Specifications

28-5
Catalyst 2970 Switch Software Configuration Guide
78-15462-03
Chapter 28 Configuring EtherChannels
Understanding EtherChannels
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 interface to a channel group, and to use the interface 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 interfaces 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.
PAgP sends and receives PAgP PDUs only from interfaces 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 interfaces.
By using LACP, the switch learns the identity of partners capable of supporting LACP and the
capabilities of each interface. It then dynamically groups similarly configured interfaces into a single
logical link (channel or aggregate port). Similarly configured interfaces are grouped based on hardware,
administrative, and port parameter constraints. For example, LACP groups the interfaces 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.