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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
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Chapter 30 Configuring EtherChannels
Understanding EtherChannels

Exchanging LACP Packets

Both the active and passive LACP modes allow interfaces to negotiate with partner interfaces to
determine if they can form an EtherChannel based on criteria such as interface speed and, for Layer 2
EtherChannels, trunking state, and VLAN numbers.
Interfaces can form an EtherChannel when they are in different LACP modes as long as the modes are
compatible. For example:
• An interface in the active mode can form an EtherChannel with another interface that is in the active
mode.
• An interface in the active mode can form an EtherChannel with another interface in the passive
mode.
An interface in the passive mode cannot form an EtherChannel with another interface that is also in the
passive mode because neither interface starts LACP negotiation.
Note An EtherChannel cannot be configured in both the PAgP and LACP modes.
Physical Learners and Aggregate-Port Learners
Network devices are classified as PAgP physical learners or aggregate-port learners. A device is a
physical learner if it learns addresses by physical ports and directs transmissions based on that
knowledge. A device is an aggregate-port learner if it learns addresses by aggregate (logical) ports.
When a device and its partner are both aggregate-port learners, they learn the address on the logical
port-channel. The device sends packets to the source by using any of the interfaces in the EtherChannel.
With aggregate-port learning, it is not important on which physical port the packet arrives.
The switch uses source-MAC address distribution for a channel if it is connected to a physical learner
even if you configure the switch for destination-MAC address distribution.
These frame distribution mechanisms are possible for frame transmission:
• Load distribution based on the source-MAC address of the packet
• Load distribution based on the destination-MAC address of the packet
The switch supports up to eight ports in a PAgP group.
PAgP and LACP Interaction with Other Features
The Dynamic Trunking Protocol (DTP) and Cisco Discovery Protocol (CDP) send and receive packets
over the physical interfaces in the EtherChannel. Trunk ports send and receive PAgP and LACP protocol
data units (PDUs) on the lowest numbered VLAN.
Spanning tree sends packets over a single physical interface in the EtherChannel, regarding the
EtherChannel as one port.
PAgP sends and receives PAgP PDUs only from interfaces that have PAgP enabled for the auto or
desirable mode. LACP sends and receives LACP PDUs only from interfaces that have LACP enabled for
the active or passive mode.