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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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Chapter31 Configuring EtherChannels Understanding EtherChannels
PAgP Modes
Table31-1 shows the use r-configurab le Et herC ha nnel PAgP modes for the channel-group interface
configuration command on an NNI.
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 E therCha nnel with anothe r port th at is in th e desirable or
auto mode.
•A port in the auto mode can form an EtherChannel with another port that is 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 swit ch po rt 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.
PAgP Interaction with Other Features
Cisco Discovery Protocol (CDP) sends and receives packets over the physical ports in the EtherChannel.
Trunk ports send and receive PAgP protocol data units (PDUs) on the lowest numbered VLAN.
Note PAgP and CDP are only available on NNIs.
In Layer 2 EtherChannels, the first port in the channel tha t co mes up provide s its MAC address t o the
EtherChannel. If this port is removed from the bundle, one of the remaining ports in the bundle pr ovides
its MAC address to the EtherChannel.
PAgP sends and receives PAgP PDUs only from ports that are up and have PAgP enabled for the auto or
desirable mode.
Table31-1 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 sta rts nego tiations with other
ports by sending PAgP packets.