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40-7
Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-21521-01
Chapter 40 Configuring EtherChannels and Link-State Tracking
Understanding EtherChannels
To prevent a dual-active situation, the core switches send PAgP protocol data units (PDUs) through the
RSLs to the remote switches. The PAgP PDUs identify the active switch, and the remote switches
forward the PDUs to core switches so that the core switches are in sync. If the active switch fails or
resets, the standby switch takes over as the active switch. If the VSL goes down, one core switch knows
the status of the other and does not change state.
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
Eth
erChannel. 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
de
sirable 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 ident
ity 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 40-2 shows the user-configurable EtherChannel LACP modes for the channel-group interface
configuration command.
Both the ac
tive 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
VL
AN numbers.
Ta ble 40-2 EtherChannel LACP Modes
Mode Description
active Places a port into an active negotiating state in which the port starts negotiations with other
p
orts by sending LACP packets.
passive Places a port into a passive negotiating state in which the port responds to LACP packets
t
hat it receives, but does not start LACP packet negotiation. This setting minimizes the
transmission of LACP packets.