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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
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Chapter 40 Configuring EtherChannels and Link-State Tracking
Understanding EtherChannels
PAgP Modes
Table 40-1 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.
PAgP Interaction with Virtual Switches and Dual-Active Detection
A virtual switch can be two or more Catalyst 6500 core switches connected by virtual switch links
(VSLs) that carry control and data traffic between them. One of the switches is in active mode. The
others are in standby mode. For redundancy, remote switches, such as Catalyst 3750-E or 3560-E
switches, are connected to the virtual switch by remote satellite links (RSLs).
If the VSL between two switches fails, one switch does not know the status of the other. Both switches
could change to the active mode, causing a dual-active situation in the network with duplicate
configurations (including duplicate IP addresses and bridge identifiers). The network might go down.
Tab le 40-1 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).