Pagp Interaction With Other Features; Link Aggregation Control Protocol; Lacp Modes - Cisco Catalyst 3120 Software Manual

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 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 IEEE 802.3ad protocol. LACP facilitates the automatic creation of
EtherChannels by exchanging LACP packets between Ethernet ports.
By using LACP, the switch or switch stack learns the identity of partners capable of supporting LACP
and the capabilities of each port. It then dynamically groups similarly configured ports 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.

LACP Modes

Table 37-2
configuration command.
Table 37-2
Mode
active
passive
Both the active and passive LACP modes enable ports to negotiate with partner ports to an
EtherChannel based on criteria such as port speed and, for Layer 2 EtherChannels, trunking state and
VLAN numbers.
Cisco Catalyst Blade Switch 3120 for HP Software Configuration Guide
37-6
shows the user-configurable EtherChannel LACP modes for the channel-group interface
EtherChannel LACP Modes
Description
Places a port into an active negotiating state in which the port starts negotiations with other
ports by sending LACP packets.
Places a port into a passive negotiating state in which the port responds to LACP packets
that it receives, but does not start LACP packet negotiation. This setting minimizes the
transmission of LACP packets.
Chapter 37 Configuring EtherChannels and Link-State Tracking
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