Port Aggregation Protocol; Pagp Modes - Cisco WS-C2960-24LC-S Software Configuration Manual

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

Table 31-1
configuration command.
Table 31-1
Mode
auto
desirable Places a port into an active negotiating state, in which the port starts negotiations with other
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 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.
Catalyst 2960 Switch Software Configuration Guide
31-4
shows the user-configurable EtherChannel PAgP modes for the channel-group interface
EtherChannel PAgP Modes
Description
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.
ports by sending PAgP packets.
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.
Chapter 31 Configuring EtherChannels and Link-State Tracking
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