Cisco Systems 3750E PAgP Modes, PAgP Interaction with Other Features, Mode Description, Auto

Chapter 37 Configuring EtherChannels and Link-State Tracking

Understanding EtherChannels

PAgP Modes

Table 37-1shows the user-configurable EtherChannel PAgP modes for the channel-groupinterface configuration command.

Table 37-1	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. This mode is not supported when the EtherChannel
	members are from different switches in the switch stack (cross-stack EtherChannel).
desirable	Places a port into an active negotiating state, in which the port starts negotiations with other
	ports by sending PAgP packets. This mode is not supported when the EtherChannel
	members are from different switches in the switch stack (cross-stack EtherChannel).

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-silentkeyword. If you do not specify non-silentwith 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.

PAgP Interaction with Other Features

The Dynamic Trunking Protocol (DTP) and the Cisco Discovery Protocol (CDP) send and receive packets over the physical ports in the EtherChannel. Trunk ports send and receive PAgP protocol data units (PDUs) on the lowest numbered VLAN.

In Layer 2 EtherChannels, the first port in the channel that comes up provides its MAC address to the EtherChannel. If this port is removed from the bundle, one of the remaining ports in the bundle provides its MAC address to the EtherChannel. For Layer 3 EtherChannels, the MAC address is allocated by the stack master as soon as the interface is created (through the interface port-channelglobal configuration command).

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

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