30-4
Catalyst 2960 Switch SoftwareConfiguration Guide
78-16881-01
Chapter30 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.
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 in to a single logical link (channel or
aggregate port). Similarly configured ports are grouped based on ha rdware, 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

Table30-1 shows the use r-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 sta te 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 Ether Chann el with anoth er port that i s in the desirable or
auto mode.
•A port in the auto mode can form an EtherChannel with another port in the desirable mo de.
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.
Use the silent mode when the switch is connected to a device that is not PAgP-capable and seldom, if
ever, sends packets. An example of a silent partner is a file server or a packet analyzer that is not
generating traffic. In this case, running PAgP on a physical port connected to a silent partner prevents
that switch port from ever becoming operational. However, the silent setting allows PAgP to operate, to
attach the port to a channel group, and to use the port for transmission.
Table30-1 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.