Physical Learners And Aggregate-Port Learners - Cisco Catalyst 3550 series Software Configuration Manual

Multilayer switch
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Chapter 30
Configuring EtherChannels
Note
An Etherchannel cannot be configured in both the PAgP and LACP modes.
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 passive mode cannot form an EtherChannel with another interface that is also in the
passive mode because neither interface starts LACP negotiation.
An interface in the on mode that is added to a port channel is forced to have the same characteristics as
the already existing on mode interfaces in the channel.
Note
An Etherchannel cannot be configured in both the PAgP and LACP modes.
Caution
You should exercise care when setting the mode to on (manual configuration). All ports configured in
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.

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.
PAgP cannot automatically detect when the partner device is a physical learner and the local device is
an aggregate-port learner. Therefore, you must manually set the learning method on the local device or
source-based distribution by using the pagp learn-method interface configuration command. With
source-based distribution, any given source MAC address is always sent on the same physical port.
You can also configure a single interface within the group for all transmissions and use other interfaces
for hot standby. The unused interfaces in the group can be swapped into operation in just a few seconds
if the selected single interface loses hardware-signal detection. You can configure which interface is
always selected for packet transmission by changing its priority by using the pagp port-priority
interface configuration command. The higher the priority, the more likely that the port will be selected.
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An interface in the active mode can form an EtherChannel with another interface that is in the active
or passive mode.
An interface in the active mode can form an EtherChannel with another interface in the passive
mode.
Catalyst 3550 Multilayer Switch Software Configuration Guide
Understanding EtherChannels
30-5

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