Port Aggregation Protocol; Pagp Modes - Cisco WS-C3750-48PS-S Software Configuration Manual

Chapter 33 Configuring 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
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
configuration command.
Table 33-1 EtherChannel PAgP Modes
Mode
auto
desirable Places a port into an active negotiating state, in which the port starts negotiations with other
on
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.
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.
78-16180-02
shows the user-configurable EtherChannel PAgP modes for the channel-group interface
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.
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).
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.
"EtherChannel Configuration Guidelines" section on page
Catalyst 3750 Switch Software Configuration Guide
Understanding EtherChannels
33-11.
33-5