EtherChannel PAgP Modes
Mode Description
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.
on Forces a port to channel without PAgP (or the Link Aggregation Control Protocol [LACP]).
In the on mode, a usable EtherChannel exists only when a port group in the on mode is
connected to another port group in the on mode. This is the only setting that is supported
when the EtherChannel members are from different switches in the switch stack
(cross-stack EtherChannel).
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