32-4
Catalyst 2928 Switch Software Configuration Guide
OL-23389-01
Chapter 32 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 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
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.
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.
Table 32-1
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.
