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Understanding EtherChannels
You should exercise care when setting the mode to on (manual configuration). All ports configured in
Caution
the on mode are bundled in the same group and are forced to have similar characteristics. If the group is
misconfigured, packet loss or spanning-tree loops might occur.
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 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-channel global configuration
command).
PAgP sends and receives PAgP PDUs only from ports that are up and have PAgP enabled for the auto or
desirable mode.
Link Aggregation Control Protocol
The LACP is defined in IEEE 802.3ad and enables Cisco switches to manage Ethernet channels between
switches that conform to the 802.3ad protocol. LACP facilitates the automatic creation of EtherChannels
by exchanging LACP packets between Ethernet ports. You can use LACP only in single-switch
EtherChannel configurations; LACP cannot be enabled on cross-stack EtherChannels. For more
information, see the
By using LACP, the switch stack learns the identity of partners capable of supporting LACP 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, LACP 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, LACP adds the group to the spanning tree as a single switch port.
Catalyst 3750 Switch Software Configuration Guide
33-6
"EtherChannel Configuration Guidelines" section on page
Chapter 33
Configuring EtherChannels
33-11.
78-16180-02
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